IMPACT OF DROUGHT AND WATER CONSERVATION ON
H2S FORMATION IN SEWER PIPES
A Thesis submitted in fulfilment of the requirements for the degree of
Master of Engineering
Chunyi Yuan
B. Eng.
School of Civil, Environmental and Chemical Engineering
College of Science, Engineering and Technology
RMIT University
August 2010
Declaration
I hereby declare that except where due acknowledgement has been made; the work is that of
the author alone; the work has not been submitted previously, in whole or in part, to qualify
for any other academic award; the content of the thesis is the result of work which has been
carried out since the official commencement date of the approved research program; and, any
editorial work, paid or unpaid, carried out by a third party is acknowledged.
Chunyi Yuan
23rd August
ii
Acknowledgements
First of all, I would like to thank my supervisor, Dr Maazuza Othman from the School of
Civil, Environmental and Chemical Engineering, for her continuous support during my
Master’s program. Dr Othman was always there to listen and give advice. She taught me ways
to approach research problems and how to solve them to accomplish any goal. I also would
like to thank my associate supervisor, Dr Niranjali Jayasuriya from the school of Civil,
Environmental and Chemical Engineering for help me in the research study.
Also many thanks go to City West Water for their support in this research for allowing me
access to the sewer manholes and to take samples. In particular, I wish to thank Matt White
and all other staff in City West Water. Also I would like to thank all staff operate the Melton
and Sunbury Wastewater Treatment Plants for their help.
I would like to thank Yenni Yenni for her computer program support during this project.
Special thanks must also go to colleagues and technicians in Civil Engineering and Chemical
Engineering in RMIT. Those that deserve special mention are: Peg Gee Zhang, Pavel Ryjkov,
Cameron Crombie, Madhu Mohan, Ray Tracy, Sandro Longano, Bao Thach Nguyen. Other
staff members in the school of Civil, Environmental and Chemical Engineering that I extend
my thanks to: Professor John Buckeridge, Assoc Professor Sujeeva Setunge, Professor Mike
Xie, Marlene Mannays and Sharon Taylor.
Last, but not least, I would like to thank my parents for supporting me during the whole study.
They always stood by me and encourage me throughout the study. I also would like to thank Yi Jiao, Jun Guo, Shuo Chen and other friends for all their encouragement.
iii
Abstract
One of the main problems associated with the transportation of sewage in sewer pipes has
been the formation of hydrogen sulphide (H2S). In addition to being an odour nuisance, H2S
emissions exceeding 1 mg/L are categorised as a health risk and can enhance corrosion
potential in concrete sewer pipes.
The main processes involving sulphur in gravity sewers are sulphide generation and emission
of hydrogen sulphide into the sewer atmosphere. H2S is produced from sulphate present in
sewage through reduction reactions by sulphate-reducing bacteria. As the average pH of sewage is normally around 7, sulphide usually exists in the form of HS- ions and aqueous H2S.
When certain conditions apply, aqueous H2S crosses the air-water interface and diffuses into
the sewer pipe atmosphere. Many researchers have examined the formation and emission of
H2S and have developed models to predict the concentration of sulphide both in sewage and
sewer pipe atmosphere. The formation and emission of H2S in sewer systems is governed by a
large number of factors. These factors include temperature, pH, hydraulic conditions (i.e.
sewage velocity), sewage characteristics and ventilation.
In recent years, Australia has suffered from drought, which has led to a number of water
conservation practices being implemented throughout the country. In Melbourne alone, a
number of water restrictions have been put in place by the government. These new restrictions
have led to reductions in quantities of sewage flowing through the sewers, which in turn have
had a major impact on hydraulics in sewer pipes. This reduction in quantity of sewage has
reduced flushing of the system and is likely to affect the characteristics of sewage and
consequently increase potential problems of safety, odour and corrosion due to the build-up of
hydrogen sulphide within the sewer.
The aim of this research was to investigate the formation of hydrogen sulphide for different
sewage characteristics and flow rates. An experimental set-up was developed to simulate a
section of a gravity sewer pipe, the set-up comprised of a pipe of 2 m long and 155 mm inside
diameter, referred to in this thesis as a laboratory sewer pipe. The first stage of the
experimental program involved developing a biological growth rich in sulphate-reducing
iv
bacteria inside the laboratory sewer pipe using synthetic sewage. To enhance and promote
film growth, sludge rich in anaerobic bacteria (sulphate-reducing bacteria) was collected from
the anaerobic digester at a local wastewater treatment plant. The second stage of the
experimental program involved monitoring of the laboratory sewer pipe’s aqueous and air
atmosphere inside the pipe. The aqueous phase was monitored for the concentration of
sulphate, sulphide, pH, soluble chemical oxygen demand (CODs) and chemical oxygen
demand (COD). The pipe’s atmosphere was monitored for oxygen, H2S gas concentrations
and temperature. These parameters were monitored at the inlet and outlet of the laboratory
sewer pipe and the feed tank for different flow rates and sewage characteristics. Synthetic
toilet sewage was used during this stage. A two-phase mathematical model was developed
and used to predict sulphide concentration in sewage and H2S concentration in sewer pipe
atmosphere at different flow conditions.
A one-week period of field monitoring was held in two manholes as a part of this project. H2S
concentrations were logged by gas detectors inside the manholes. Raw sewage samples were
collected using two auto-samplers and analysed for COD, CODs and sulphide. The aim was to
determine the concentration and variation of H2S inside the manholes. The results showed that
the levels of H2S inside the manholes were around 1 mg/L.
Results showed that using synthetic toilet sewage that contained 29.5 mg/L sulphate, a higher
aqueous sulphide concentration was measured compared to that at 18.2 mg/L. The aqueous
sulphide concentration increased by 89.3% with the 11.3 mg/L increase in the sulphate
concentration. Similarly, a higher CODs concentration, 36.8% increase was obtained with a
21.4% increase in the aqueous sulphide concentration. Increasing the sewage velocity by
85.7% increased the sulphide build-up rate by 15.4%.
A model that can predict sulphide concentration in the liquid phase and H2S in the air phase in the sewer at different conditions was developed based on two-phase model using MATLAB®
software. The model was calibrated using experimental data and used to compare sulphide
concentrations predicted using the model with those obtained experimentally.
The concentrations of sulphide predicted using the two-phase model were in agreement with
those measured using the laboratory sewer pipe in terms of trend but agreement in terms of
value varied. The predictions of H2S in the atmosphere were higher by 50 to 85% than
v
measured concentrations. The model was also used to predict the sulphide concentrations at
different sewage conditions. At the experimental time of 24 hours, sulphide concentration in
the water phase increased 38% when the velocity increased from 0.034 to 0.238 m/s while
H2S concentration up by 65.8%. A higher initial sulphate concentration in the aqueous phase
led to more sulphide generation, 20.2%, but the concentrations of H2S decreased by 5.6% at
the conditions tested. Increased initial CODs concentrations from 242 to 383 mg/L resulted in
increased sulphide concentrations, 14% more for aqueous sulphide and 33% for H2S in the
pipe atmosphere. The two-phase model also showed that increasing the pipe slope, the
sulphide in aqueous phase decreased while H2S went up slightly. The same trend was
observed for changes in pH. However, the increase in sewage temperature resulted in a net
increase in the sulphide concentration. It could be concluded from the results that pH and flow
rate were the major factors affecting hydrogen sulphide emission rate.
vi
Contents
Declaration.................................................................................................................................ii
Acknowledgements ..................................................................................................................iii
Abstract.....................................................................................................................................iv
Contents ...................................................................................................................................vii
List of Figures............................................................................................................................x
List of Tables ..........................................................................................................................xiv
Chapter One: Introduction .........................................................................................................1
1.1 Introduction .....................................................................................................................1
1.2 Thesis Structure ...............................................................................................................2
Chapter Two: Literature Review ...............................................................................................3
2.1 Introduction .....................................................................................................................3
2.2 Sulphide Build-up in Sewer Systems ..............................................................................3
2.2.1 Bacteria and Sewer Pipe Slime Layer ......................................................................4
2.2.2 Factors Affect Sulphide Build-up in Sewer Pipes ....................................................5
2.3 Hydrogen Sulphide Emission in Sewer Pipes .................................................................6
2.4 Sewer Pipe Corrosion ......................................................................................................9
2.5 Models for Prediction of H2S in Sewer Pipes................................................................11
2.5.1 Models for Sulphide Build-up ................................................................................11
2.5.2 Hydrogen Sulphide Emission Prediction................................................................14
2.5.3 Sulphide Oxidation Rate.........................................................................................17
2.5.4 Concrete Corrosion Rate ........................................................................................18
Chapter Three: Materials and Methods ...................................................................................20
3.1 Experimental Set-up ......................................................................................................20
3.1.1 Preliminary Experimental Set-up ...........................................................................20
3.1.2 Final Experimental Set-up ......................................................................................24
3.2 Materials ........................................................................................................................27
3.3 Methods .........................................................................................................................27
3.3.1 Development of Biological Growth .......................................................................27
3.3.2 Laboratory Sewer Pipe Runs ..................................................................................29
3.4 Analysis .........................................................................................................................31
vii
3.5 Field Monitoring............................................................................................................32
3.5.1 Monitoring Locations .............................................................................................32
3.5.2 Sampling Procedure................................................................................................35
Chapter Four: Results and Discussion.....................................................................................37
4.1 Introduction ...................................................................................................................37
4.2 The Development of Biological Growth in the Laboratory Sewer Pipe .......................37
4.2.1 Experimental Results in the Initial Stage ...............................................................38
4.2.2 Experimental Results during Postgate’s Grow Medium ........................................39
4.2.3 Experimental Results when Using Synthetic Sewage ............................................43
4.3 Performance of the Laboratory Sewer Pipe...................................................................48
4.3.1 Sulphur Processes in the Laboratory Sewer pipe ...................................................48
4.3.2 Sulphate Calibration ...............................................................................................48
4.3.3 Sulphate Reduction and Sulphide Build-up ...........................................................51
4.3.4 Mass Balances of Sulphate and Sulphide Concentration .......................................66
4.4 Factors Affecting the Concentration of Sulphide in the Liquid Phase and H2S in the Air
Phase....................................................................................................................................69
4.4.1 Effect of Velocity on the Generation of Sulphide in Sewage and H2S in the
Atmosphere......................................................................................................................69
4.4.2 Effect of Sulphate Concentration on the Generation of Sulphide in Sewage and H2S
in the Atmosphere............................................................................................................72
4.4.3 Effect of COD on the Generation of Sulphide in Sewage and H2S in the
Atmosphere......................................................................................................................74
4.5 Model to Predict Sulphide in Sewer System and Predicted Sewer Pipe Corrosion ......76
4.5.1 Two-phase Model ...................................................................................................77
4.5.2 Comparison of Prediction and Experimental Results.............................................78
4.5.3 Comparison of Sulphide Concentration Using the Two-Phase Model under
Different Sewer Conditions .............................................................................................83
4.5.4 Sulphide Oxidation Rate in the Liquid Phase.........................................................91
4.5.5 Predict Concrete Sewer Pipe Corrosion Rate .........................................................92
4.6 Field Monitoring Results ...............................................................................................94
4.6.1 Two-phase Model to Predict H2S in Sewer Pipes ..................................................94
4.6.2 Effect of Temperature on H2S Concentration ........................................................97
4.6.3 Effect of Sewage Constituent on H2S Concentration .............................................98
Chapter Five: Conclusions ....................................................................................................101
viii
5.1 Biological Growth of Sulphate Reducing Bacteria .....................................................101
5.2 Laboratory Performance of Sewer Pipe.......................................................................101
5.3 Model for Prediction of Sulphide Concentrations.......................................................102
References .............................................................................................................................104
Appendix ...............................................................................................................................112
ix
List of Figures
Figure 2.1: Schematic Diagram of Slime Layer Growth on Sewer Pipe Wall (Pomeroy and
Boon, 1990) ...............................................................................................................................4
Figure 2.2: Predicted Hydrogen Sulphide Initial Emission Rate of Sewer Pipe with Diameter
0.3 m under Different pH and Slope (Yongsiri et al., 2003).....................................................8
Figure 2.3: Cross-section of a Sewer Pipe that Shows the Sulphur Process both in Sewage and
Atmosphere................................................................................................................................9
Figure 3.1: Schematic Diagram of the Preliminary Experimental Set-up...............................23
Figure 3.2: Schematic Diagram of the Final Experimental Set-up..........................................25
Figure 3.3: Final Experimental Set-up Picture ........................................................................26
Figure 3.4: Monitoring Location that Shows on Google® Maps.............................................33
Figure 3.5: Location of Manhole WIL54A. Located in Field, Kororoit Creek road...............33
Figure 3.6: Location of Manhole NWI1. Located Opposite 4–14 Orange Street ...................34
Figure 3.7: Sampling Cabinet and Testing Auto-sampler Located in Manhole NWI1...........35
Figure 4.1: Sewer Pipe at the End of the Initial Stage Showing Bacteria Growth on the Inside
Walls of the Laboratory Sewer Pipe........................................................................................39
Figure 4.2: Temperature and O2 Concentration of Gas Samples during Test B Period ..........41
Figure 4.3: H2S Concentration during Test A Period..............................................................42
Figure 4.4: Time at which Gas Detector recorded H2S Concentration above 0.0 ppm (Test B)
.................................................................................................................................................42
Figure 4.5: Measured Sulphide and Sulphate Concentration when Using Synthetic Sewage 45
Figure 4.6: H2S Concentration Inside the Laboratory Sewer Pipe During the Last Stage of
Biological Growth Development.............................................................................................46
Figure 4.7: Outside of the Laboratory Sewer Pipe Wall during the Last Stage ......................47
Figure 4.8: Inside of the Laboratory Sewer Pipe Wall during the Last Stage.........................47
Figure 4.9: Sulphur Cycle Processes during the Experimental Period....................................48
Figure 4.10: Sulphate Calibration Curve.................................................................................49
x
Figure 4.11: Measurement of the Sulphate Concentration. The Machine Shown in the Picture
is HACH DR 4000...................................................................................................................50
Figure 4.12: Temperature, pH and DO Concentration of Sewage Samples............................51
Figure 4.13: Calculation of the Parameters for Laboratory Sewer Pipe..................................54
Figure 4.14: DO Concentration during the Experimental Period............................................65
Figure 4.15: Initial Sulphate and CODs Concentrations on Measured and Calculated Sulphide
in the Liquid Phase ..................................................................................................................68
Figure 4.16: Effect of Initial Sulphate and CODs Concentration on H2S/H2S(aq)....................68
Figure 4.17: Comparison of pH, Temperature, Concentrations of Initial Sulphate and CODs of
Experiments with Velocity of 0.034 m/s and 0.238 m/s .........................................................70
Figure 4.18: Sulphide Concentrations at Different Velocities ................................................70
Figure 4.19: H2S Concentration at Different Velocities..........................................................71
Figure 4.20: DO Concentration under Different Velocity.......................................................72
Figure 4.21: Sulphide Concentration under Synthetic Sewage with Different Initial Sulphate
Concentration ..........................................................................................................................73
Figure 4.22: H2S Concentration under Synthetic Sewage with Different Original Sulphate
Concentration ..........................................................................................................................74
Figure 4.23: Comparison of pH, Temperature, Concentrations of DO and Original Sulphate of
Experiments with CODs of 242 mg/L and 383 mg/L..............................................................75
Figure 4.24: Sulphide Concentration under Synthetic Sewage with Different CODs
Concentration ..........................................................................................................................75
Figure 4.25: H2S Concentration Using Synthetic Sewage for Different CODs.......................76
Figure 4.26: pH, DO, Temperature, Sulphide, Sulphate and CODs of the Liquid Phase in the
Laboratory Sewer Pipe ............................................................................................................78
Figure 4.27: Comparison of Sulphide Concentrations in the Liquid Phase Predicted using the
Two-phase Model and Experimental Data Reported in Table 4.6. .........................................79
Figure 4.28: Comparison of H2S in the Air Phase Predicted using the Two-phase Model and
Experimental Data Reported in Table 4.6 ...............................................................................79
Figure 4.29: Comparison of Sulphide Concentration in the Liquid Phase Predicted Using the
Two-phase Model and Experimental Data Reported in Table 4.7 ..........................................80
xi
Figure 4.30: Comparison of H2S in the Air Phase Predicted Using the Two-phase Model and
Experimental Data Reported in Table 4.7 ...............................................................................80
Figure 4.31: Comparison of Sulphide Concentration in the Liquid Phase Predicted Using the
Two-phase Model and Experimental Data Reported in Table 4.8 ..........................................81
Figure 4.32: Comparison of H2S in the Air Phase Predicted Using the Two-phase Model and
Experimental Data Reported in Table 4.8 ...............................................................................81
Figure 4.33: Comparison of Sulphide Concentration in the Liquid Phase Predicted Using the
Two-phase Model and Experimental Data Reported in Table 4.9 ..........................................82
Figure 4.34: Comparison of H2S in the Air Phase Predicted Using the Two-phase Model and
Experimental Data Reported in Table 4.9 ...............................................................................82
Figure 4.35: Predicted Sulphide Concentrations in the Liquid Phase at Sewage Velocity of
0.034 m/s and 0.238 m/s..........................................................................................................83
Figure 4.36: Predicted H2S Concentrations in the Air Phase under Velocity 0.034 m/s and
0.238 m/s .................................................................................................................................84
Figure 4.37: Predicted Sulphide Concentrations in the Liquid Phase under Initial Sulphate
Concentrations 18.2 mg/L and 29.5 mg/L...............................................................................85
Figure 4.38: Predicted H2S Concentrations in the Air Phase under Initial Sulphate
Concentrations 18.2 mg/L and 29.5 mg/L...............................................................................85
Figure 4.39: Predicted Sulphide Concentrations in the Liquid Phase under Initial CODs
Concentrations 242 mg/L and 383 mg/L (Initial Sulphide Concentrations 0.024 mg/L and
0.015 mg/L, Respectively).......................................................................................................86
Figure 4.40: Predicted H2S Concentrations in the Air Phase under Initial CODs
Concentrations 242 mg/L and 383 mg/L (Initial Sulphide Concentrations 0.024 mg/L and
0.015 mg/, Respectively) .........................................................................................................86
Figure 4.41: Predicted Sulphide Concentrations in the Liquid Phase under Initial CODs
Concentrations 242 mg/L and 383 mg/L (Same Initial Sulphide Concentrations 0.024 mg/L)
.................................................................................................................................................87
Figure 4.42: Predicted H2S Concentrations in the Air Phase under Initial CODs
Concentrations 242 mg/L and 383 mg/L (Same Initial Sulphide Concentrations 0.024 mg/L)
.................................................................................................................................................87
Figure 4.43: Predicted Sulphide Concentrations in the Liquid Phase under Sewer Slope of
0.004, 0.008 and 0.01 ..............................................................................................................88
xii
Figure 4.44: Predicted H2S Concentrations in the Air Phase under Sewer Slope of 0.004,
0.008 and 0.01 .........................................................................................................................89
Figure 4.45: Predicted Sulphide Concentrations in the Liquid Phase under Temperatures of
10°C, 20°C and 30°C...............................................................................................................89
Figure 4.46: Predicted H2S Concentrations in the Air Phase under Temperatures of 10°C,
20°C and 30°C.........................................................................................................................90
Figure 4.47: Predicted Sulphide Concentrations in the Liquid Phase at pH 6.75, 7 and 8......90
Figure 4.48: Predicted H2S Concentrations in the Air Phase at pH 6.75, 7 and 8...................91
Figure 4.49: Chemical Sulphide Oxidation Rate Calculated Compared with the Rate in our
System for Results Reported in Tables 4.6 to 4.9 ...................................................................92
Figure 4.50: Comparison of H2S Concentrations in the Air Phase using the Two-phase Model
with Real Data for Manhole WIL54A.....................................................................................95
Figure 4.51: Comparison of H2S Concentrations in the Air Phase using the Two-phase Model
with Real Data for Manhole NWI1 .........................................................................................96
Figure 4.52a: Temperature Value for Two Different Days in Same Manhole WIL54A ........97
Figure 4.52b: Comparison of H2S Concentrations under Different Temperatures in the Same
Manhole WIL54A ...................................................................................................................97
Figure 4.53: Measured H2S Concentrations of Manholes WIL54A and NWI1 ......................99
Figure 4.54: Predicted H2S Concentrations of Manholes WIL54A and NWI1.......................99
xiii
List of Tables
Table 2.1: Corrosion Rate and Lifetime of Sewer Pipes (Zhang et al., 2008) ........................10
Table 2.2: Z-Formula to Evaluate the Magnitude of the Sulphide Problem in Gravity Sewer
Pipes (Pomeroy and Parkhurst, 1977;ASCE, 1989;ASCE and WPCF, 1982)........................11
Table 3.1: Summary of Experimental Program.......................................................................22
Table 3.2: Postgate’s Grow Medium Constituents (Hauser and Holder, 1986) ......................27
Table 3.3: Composition of the Synthetic Sewage Used to Feed the Laboratory Sewer Pipe,
with a Ratio of COD: N: P=100:5:1 (Weidhaas et al., 2007)..................................................29
Table 3.4: Synthetic Sewage Constituents (Van Manh and Anh, 2005) .................................30
Table 3.5: Parameters of Manhole WIL54A and NWI 1 ........................................................34
Table 4.1: Parameters of Gas Samples M................................................................................38
Table 4.2: Sulphide Concentration during the Second Stage when Using Postgate’s Grow
Medium....................................................................................................................................40
Table 4.3: Characteristics of Constituent of Postgate’s Grow Medium and Synthetic Sewage
.................................................................................................................................................44
Table 4.4: Sulphate Stock Solutions........................................................................................49
Table 4.5: Initial Sulphate Concentration in the Different Solutions Used in Preparing
Synthetic Toilet Sewage ..........................................................................................................50
Table 4.6: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and the
Feed/Effluent Tank (flow rate=1 L/min, slope=1%, initial sulphate concentration =18.2 mg/L)
.................................................................................................................................................53
Table 4.6a: Calculation of Coefficient A for Table 4.6...........................................................55
Table 4.6b: H2S Concentration at the Inlet (Table 4.6) ...........................................................57
Table 4.7: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and
Feed/Effluent Tank (flow rate=1 L/min, slope=1%, initial sulphate concentration = 29.5 mg/L,
initial CODs = 242 mg/L)........................................................................................................58
Table 4.7a: Calculation of Coefficient A for Table 4.7...........................................................59
Table 4.7b: H2S Concentration at the Inlet (Table 4.7) ...........................................................59
Table 4.7c: Comparison of Sulphate, Sulphide and H2S Concentration .................................60
xiv
Table 4.8: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and
Feed/Effluent (flow rate=1 L/min, slope=1%, initial sulphate concentration = 29 mg/L, initial
CODs = 297 mg/L) ..................................................................................................................61
Table 4.8a: Calculation of Coefficient A for Table 4.8...........................................................62
Table 4.8b: H2S Concentration of the Inlet (Table 4.8)...........................................................62
Table 4.9: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and
Feed/Effluent Tank (flow rate=7 L/min, slope=1%, initial CODs = 258 mg/L).....................63
Table 4.9a: Calculation of Coefficient A for Table 4.9...........................................................64
Table 4.9b: H2S Concentration at the Inlet during this Period ................................................64
Table 4.10: Coefficient A for Sulphide Build-up Equation for the Different Experimental
Results .....................................................................................................................................65
Table 4.11: Mass Balances of Sulphate and Sulphide Concentrations for Tables 4.6, 4.7, 4.8
and 4.9 .....................................................................................................................................66
Table 4.12: Proportion of Measured Sulphide Concentration, Calculated Sulphide and H2S of
Calculated H2S (aq) ...................................................................................................................67
Table 4.13: Parameters of Experimental Results under Synthetic Sewage with Different
Original Sulphate Concentration .............................................................................................72
Table 4.14: Concrete Sewer Pipe Corrosion Rate under Results Reported in Table 4.6 to 4.9
and Field Study........................................................................................................................94
Table 4.15: Temperature, pH, CODs and Sulphide Concentration of Manhole WIL54A and
NWI1 .......................................................................................................................................98
xv
Chapter One: Introduction
1.1 Introduction
Australia has been suffering from drought for many years. Most major cities in Australia are
facing a water shortage with main water storage reservoirs less than half-full. High
temperatures caused a reduction in water levels in dams and reservoirs of 0.1% a day in
Melbourne in 2007 (Melbourne Water, 2009).
Due to the lack of water, Victoria has set a water consumption target of 155 L per person per
day and developed several programs to reduce water usage and achieve the target water
consumption goal. Government reports show that in 2008/2009, Melbourne reduced its
household water use by 35% and its industry water use by 41% when compared to the 1990s
(Victoria Government, 2010).
Drought and water conservation efforts also affected the sewer system. According to Ker and
Morton (2008), the city’s sewers were carrying around 20% less water than seven years ago,
but the reduction was unrelated to the sewer pipe bursts and sewer blockages.
The change of total volume of sewage may change the formation of a complex mixture of
objectionable odour-causing substances such as hydrogen sulphide (H2S), volatile organics,
sulphur compounds and nitrogenous compounds.
Hydrogen sulphide formation in sewer pipes has been studied for many years because of its
odour nuisance, health risks to human beings and because it can lead to the corrosion of
concrete pipes (Yongsiri et al., 2004; Pomeroy and Bowlus, 1946; Boon and Lister, 1975;
Nielsen and Hvitved-Jacobsen, 1988). The generation of gaseous hydrogen sulphide, H2S(g),
in the sewers’ atmosphere results from the transfer of molecular hydrogen sulphide, H2S(aq),
from the water phase to the overlaying atmosphere (Yongsiri et al., 2004). In sewer systems,
sulphide is generated through the biological reduction of sulphate under anaerobic conditions.
Fundamental processes related to the sulphur cycle in gravity sewer networks are: 1)
1
formation of sulphide in sediments and biofilms; 2) transfer of H2S(aq) at the air-water
interface; 3) chemical and biological oxidation of sulphide in the biofilms, sediments, and
water phase; 4) adsorption and biological oxidation of H2S(g) at sewer surfaces exposed to the
sewer atmosphere; and 5) precipitation of sulphide in the water phase (Boon and Lister,
1975;Hvitved-Jacobsen et al., 1988;Pomeroy and Parkhurst, 1977;Thistlethwayte and Goleb,
1972;Yongsiri et al., 2004).
Modelling sulphide formation has been included in a conceptual sewer process model that
takes into account organic matter transformation under aerobic and anaerobic conditions
(Tanaka and Hvitved-Jacobsen, 1998;Tanaka et al., 2000;Yongsiri et al., 2004). However,
information on other processes with respect to sulphide reactions and transfer in gravity sewer
networks (for example, emission and oxidation) is limited. Therefore, there is a need to work
on more processes and prediction models for real sewer networks when addressing the design,
operation and maintenance of sewer networks.
The objectives of this study were to investigate sulphide generation at different conditions in
the sewer pipe, (e.g. flow rate, initial concentration of sulphate). An associated objective was
to develop a model based on the two-phase model that can be used to predict sulphide
generation and concentration.
1.2 Thesis Structure
Chapter Two of this thesis presents a literature review summarising former research on the
anaerobic environment of sewer pipes, sulphate reduction and sulphide build-up cycles,
hydrogen sulphide emission, and models to predict hydrogen sulphide concentration and
assess the potential corrosion of sewer pipes. Chapter Three describes the materials and
methodology used in this research. Chapter Four contains experimental and field monitoring
results, prediction of model and discussion of these results. Chapter Five presents the
conclusions based on the results in Chapter Four.
2
Chapter Two: Literature Review
2.1 Introduction
Odour generation due to the emission of hydrogen sulphide (H2S) is a common problem in
sewer networks. Hydrogen sulphide presence in sewer systems can cause many problems
such as posing a health risk to human beings exposed to the gas and the corrosion of pipes
that may reduce the lifetime of the sewers (ASCE, 1989; Nielsen and Keiding, 1998; Nielsen
et al., 1998; USEPA, 1974).
Hydrogen sulphide is a poisonous gas that has an unpleasant smell similar to that of a rotten
egg. Evidence shows that 0.03% (300 ppm) of H2S can cause death after lengthy exposure
(Pomeroy, 1941). H2S can kill a human in only few minutes if the concentration is over 2000
ppm (Allen and Yang, 1991).
H2S has a molecular weight of 34.08 g/mole, a little heavier than air. At the low temperature
of -60°C, it compresses to a liquid. While at 20°C, it is soluble at 3850 mg/L in pure water
(Pomeroy and Boon, 1990).
2.2 Sulphide Build-up in Sewer Systems
In sewer pipes, sulphide could be present in four forms, these are:
1) H2S gas in the sewer atmosphere;
2) Dissolved H2S gas in the liquid phase; 3) Sulphide ions (HS- and S2-) in the liquid phase;
4) Sulphide precipitates such as ZnS, FeS (Spooner, 2007), deposited on the bottom of
the sewer pipe with sediments.
According to the fundamental processes for sulphur cycle in sewer networks, sulphide is
generated in sediments and biofilms by sulphate-reducing bacteria. Then, dissolved H2S gas
diffuses to the sewer atmosphere at certain conditions. In the pipe atmosphere, sulphide is
3
oxidised in the biofilms growing on the pipe’s wall. This could be followed by the
precipitation of sulphide in the water phase (Yongsiri et al., 2004).
2.2.1 Bacteria and Sewer Pipe Slime Layer
When sewage flows, a layer of slime grows on the sewer pipe walls (referred to as biofilm).
The thickness of the biofilm is thin when the flow rate is fast, while thicker at slow flow rates.
The depth of the layer that contains oxygen is only 0.4 mm (i.e. aerobic film). Thicker
biofilms, for example, 0.3–3 mm, comprise an anaerobic layer. It is in this anaerobic layer
where sulphide is produced (Pomeroy and Boon, 1990). A schematic diagram of a typical
biofilm growth on a sewer pipe wall is represented in Figure 2.1.
S ew ag e F lo w
A ero b ic Z on e
A n aero b ic Z on e S u lph ide P rod u cing A rea
S lim e lay er typ ically 0.3 m m to 3 m m
Inn er A n aero b ic Z o ne
P ip e W all
Figure 2.1: Schematic Diagram of Slime Layer Growth on Sewer Pipe Wall (Pomeroy
and Boon, 1990)
Sulphide species sometimes can be presented in sewage due to the discharge of industrial
wastewater, which could range from 16.7–30 mg/L (Spooner, 2007;Tomar and Abdullah,
1994). However, the main source of sulphide is usually due to biological activity because 2-) can be reduced to sulphide due to sulphate-reducing bacteria (SRB) (see Eq.
sulphate (SO4
2.1 and 2.2). The typical species of sulphate-reducing bacteria are desulfovibrio and
desulfuricans (Larry and Hamilton, 2007). In typical domestic sewage, the concentration of
4
sulphate is in the range of 40–200 mg/L (Araùjo et al., 2000; Kalogo and Verstraete, 1999;
Paing et al., 2000).
−
−
SO
organic
matter
anaerobic
bacteria
CO
(2.1)
+
+
2 S +→
+
2 4
OH 2
2
2
−
S
H
(2.2)
+
+ 2 →
SH 2
According to the literature, sulphide concentrations of 0.5, 3 and 10 mg/L in sewage are
classified as low, medium and high, respectively (Boon and Lister, 1975; Hvitved-Jacobsen,
2002).
2.2.2 Factors Affect Sulphide Build-up in Sewer Pipes
Sulphide formation rates in gravity sewer pipes are affected by many factors, including re-
aeration conditions, concentration of organic matter and oxygen in sewage, and temperature
(USEPA, 1974). Sulphide generation was observed in locations with lower velocity,
insufficient re-aeration condition and high temperature (Hvitved-Jacobsen et al., 2000;
Nielsen et al., 1992; Tomar and Abdullah, 1994) .
Norsker et al. (1995) found that the highest sulphide generation rate in biofilms was 80 mg/(m2·h), which occurred at oxygen levels below 1 ppm.
The velocity of sewage is one of the factors for sulphide generation. When the flow is slow,
solids and organic matters may settle down near the bottom of the sewer pipes and consume
dissolved oxygen. The process of sulphide build-up could continue until the depletion of
sulphate organic nutrients occurs. However, if the stream is swift, it will increase the oxygen
level in the water phase and prevent sulphide generation (Pomeroy and Boon, 1990).
Kuhl and Jorgensen (1992) found that for trickling filters, up to 50% of organic matter was
removed due to sulphate reduction. They also found that for biofilm incubated in high
sulphate and glucose concentration, sulphide reduction rates were the highest (0.05 mg/(cm3·h)) but it was 1.5–5 times lower than Nielsen and Hvitved-Jacobsen’s (1988) results.
5
Laboratory experiments and field investigations show that sewage that has a higher
biodegradable substrate could cause higher sulphide build-up rates. Moreover, additional
organic matters could promote the growth of sulphate-reducing bacteria and when the
population increases, sulphide generation is at a high rate (Larry and Hamilton, 2007; Norsker
et al., 1995; Nielsen et al., 2008a).
Dissolved oxygen (DO) concentration above 0.5 mg/L can prevent the generation of sulphide
while in some conditions, more than 1.0 mg/L may required (Pomeroy and Boon, 1990;
USEPA, 1991). When DO concentration is below this level, sulphate reduction can take place
in the anaerobic biofilm and dissolved sulphide can present as dissolved hydrogen sulphide
gas (H2S) in bulk sewage (USEPA, 1991; Yongsiri et al., 2005).
In partly filled sewer pipes, i.e. gravity sewers, the water surface that is exposed to the air can
absorb oxygen but the absorption rate is slow. If the DO is present in sewage, dissolved
sulphide will oxidise mostly to elemental sulphur (Pomeroy and Parkhurst, 1977;Nielsen et al.,
2003;Nielsen et al., 2004e;Yongsiri et al., 2005). In biofilm, dissolved sulphide oxidised to
elemental sulphur, 0.5 g O2 rate of 1.0 g dissolved sulphide is required (Nielsen et al., 2004b).
2.3 Hydrogen Sulphide Emission in Sewer Pipes
Field investigation clearly showed that the concentration of hydrogen sulphide in gravity sewers and sewer infrastructures was up to about 300 mg/m3 in the air (Pomeroy and Bowlus,
1946; Thistlethwayte and Goleb, 1972; Matos and Aires, 1995).
As the pH for sewage is around 7, the main species of hydrogen sulphide in water are H2S(aq) and HS- (see Eq. 2.3) (Yang et al., 2005). In typical water, H2S is half-ionised, which is 50% present as HS- and 50% of H2S(aq) at a pH 7 at 20°C (Yongsiri et al., 2005). S2- can be
presented when pH is high (see Eq. 2.4) (Fu and Shen, 1990).
+
−
HS
pH
(2.3)
,
04.7
H +↔
≈
SH aq 2
(
)
−
+
HS
S
pH
,2 −
96.11
(2.4)
H +↔
≈
6
Sewer systems are designed with pressure mains and gravity pipes. In principle, pressure
mains provide anaerobic conditions for sulphide build-up and gravity pipes contribute to the
emission for hydrogen sulphide (Yongsiri et al., 2005). H2S(aq) can cross the interface of
water-air and diffuse into the atmosphere when there is a free water surface from which gas
can escape such as gravity sewers, pumping stations and manholes (Fu and Shen, 1990;
Yongsiri et al., 2005).
The emission rate for hydrogen sulphide in sewage was 32% to 46% slower than in de-ionised
water, mainly due to the organic and inorganic matters that exist in sewage and air-water
surface. Moreover, temperature, pH, hydraulic conditions in the water phase, and ventilation
of the air phase also could affect the emission of hydrogen sulphide (Yongsiri et al.,
2004a,2004b,2005). Turbulence also could affect the transfer of H2S(aq). When turbulence
increase, the air-water interface is not only larger, but it could also bring more hydrogen
sulphide molecules to the interface to be transferred (Liss and Slater, 1974).
Yongsiri et al. (2003) summarised the temperature effect as a function of H2S emission rate in
Eq. 2.5.
(
)20
−T
.1
024
(2.5)
Temperature effect term=
In which
T = temperature, °C.
pH is one factor that controls the proportion of H2S(aq), which affects the hydrogen sulphide
concentration because only H2S(aq) could transfer across the air-water interface. According to
USEPA (1974), when pH is below 5, almost all sulphide was present as H2S(aq), the proportion
of 45–50% when pH is 7 and 5–10% when pH is 8 of total sulphide. The equation of H2S
listed below:
T
(2.6)
=
]
[ SH 2
aq
pH
pH
2
−
−
1
10
10
+
S +
( K
)
)
s
s
1
( KK s 1
2
Where
7
[H2S]aq = H2S(aq) concentration, mg/L;
Ks1, Ks2 = thermodynamic equilibrium constants for the sulphide weak-acid system adjusted
for Debye-Huckel effects;
ST = total sulphide concentration, mg/L.
At pH= 7.05, H2S(aq) could transfer 42% as fast as pH =4.45 (Yongsiri et al., 2004), which
USEPA (1974) was suggested 50% and H2S(aq) could transfer 17%, as fast as at pH =4.45.
Conversely, Nielsen et al. (2008b) found the proportion of H2S(aq) of total sulphide to be 44%
and 12% at pH of 7.2 and 7.9, respectively. When pH dropped from 7.9 to 7.2, hydrogen
sulphide increased from 0 ppm to 11 ppm.
Yongsiri et al. (2003) created a model to predict the emission rate of hydrogen sulphide and
they found the pH affects sewers with a higher slope more than sewers with a lower slope (see
Figure 2.2).
4.00
3.00
e t a R
n o
pH=6
i s s i
pH=7
2.00
m E
pH=8
l a i t i
n
I
1.00
S 2 H
0.00
0.004
0.008
0.010
0.000
slope (m/m)
Figure 2.2: Predicted Hydrogen Sulphide Initial Emission Rate of Sewer Pipe with Diameter 0.3 m under Different pH and Slope (Yongsiri et al., 2003)
Lahav et al. (2004) developed a model that shows that sulphide concentration decreases from
5 mg/L to 2 mg/L after 10 mins (or 1500 m) of a straight flow (d/D=0.43, s=1% pH=7).
8
Moreover, when other conditions were same (∆S= 2 mg/L D=0.8 m, Q=0.5 m3/s), stripping
rate of slope of 0.5% was 13% under slope of 2%.
2.4 Sewer Pipe Corrosion
When hydrogen sulphide diffuses to the atmosphere of the sewer pipes, it can react with
oxygen to generate sulphuric acid through biological processes (see Eq. 2.7). Nielsen et al.
(2008a) found that hydrogen sulphide concentrations that are detected by gas sensors were
much lower than the concentration in equilibrium with the sulphide concentration in the
sewage, proving that the process of absorption of hydrogen sulphide and oxidation to
sulphuric acid onto the sewer walls was fast.
(2.7)
O 2
+
→
SH 2
2
SOH 2
4
H S2
H SO2 4
H S2
H S 2
2- SO4
-
HS
Figure 2.3: Cross-section of a Sewer Pipe that Shows the Sulphur Process both in
Sewage and Atmosphere
9
On the moist surface of pipe walls, there exist a thin liquid film that provides a location for
the aerobic and autoreophic Thiobacillus sp. oxidise H2S to sulphuric acid (see Figure 2.3)
(Devereux et al., 1989;Gadekar et al., 2006;Islander et al., 1991). Sulphuric acid can react
with the sewer pipe material, such as cement of concrete pipes. Continuous reaction can
generate a corroding layer that consists of gypsum and moisture on the surface of the concrete
and the layer expand into pipe walls (Mori et al., 1991). When sulphuric acid reacts with
cement,
one
of
the
formation
is
ettringite
or
(3CaO·Al2O3·CaSO4·12H2O
3CaO·Al2O3·3CaSO4·31H2O), which is expensive and can cause internal cracking and pitting
in the concrete (Redner et al., 1991,1994).
Even a small concentration of sulphide could cause serious corrosion. When a gravity sewer
pipe encounters a drop or junction that has high turbulence, a concentration of 0.01 mg/L of
sulphide can cause a significant corrosion. It is reported that under immediate velocity, total
sulphide concentration within the range of 0.1–0.5 mg/L causes minor problems in large
sewers while in a small sewer, the concentration of sulphide is 0.03–0.05 mg/L. Moreover,
sulphide concentrations over 2.0 mg/L can cause severe corrosion (Pomeroy and Boon,
1990;Hvitved-Jacobsen et al., 2002). In some extreme cases, the concrete pipes could break
within a few years (Pomeroy and Boon, 1990). The corrosion rate and lifetime for sewer pipes
are listed in Table 2.1.
Table 2.1: Corrosion Rate and Lifetime of Sewer Pipes (Zhang et al., 2008)
Corrosion rate
Lifetime
References
mm/year
(year)
2.5–10
20–70
(USEPA, 1991)
2.7
65
(Morton et al., 1991)
4.3–4.7
35–45
(Mori et al., 1992)
2–4
45–90
(Ismail et al., 1993)
3.1
55
(Davis et al., 1998)
1.0–1.3
130–170
(Monteny et al., 2001)
1.1–1.8
90–160
(Vincke et al., 2002)
10
2.5 Models for Prediction of H2S in Sewer Pipes
When the diameter of the sewer pipe is smaller than 0.6 m, a simple equation called the Z-
formula given by Eq. 2.8 cab be used for predict potential sulphide related problems in sewer
networks (see Table 2.2) (Pomeroy and Parkhurst, 1977;ASCE, 1989;ASCE and WPCF,
1982).
1 −
33.0
−
( T
)20
Z
bP
3
(2.8)
=
(
) 07.1
( 5.0 Qs
)
BOD 5
Where
BOD5 = biochemical oxygen demand concentration, mg/L;
T = temperature, °C;
s = slope, m/m;
Q = flow rate, L/s;
P = wetted pipe wall perimeter, m;
b = pipe width at the water surface, m.
.
Table 2.2: Z-Formula to Evaluate the Magnitude of the Sulphide Problem in Gravity
Sewer Pipes (Pomeroy and Parkhurst, 1977;ASCE, 1989;ASCE and WPCF, 1982)
Z-Formula
Estimated Magnitude of the Sulphide Problem
Z<5000
Problems occur rather infrequently
5000 Risk of sulphide problems Z>10000 Risk of sulphide problems frequent Pomeroy and Parkhurst (1977) developed an empirical equation for sulphide build-up in gravity sewers (see Eq.2.9). 11 3 ' 20 − 1
− (
T ) 8 M r (2.9) = − (
suN ) [
BOD ]
07.1 [ ]
dS 1
−
m [ ]
Sd
dt = change rate of sulphide concentration, mg/(L·hr); Where
[ ]
Sd
dt [BOD] = biochemical oxygen demand concentration, mg/L; M’ = coefficient, m/h; T = temperature, °C; N = coefficient; s = slope, m/m; u = stream velocity, m/s; dm = hydraulic mean depth of sewage, the cross-sectional area of the water volume divided by the water surface width, m; r = hydraulic radius, the cross-sectional area of the water volume divided by the wetted perimeter, m; [S] = sulphide concentration, mg/L. Pomeroy and Boon (1990) developed equations that could forecast the sulphide build-up rate in gravity sewers according to the concentrations of biological oxygen demand (BOD) or chemical oxygen demand (COD) (see Eq. 2.10 to Eq. 2.13). The conservative equations are: T 20 83 − ) 3 − 1
− r su d 32.0 10 64.0 (2.10) = × − )( ( ) [
BOD ](
07.1 1
−
m T 20 83 − ) 3 − 1
− r su d .0 073 10 64.0 (2.11) = × − )( ( ) [
COD ](
07.1 1
−
m [ ]
Sd
dt
[ ]
Sd
dt While reasonable expectation equations are: T 20 83 − ) 3 − 1
− r su d 32.0 10 96.0 (2.12) = × − )( ( ) [
BOD ](
07.1 1
−
m T 20 83 − ) 3 − 1
− r su d (2.13) .0 073 10 96.0 = × − )( ( ) [
COD ](
07.1 1
−
m [ ]
Sd
dt
[ ]
Sd
dt 12 = sulphide build-up rate, mg/(L·hr); Where
[ ]
Sd
dt [BOD] = biochemical oxygen demand concentration, mg/L; [COD] = chemical oxygen demand concentration, mg/L; u = sewage velocity, m/s; s = slope, m/m; T = temperature, °C; r = hydraulic radius, m; dm = hydraulic mean depth, m. Hvitved-Jacobsen et al. (2002) developed the sulphide formation rate equation (see Eq. 2.14). 5.0 −20 (
T ) (2.14) 50 03.1 = − (
COD
001.0 ) S [ ]
Sd
dt K S K
O
+ A
V
W O O = sulphide build-up rate, mg/(L·hr); Where
[ ]
Sd
dt CODs = soluble chemical oxygen demand, g/m3; T = temperature, °C;
A/Vw = ratio of biofilm area to bulk sewage volume, m2/m3;
Ko = half-saturation concentration for dissolved oxygen, g/m3;
So = dissolved oxygen concentration, g/m3. This equation clearly showed that the sulphide build-up rate has a close relationship with the DO concentration in sewer systems. If DO that was used for sulphide oxidation is not considered, the balance can be defined as Eq. 2.15 (Hvitved-Jacobsen, 2002;Yongsiri et al., 2004b). aK S S = − − + ( ) (
R SHL OS O uw )uf
R 2 dS
O
dt R k = uw K S S
O
+ O O 13 (2.15) uf 5.0
O 21= Where = overall mass-transfer coefficient of H2S, h-1; SHLaK 2 Sos = saturation concentration of oxygen, g/m3;
Ruw = rate of oxygen uptake in the water phase, g/(m3·h);
Ruf = rate of oxygen uptake in the biofilm, g/(m3·h);
k = oxygen uptake constant in the water phase, g/(m3·h);
k1/2 = oxygen uptake constant in the biofilm, g2/(m·h2);
Ko = half-saturation concentration for dissolved oxygen, g/m3;
So = dissolved oxygen concentration, g/m3. R Sk A
V
W In sewer conditions, the emission rate of H2S is determined by the overall mass-transfer coefficient and difference between total sulphide concentration in the water phase and saturation concentration. Therefore, the emission rate can be described as Eq. 2.16. A R K C (2.16) = − e α
SHL SHW
, 2 2 C
H
C Where Re = H2S emission rate, g/(m3/h); = overall mass-transfer coefficient of H2S, h-1; SHLaK 2 = H2S concentration in the water phase, g/m3; SHWC
, 2 CA = H2S concentration in the air phase, g/m3;
Hc = Henry’s law constant of H2S, m3 of water phase m-3 of air phase. As the H2S dissociation was not always constant and the emission rate was associated with sewage characteristics and temperature, the emission rate can be included as Eq. 2.17 (Yongsiri et al., 2004a). 14 − )20 R fK C (2.17) α = − e a
SHL W 2 C
A
fH
(
T
θ
e
C Where Re = H2S emission rate, g/(m3/h); α = ratio of for domestic sewage to that for clean water (-); SHLaK 2 f = ratio of H2S(aq) to total sulphide; = overall mass-transfer coefficient of H2S, h-1; SHLaK 2 Cw = H2S concentration in the water phase, g/m3;
CA = H2S concentration in the air phase, g/m3; eθ = temperature correction factor for H2S emission (-). fK It is reported that the ratio of to re-aeration coefficient SHLa 2 2OLK α was 0.36± 0.11 at pH 7 and 2OLK α was defined as Eq. 2.18 (Pomeroy and Parkhurst, 1977;Jensen, 1995). 83 − )20 K su d (2.18) 22.0186.0
F
+ ) ( )( m (
1
−
T
θ
r α
OL
2 Which 2OLK α = overall mass-transfer coefficient of O2; 5.0− ; F = Froude number (-), = ( ) mgdu s = slope of sewers, m/m; u = mean flow velocity, m/s; dm = hydraulic mean depth, m; rθ = temperature correction factor for re-aeration. For an ideal closed sewer pipe, the mass transfer inside are inflow, outflow and transfer flow on the air-water interface so a compartment of sewer pipe can be defined as continuous-flow stirred-tank reactors (CFSTRs). To predict the sulphide concentration both in the water phase and air phase, Yongsiri et al. (2004b) developed a two-phase model (see Eq. 2.19 and Eq. 2.20). 15 = dC C − V CQ CQ VR fK C )
20
V α = − + − − (2.19) nWW nWW nWf a
SHL nW
, , , , nW
, nW
, 1
− 2 nW
,
dt nA
,
fH
(
T
θ
e
c dC C nA
, 20 − V fK C )
V (2.20) α = − + − CQ
A CQ
A a
SHL nA
, nA
, nA
, nW
, nW
, 1
− 2 dt nA
,
fH
(
T
θ
e
c Where
nWC , = H2S concentration in the water phase, g/m3; VW,n = volume of sewage water phase of CFSTR compartment, m3;
QW = flow rate in sewage water phase of CFSTR, m3/h;
Rf = sulphide formation rate, g/(m3/h); α = ratio of for domestic sewage to that for clean water (-); SHLaK 2 f = ratio of H2S(aq) to total sulphide; = overall mass-transfer coefficient of H2S, h-1; SHLaK 2 nAC , = H2S concentration in the air phase, g/m3; Hc = Henry’s law constant of H2S, m3 of water phase m-3 of air phase; eθ = temperature correction factor for H2S emission (-); T = temperature, °C;
VA,n = volume of air phase of CFSTR compartment, m3;
QA = flow rate in air phase of CFSTR, m3/h. For gravity sewer pipes, if we consider that the main source of H2S gas comes from the strip, such as fast velocity and high turbulence, Lahav et al. (2006) developed a predicting model for H2S emission rate in gravity sewers (see Eq. 2.21). S − ] 7 20 − − T ) 10 (
T
024 .1 su
µγ 8
×= × − (2.21) HP
S
pH c pH pH 2 − − 2 [
Sd
dt 1 10 10 + T
+
ω
A
cs s s
1 2 Where ST = total sulphide concentration in the sewage water phase, mg/L;
γ = the unit weight of liquid, N/m3; s = the sewer slope, m/m; 16 u = the mean velocity, m/s;
µ = the dynamic viscosity, Ns/m2; ω = the flow surface width, m;
Acs = the cross-section area, m2; T = temperature, °C; Ks1, Ks2 = thermodynamic equilibrium constants for the sulphide weak-acid system adjusted for Debye-Huckel effects; PpH2S = the partial pressure of H2S in the sewer atmosphere, atm; Hc = Henry’s constant, mol/(L/atm). When considering the effect of sewage constituents on H2S emission rate, Yongsiri et al. (2005) found a model application for predicting H2S gas concentration (see Eq. 2.22). 3 2 8 su + ) − A )( (
T )20 f R (2.22) −= = × − [
86.0
α SH
2 C
β
fH dC
W
dt (
F
2.0186.0
dm
C
W
034.1
c
]
Where
RH2S = H2S emission rate, g/(m3·h);
Cw = dissolved sulphide concentration in the water phase, g/m3;
CA = H2S concentration in the air phase, g/m3; α = correction factor for overall mass-transfer coefficient in sewage; f = H2S (in the liquid phase) fraction relative to total sulphide; F = Froude number; s = sewer slope, m/m; u = mean velocity, m/s; dm = mean hydraulic depth, m; β = correction factor for saturation concentration in sewage; Hc = Henry’s law constant for H2S; T = temperature, °C. 17 Nielsen et al. (2003, 2004d, 2004e) developed the sulphide oxidation rate equation both in chemical and biological reactions (see Eq. 2.23 and 2.24). − (
T )20 k S S Chemical sulphide oxidation rate (2.23) 07.1 = S s 1.0
O co
, ∏− ∏− ( )
, ( ) − (
T )20 k S S Biological sulphide oxidation rate= (2.24) 10.1 = S s 1.0
O bo
, ∏− ∏− ( )
, ( ) In which pH 04.0 5.0 + ) k = S co
, ∏− ( )
, K 1 + (
K
101
a
1
(
) pH
101 a
1 Where
Ks(-п),o,c = rate constant for chemical oxidation, (g/m3)-0.1h-1;
Ks(-п),o,b = rate constant for biological oxidation, (g/m3)-0.1h-1;
Ss(-п) = dissolved sulphide concentration, g/m3;
So = DO, g/m3; T = Temperature, °C; Ka1 = the fist dissociation constant for hydrogen sulphide (-), the value of Ka1 at 25 °C is
reported at 8.913*10-8 (Lide, 2003) The biological sulphide oxidation rate could be assumed constant and Nielsen et al. (2008b) assumed that 0.5 g of O2 is consumed for the oxidation of 1.0 g of S. As the main cause of the corrosion of concrete is sulphur acid, USEPA (1974) developed an equation to predict the release rate of H2S gas (see Eq. 2.25). 83 j (2.25) 7.0=φ (
su ) ]DS
[ sf In which 1 j k 2 1 = k
×
pH
2 k
1
pH
− adj adj 1
−
10 10 18 + + 8 − k 1 + [
]
SH
2
][
[
−
HS
H − 12
− k 10 = 1
×= 2 + 94.7 10 = = × pH adj = )5− f When the processes of H2S gas transfer to the sulphuric acid is mainly biological and the 83 su j transfer time is fast, the equation could be concluded to be Eq. 2.26. 7.0φ
= ( ) [
](
)PbDS
′ sw (2.26) swφ = average flux of H2S (expressed as sulphide) at the air/wall interface, g/(m2·hr); In which
sfφ = the flux of sulphide in g/m2-hr at the sewage/air interface, g/(m2·hr); s = slope of sewers, m/m; u = mean velocity, m/s; j = pH-dependent factor for proportion of H2S; [DS] = dissolved sulphide concentration, mg/L; b = surface width of the stream, m; P’ = exposed perimeter (π-P/D)*D, P- wetted perimeter c The ratio of corrosion of the pipe walls is calculated by Pomeroy and Boon (1990) as Eq. 2.27. k
5.11 φ )A (
sw 1 (2.27) = Where c= average rate of corrosion of pipe walls (mm/year) k = factor representing the proportion of acid reacting, taken as 0.8 after Pomeroy A= constant representing the alkalinity of pipe material, taken as 0.8 for concrete with 19 limestone aggregate The aim of this project was to investigate sulphide generation and H2S formation in sewer networks, especially in gravity sewer pipes. Therefore, to be able to achieve the aims of the project and experimental set-up was developed to simulate a gravity sewer pipe. This project was carried in two stages. The first stage was building up a biological growth rich in sulphate-reducing bacteria on the inside wall of the laboratory sewer pipe. The second stage was monitoring of the sulphide concentration in the liquid phase and H2S in the atmosphere of laboratory sewer pipe for different sewage characteristics and flow rates. A summary of the experimental progress is shown in Table 3.1. The experimental set-up was designed to simulate a section of a sewer system, specifically a gravity sewer pipe. A 2 m long PVC sewer pipe with an internal diameter of 0.155 m was used in this experimental set-up. The pipe was wrapped by aluminium foil to simulate the environment of the sewer pipe, i.e. to shield the contents from light or sunlight. A schematic diagram of the preliminary experimental set-up is shown in Figure 3.1. During the first stage of the experimental program, a 5 mm diameter probe was installed on the centre of the top of
the clear PVC pipe. The probe was connected to a gas detector (YESAIR®), which measured temperature (°C), relative humidity (RH, %), oxygen concentration (O2, %, 0–25.0% range), hydrogen sulphide (H2S, ppm, 0–50ppm range), combustible gases (CH4, %, 0–100% range) and sulphur dioxide (SO2, ppm, 0–20ppm range). Gas samples were transferred from the pipe
atmosphere to the YESAIR® gas detector by a draw pump built in inside the detector. The whole system was sealed properly, using rubber and glue. The inlet and outlet of the pipe were closed on both sides using PVC caps of 0.155 mm in diameter. Synthetic sewage was stored in a tank of 50 L volume and circulated through the pipe for a designated time using a universal pump with a maximum capacity of 56.6 L/min. The tank was sealed with a cap and 20 a sample collection point controlled by a gate valve was placed on the outlet of the tank. In addition, a gate valve was placed on the inlet to the pipe to control the flow rate. The whole system was mounted on a moveable steel frame with adjustable incline, which enabled 21 varying slope. Stage Feed Gas phase Initial stage Domestic sewage Second stage Postgate's grow media Development of biological growth
inside the laboratory-sewer-pipe Last stage Synthetic sewage (Weidhaas et
al., 2007) H2S, O2, T,
humidity,
SO2, CH4 Final stage Monitoring of laboratory-sewer-
pipe Synthetic toilet sewage (Van
Manh and Anh, 2005) Monitoring
Liquid
phase
HS- H2S(aq)
Temperature
pH
HS- H2S(aq)
COD
Temperature
pH
HS- H2S(aq)
−2
4SO
COD
Temperature
pH
DO
HS- H2S(aq)
−2
4SO
COD
CODs 22 m u lt i- g a s d e t e c t o r v a lv e t a n k
5 0 L p u m p s a m p lin g
p o in t 23 The above set-up was modified as follows: 1) The frame was adjusted. The height of one side was 0.02 m higher than the other side to obtain a 1% slope; 2) One flow meter (0 to 10 L/min) was installed at the inlet to the pipe to measure the flow rate; 3) The YESAIR® gas detector was moved to the inlet of the pipe. A new probe of 5 mm
diameter was installed on the outlet of the pipe and connected to the QRAEII® gas detector, which has the capacity to measure the combustible gases, H2S, CO and O2 in the pipe’s atmosphere; 4) Two pH sensors were installed on both sides of the pipe besides the gas detectors and submerged in sewage. Each sensor was connected to the pH and temperature transmitter, which monitored the pH and temperature value; and 5) Two tubes were installed at each end of the pipe (next to the pH sensors). The end of the tubes was under the sewage level to allow for the collection of sewage samples. 24 A schematic diagram of the final experimental set-up is shown in Figure 3.2. t r a n s m i t t e r t r a n s m i t t e r g a s d e t e c t o r s a m p l i n g
t u b e s a m p l i n g
t u b e g a s d e t e c t o r f l o w m e t e r p H p H s e n s o r s e n s o r v a l v e t a n k p u m p s e w a g e
f e e d d i s c h a r g i n g
p o i n t 25 26 Anaerobic sludge was sourced from an anaerobic digestion reactor at a wastewater treatment plant because it contains a mixed culture of anaerobic micro-growing including sulphate- reducing bacteria. Domestic sewage was collected from the influent to the Sunbury Wastewater Treatment Plant. Initially, 20 L of sludge was pumped into the laboratory sewer pipe and 20 L of domestic sewage was stored in the feed tank. The sewage was circulated inside the pipe at a flow rate of 56.6 L/min. The concentration of sulphide in real sewage was measured in chemistry lab of RMIT University as 1.17 mg/L. For the following four weeks (6 March to 9 April 2009), the laboratory sewer pipe was fed Postgate’s grow medium (Hauser and Holder, 1986). The composition of Postgate’s grow medium is given in Table 3.2. This medium was used because Hauser and Holder (1986) found that this medium was suitable for anaerobic bacteria especially sulphate-reducing bacteria to grow. Constituent Concentration (mg/L) 500 KH2PO4 1000 NH4Cl 60 CaCl2·2H2O 60 MgSO4·7H2O 4 FeSO4·7H2O Glucose 12000 4500 Na2SO4 27 300 Sodium citrate 2H2O The feed was circulated for a week then replaced with fresh feed. During the week, samples were collected on a regular basis, on the first, fourth and seventh day. The laboratory sewer pipe was monitored for sulphide, pH, temperature and COD. As sulphide production rates are affected by temperature, re-aeration condition, sewage characteristics and oxygen level, it is expected that sulphate-reducing bacteria will reduce sulphate to sulphide. The activity of bacteria was monitored in term of sulphide concentration, temperature and COD. The results indicated that the COD and sulphide both reached a minimum concentration after one week. Hence, it was decided that the feed would be changed on a weekly basis. In order to investigate the sulphide cycle, two groups of testing (Test A and Test B) were carried out during following weeks using Postgate’s grow medium (11 March to 31 March). Test A lasted for eight days. There were five samples on the first day (one 100 mL sample per hour) and one sample on the second, third, seventh and eighth day, respectively. For Test B, one sample was taken on the first, second and seventh day, respectively. During this period, the gas detector logged temperature, humidity, concentrations of H2S, O2, SO2 and CH4 every minute. Synthetic sewage was used to grow bacteria. The composition of the synthetic sewage used during last few weeks of the preliminary experiment is shown in Table 3.3. The sewage inside the pipe was discharged weekly through the gate valve and new feed was added to the tank. A small amount of sludge was added to the tank every two weeks because some bacteria may be lost when discharging the sewage. To enhance anaerobic bacteria growth, the pump was stopped for 5–12 hours every week to create standing conditions, hence creating a suitable environment for anaerobic growth. The results from Test A and Test B showed that sulphide concentration could achieve its highest point during the first 24 hours. Therefore, during these last few weeks of the preliminary experiment, three samples were taken on the first, second and eighth day (one 28 sample per day) to analyse temperature, pH, concentrations of sulphide, sulphate and COD. Constituent Concentration (mg/L) Carbon Source Glucose 120 Sodium acetate 120 CH3COONa Peptone 120 Nutrient Source Ammonium Chloride 80 NH4Cl Potassium Phosphate 40 K2HPO4 Magnesium Sulphate 20 MgSO4.7H2O Calcium Chloride Gel 40 CaCl2.2H2O Trace Metals Ferric Chloride 0.5 FeCl3.6H2O Zinc Sulphate 0.04 ZnSO4.7H2O Copper Sulphate Penthydrate 0.02 CuSO4.5H2O 0.04 MnCl2.4H2O Manganese Chloride Tetrahydrate Cobalt Chloride Sol. 0.05 CoCl2.6H2O Sodium Molybate 0.04 Na2MoO4.2H2O Boris Acid 0.1 H3BO3 KI Potassium Iodide 0.02 During the final stage, the laboratory sewer pipe received synthetic toilet sewage, which was used by Van Manh and Anh(2005) to simulate toilet sewage. The constituents are shown in Table 3.4. The synthetic swage was prepared as follow. Pork meat bought from a local supermarket was grinded and boiled in the tap water for six hours before use. The cooked meat was mixed using a blender. A slurry texture mixture was allowed to cool to room 29 temperature then stored in the freezer. Material Concentration (30L) 6.75g C2H3NaO2 2.5g CH4N2O Chemicals 0.875g NH4Cl 0.875g KH2PO4 Starch 7.5g Pork meat 3g Other ingredients Peptone 1g MgSO4.7H2O* 0.6g * used in the preparation of sewage rich in sulphate 7.5g of starch was boiled in tap water for 3–5 min before mixing it with the chemicals stock solution. This starch solution was prepared fresh just before preparing the synthetic sewage. The synthetic sewage was changed weekly. The high sulphate concentration synthetic sewage was prepared by adding 0.6 g MgSO4.7H2O to 30 L chemical stock solution. After each week, the old sewage was discharged completely. Synthetic sewage was pumped into the sewer pipe at a flow rate of 1 L/min, which has a velocity of 0.034 m/s. The sewage was allowed to circulate for one week, during a period of one week the sewage travelled 20.56 km. Sewage samples were collected from the inlet, the outlet and the storage tank using a 50 ml syringe and a small pump. The inlet and outlet sampling tubes were sealed with rubber caps therefore syringe was used to collect samples to ensure gases were not released to the atmosphere during sampling. Emission gases including H2S and volatile organic gases (VOGs) in addition to oxygen were
measured at the inlet and outlet of the sewer pipe using the YESAIR® and QRAEII® gas 30 detectors. The samples that were taken during the preliminary experimental period and the final experimental period were analysed for temperature, DO, pH, sulphide, sulphate, COD and soluble CODs. The samples were transferred to be analysed immediately after collection. The pH of the inlet and outlet was measured using a BAT industrial pH sensor with a EUTECH Alpha 500 series transmitter. These pH sensors were suitable for sewage use. DO was measured using HACH DO meters. When transferring samples, it is important to avoid shaking samples. The temperature of sewage was measured using HACH DO meters. The gas phase
temperature was measured using a YESAIR® gas detector. The concentration of sulphide (S2-) in sewage was measured according to the modified Methylene Blue Method, which was adapted from standard methods for examination of water and wastewater using a sulphide reagent (HACH, 1984). This procedure is equivalent to the
USEPA method 376.2 and Standard Method 4500-S2- D for wastewater. The interfering −2 substances were considered. 4SO ) in sewage samples was determined according to a The concentration of sulphate ( modified barium sulphate turbudimetric method adapted from standard methods for the examination of water and wastewater using sulfaver 4 sulphate reagent (HACH, 1984). This procedure is equivalent to the USEPA method 375.4 for wastewater. The calibration curve was made using the sulphate standard solution, which contains 1000 mg/L sulphate. The concentration of total COD was measured according to the HACH high range plus reactor digestion method and the HACH high range reactor digestion method (Jirka and Carter, 1975) which USEPA approved for wastewater analyses. Before testing, samples were 31 mixed well by shaking the sample bottles. The concentration of soluble COD was measured as follows: 10 ml of each sample were centrifuged at the speed of 4000 rpm for 20 mins. Then samples were filtered using the Advantec Ф42.5mm filter paper, for which the filtration unit was 6 µm. The concentration of CODs of treated samples was measured according to the HACH high range reactor digestion method (Jirka and Carter, 1975). The aim of this monitoring was to analyse the parameters of the real sewage and monitor the H2S gas in manholes. The monitoring was set up in two manholes, WIL54A and NWI1, which were located in Williamstown North, Melbourne (see Figures 3.4 to 3.6). Both of the manholes belong to the Kyle Road Catchment, City West Water. Table 3.5 shows basic information about the two 32 manholes. 33 Parameters WIL54A NWI1 Manhole depth (m) 3.05 5.03 Average Sewage depth (mm) 110 51 Average velocity (m/s) 0.5 0.22 Pipe shape Circular 364 382 Pipe height (mm) 364 382 Pipe width (mm) Traffic condition none none Sewage type domestic industrial 34 Pipe type Plastic clay The sewage sampling was carried from 17 December to 22 December 2008. The samples were analysed for a variety of parameters including temperature, pH, sulphide, total COD and CODs. A VST-7750 Auto Sampler (Manning Environmental Inc.) was installed in manhole WIL54A. The function has been set: • Multi-bottle model • 24 HDPC bottles in the sampler • Purge time: 10 secs • Draw time: 60 secs • Measure time: 30 secs 35 • Deposit time: 40 secs • Auto start • Default test cycle model A PST-5500 Auto Sampler (Manning Environmental Inc.) was installed in manhole NWI1. The function has been set: • Multi-bottle model • 24 HDPC bottles in the sampler • Sample volume: 200ml • Tube length: 10m • Draw height: 10m • Rinses per sample: 0 • Bottle volume: 500ml • Purge time: 10 secs • Draw time: 60 secs • Auto start • Default test cycle Both of the samplers were set to start at around 10 am and collect samples every hour and deposit to each bottle over 24 hours. Samples were collected around 9 am everyday (except on weekends). When collecting samples, 50 ml of each bottle was measured and mixed into clearly labelled HDPC bottles by following the methods of composite sampling (Patil, 2002). Some sulphide may be lost during this procedure. Samples were kept in ice after being collected and delivered to RMIT University. HDPC bottles of auto-samplers were rinsed using clean water and discharge properly without contaminating the local soils and water sources. All the solid waste such as gloves and tissues were collected in rubbish bags and brought to the chemistry laboratory at RMIT University for dispose. Samples were measured directly when brought back to RMIT University. pH, sulphide, total COD and CODs were measured at the RMIT University. Data for 36 temperature and concentration of H2S in air phase were obtained from City West Water. In gravity sewer pipes, biological reactions, mainly through the activities of sulphate-reducing − − SO organic matter anaerobic bacteria CO bacteria, reduce sulphate to sulphide (see Eq. 2.1–Eq. 2.2). 2
S
+→ 2
4 OH
2 2 2 − S H (2.1) + + + +
2 → SH
2 (2.2) + Depending on sewage pH, sulphide species could exist as S2-, HS- and H2S(aq) (see Eq. 2.3–Eq. 2.4). H2S(aq) could cross the air-water interface, and be emitted into the atmosphere to form + − HS pH H2S gas. , 04.7 H
+↔ ≈ SH aq
2 ( ) − + HS S pH (2.3) H
+↔ ,2
− 96.11 (2.4) ≈ H2S gas exists in the atmosphere of the gravity sewer pipe could oxidise under certain conditions to generate sulphuric acid, which causes corrosion of concrete pipes (see Eq. 2.5). O
2 + → SH
2 2 SOH
2 4 (2.5) This project was carried out in two stages as discussed in chapter three. During the biological growth period, the anaerobic bacteria was first acclimatised with domestic sewage for a few weeks (initial stage), then fed Postgate’s grow medium (Test A and B), and finally fed synthetic sewage (last stage). During the synthetic sewage stage, the 37 pump was stopped for 5–12 hours every week to simulate night flow or less flow. During the initial stage, the flow rate was 56.6 L/min and the pump only stopped when
changing feed. The gas samples were pumped to the YESAIR® gas detector and analysed for temperature (°C), relative humidity (%), O2 (%), H2S (ppm), CH4 (%) and SO2 (ppm). The maximum and minimum values of these parameters are listed in Table 4.1 which obtained during the experimental period. Parameters Minimum Maximum Temperature 22.9 °C 24.3 °C Humidity 40% 61% 0 ppm 0.5 ppm H2S 18.80% 20.50% O2 0% 0% CH4 0 ppm 0 ppm SO2 The optimum temperature of sulphate-reducing bacteria growth is 18°C and optimum temperature for reduction is 28°C (Knoblauch et al., 1999). Also sulphate-reducing bacteria could survive in a wide range of pH conditions (Jong and Parry, 2006). Therefore, this environment could be suitable for sulphate-reducing bacteria to grow. Figure 4.1 shows the 38 condition of growth on the walls of the laboratory sewer pipe during the initial stage. To promote the growth of sulphate reducing bacteria, Postgate’s grow medium (second stage) and synthetic sewage (last stage) were used. The results of Test A and Test B (second stage) showing the generation of sulphide during a one-week period are shown in Table 4.2. Test B is a repetition of Test A to confirm the time at which sulphide form inside the pipe and to its 39 concentration. Test A Test B Time (hour) Sulphide (µg/L) 0 6 15 1 5 2 6 - 3 7 4 5 6 4 16 12 32 32 22 - 47 15 148 12 12 171 11 - The results in Table 4.2 show that when the Postgate’s medium was added, sulphide concentration in the liquid phase remained almost at the same initial concentration during the first six hours. Then it slightly increased and reached the highest point after 16 hours for Test 40 B and 32 hours for Test A. 23.0 21.5 21 22.5 20.5 22.0 20 19.5 21.5 Temperature (°C) 19 O2 (%) 21.0 18.5 18 20.5 17.5 20.0 17 16.5 19.5 36.50 64.06 81.65 99.31 123.48 137.15 Figure 4.2: Temperature and O2 Concentration of Gas Samples during Test B Period Figure 4.2 shows the temperature and O2 of gas samples collected during Test B. The values of temperature and O2 shown in Figure 4.2 did not fluctuate during Test B period. Therefore, temperature and O2 were not the major factor affecting the emission of H2S. The gas detector also recorded the H2S concentration in the air phase during Test A and Test B (see Figures 4.3 and 4.4). The maximum concentration observed during Test A was 0.4 ppm and 0.3 ppm for 41 Test B. 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 180.00 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 According to Figures 4.3 and 4.4, the appearance of H2S followed a consistent pattern, the 42 concentrations were almost the same and time intervals showed a pattern of increasing length of time intervals at which H2S gas was detected. For example, for Test A H2S was detected every 5 hours during the first 51 hours of the test and approximately every 15 hours during the period from 51 to 145 hours of the experiment total duration. This trend indicates that H2S concentration reached the saturation level faster during the first 51 hours. As discussed above, temperature and oxygen did not show large variations therefore, H2S appearance could be attributed to the flow rate, or in other words to the retention time. That is the time required for the feed to make a complete cycle. In addition the increased length of appearance of H2S after 51 hours could be attributed to the concentration of sulphide in the aqueous phase. The sulphide concentration decreased with time. However, as the variation in H2S concentrations were low during this Test, perhaps as a result of lacking nutrition or lacking bacteria, the project moved to the synthetic sewage stage and a fixed volume of seed (anaerobic bacteria sourced from a wastewater treatment plant) was pumped into the pipe at the start of each run, i.e. every week. Feeding the laboratory sewer pipe using synthetic sewage was the last stage of developing biological growth. Table 4.3 shows the characteristics of Postgate’s grow medium and 43 synthetic sewage. Postgate’s grow medium Synthetic Sewage Constituent Concentration (mg/L) 120 Glucose 12000 40 500 KH2PO4 80 1000 NH4Cl 40 60 CaCl2·2H2O 20 60 MgSO4·7H2O - 4 FeSO4·7H2O - 4500 Na2SO4 - 300 Sodium citrate 2H2O 120 Sodium acetate - 120 Peptone - 0.5 - FeCl3.6H2O 0.04 - ZnSO4.7H2O 0.02 - CuSO4.5H2O 0.04 - MnCl2.4H2O 0.05 - CoCl2.6H2O 0.04 - Na2MoO4.2H2O 0.1 - H3BO3 0.02 KI - Compared with Postgate’s grow medium, synthetic sewage added Peptone, which served as another source of carbon, in addition to Glucose, for bacteria growth. Trace metals normally exist in real sewage, especially industry sewage. During this stage, the pump was stopped for 5–12 hours every week to simulate the night flow of real sewage. Figure 4.5 shows the 44 measured value of sulphide and sulphate concentration when using synthetic sewage. 600 2.5 500 2 400 1.5 Sulphide (mg/L) 300 Sulphate (mg/L) 1 200 0.5 100 0 0
192 23 0 During this period, the highest sulphate reduction rate was 4.17 mg/(L·hr) and meanwhile the highest recorded concentration of H2S was 18.5 ppm (see Figure 4.6). These results are in an agreement with 4.21 mg/(L·h) of sulphate reduction rate reported by Mudryk et al. (2000). Based on these results it could be concluded that sulphate-reducing bacteria existed in the 45 system. 20 18 16 14 12 H2S 10 8 6 4 2 0 0 4 8 2
1 6
1 0
2 4
2 8
2 2
3 6
3 0
4 4
4 8
4 2
5 6
5 0
6 4
6 8
6 2
7 Figure 4.6: H2S Concentration Inside the Laboratory Sewer Pipe During the Last Stage Figure 4.7 shows the outside of the laboratory sewer pipe wall during the last stage and Figure 46 4.8 shows the inside of the pipe wall. Figure 4.7: Outside of the Laboratory Sewer Pipe Wall during the Last Stage 47 Figure 4.8: Inside of the Laboratory Sewer Pipe Wall during the Last Stage The sulphur cycle in sewer systems, typically comprises five processes: 1) sulphide formation in liquid; 2) H2S(aq) transfer cross air-water interface to H2S(g); 3) oxidation of sulphide; 4) adsorption and oxidation of sulphide; and 5) precipitation of sulphide in water. Several experiments that monitored sulphide concentrations confirmed the five processes of the sulphur cycle (see Figure 4.9). However, some reactions may happen simultaneity depending on the processes of the sulphur cycle. For example, when sulphide forms inside the biofilm, 0.18 H2S may diffuse from the liquid phase into the air phase. 0.16 0.14 (3) 0.12 (2) (4) 0.1 0.08 0.06 (5) 0.04 0.02 0 0 3.5 22.5 26.5 50.5 145.5 (1) Figure 4.9: Sulphur Cycle Processes during the Experimental Period These experiments included sulphate standard solution calibration, and measured sulphate concentration in domestic tap water from different water sources and sulphate concentration 48 in synthetic sewage. During the sulphate calibration experiment, 1000 mg/L sulphate standard solution was prepared by dissolving 3.354 g Na2SO4·10H2O in 1 L of deionised water. Then, the sulphate concentration was measured in three 25 ml samples that contain 0.1 ml, 0.2 ml and 0.3 ml stock solution, separately. The results are shown in Table 4.4. The calibration curve and best- fit equation are shown in Figure 4.10. Standard solution Calculated measured by 3.354 g Volume of stock Volume after value solution, mL diluting, mL Na2SO4.10H2O HACH, mg/L SO4 mg/L SO4 0 0 0.2 0 1000 mL 0.1 25 16.5 3.99 0.2 25 25.2 7.99 14 12 y = 0.3385x
R2 = 0.9446 0.3 25 31.7 11.99 10 8 6 4 2 0 0 5 10 15 20 25 30 35 49 Sulphate concentrations of deionised water, tap water from the RMIT University civil engineering lab, tap water from the RMIT University chemical engineering lab, a pork and starch solution and a chemical solution were measured before monitoring laboratory sewer pipe performance. One example is shown in Figures 4.11. The results are shown in Table 4.5. Sample Sulphate concentration (mg/L) Deionised water 0 Civil Engineering lab tap water 12.8 Chemical Engineering lab tap water 21.2 10.4 Pork and starch solution high concentration (1:30 dilute) 0.3 Chemical solution high concentration (1:30 dilute)* 50 *chemical solution was the preparation of chemicals of synthetic toilet sewage Monitoring of the sewage during the experiment for pH, temperature and DO concentration is shown in Figure 4.12. The results show that during each week’s experiment, there were not great changes in pH, temperature and DO concentration. However, as synthetic toilet sewage was changed weekly and this period was carried on over several months from June to November, pH, temperature and DO concentration may vary in values but were stable during 25 each run. 20 15 pH DO Temperature 10 5 0 0 3.5 22.5 26.5 50.5 145.5 Tables 4.6 to 4.9 show the sewage concentrations of sulphate, sulphide and CODs at the inlet, the outlet and the feed/effluent tank under different sewage conditions (e.g. flow rate and sewage concentrations). The feed/effluent tank was used as storage of feed and circulation of the feed, i.e. collection of the pipe’s effluent then pumps it to the pipe inlet, therefore it can be assumed it simulates a sewer manhole. The sulphide build-up model, developed by Pomeroy and Boon (1990), was discussed in 51 chapter 2 and it is given below for convenience: 3 T 20 − 1
− ) 3 − 1
− 8 r su d )( (
0.96 ) [
CODs ](
07.1 m dS
dt .0 073 10 (2.13) = × − T 20 83 − ) 3 − 1
− r su d The conservation equation is given below (Pomeroy and Boon,1990): )( ( ) [
COD ](
07.1 1
−
m [ ]
Sd
dt .0 073 10 64.0 (2.11) = × − In this project, we defined the coefficient A for a sulphide build-up rate equation, which is as 3 T 20 − 1
− ) 3 − 1
− 8 r su d follows: )( ( ) [
CODs ](
07.1 m dS
dt 52 .0 073 10 A = × − Pipe Inlet Pipe Outlet Tank Time Sulphate Sulphide Sulphate Sulphide Sulphate Sulphide CODs CODs CODs (hour) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 0 - 18.2 0.019 211 3.5 205 27.3 0.032 202 22.7 0.024 23 0.026 22.5 49 21.3 0.17 58 22.7 0.171 20.2 0.154 26.5 75 19.3 0.172 21.5 0.144 55 21.6 0.147 50.5 31 16.1 0.179 16.7 0.113 34 16.8 0.117 145.5 32 13.6 0.105 52 20.6 0.003 39 17.3 0.005 53 The build-up rate of sulphide in the sewer was determined according to equation 2.11. The laboratory sewer pipe has an internal diameter of 0.155 m and a water depth of 0.022 m at the 44.26° m 55.5mm 77.5m flow rate of 1 L/min. Hydraulic conditions were calculated as follows. 2 3 2 − m Cross-sectional area: Acs
1
×−
2
π
64.1 10 2
×= = × 5.77
1000 26.44
360 54
1000 5.55
1000 Hydraulic mean depth, which equals cross-sectional area of the water volume, divided by the 3
− d m water surface width: m A
cs
W .0 01477 = = = 64.1
.0 ×
054 2 10
× 3 − Hydraulic radius, cross-sectional area of water volume divided by wetted perimeter: w 54 64.1 10 × r m .0 00713 = = = A
cs
W .0 155 .022 054 ×+×
×
π
26.44
360 Therefore, coefficient A shown in Tables 4.6a to 4.9a for the experiments were used to calculated sulphide values at the inlet, outlet and feed/effluent given in Tables 4.6 to 4.9, respectively. Time (hour) A 3.5 0.702 3.5 0.678 3.5 0.668 22.5 0.165 22.5 0.192 22.5 0.451 26.5 0.267 26.5 0.320 26.5 0.195 50.5 0.120 50.5 0.680 50.5 0.130 145.5 0.103 145.5 0.166 145.5 0.126 According to Table 4.6, as the average velocity of this period was 0.034 m/s, the synthetic toilet sewage could be assumed as travelled 0.034 m/s* 3600 s/hr*145.5 hr = 17.80 km. When sewage went through the laboratory sewer pipe, for the first 22.5 hours, the main process was sulphate reduced to sulphide. Sulphate concentration decreased by 12% during 19 hours and sulphide increased 0.128 mg/L. Meanwhile, COD dropped 156 mg/L when sulphide generated. It was noticed that two concentrations were unusual. For situation (*), at experimental time 55 22.5 hours, chemical oxygen demand in the tank was much higher than in the sewer pipes. This may be due to the inhomogeneous system. As the sewer pipe was 2 m long and the feed tank was 50 L, this sewage was mixed mainly by the pump; therefore, sometimes samples may have not got mixed well. The laboratory sewer pipe had a slope of 1% and synthetic toilet sewage was flowing through the tube. Although the outlet tube was much closer to the bottom of the sewer pipe, there were still corners remaining and this could not be avoided during construction. Therefore, situation (#), with parameters from the outlet samples, obtained extremely high results. This situation also happened in the following experiments. Table 4.6 shows that at the end of one week of experimental work, sulphide concentration was 73.7% lower than the original concentration. The loss of sulphide in the water phase might be as a result of the transfer from sulphide in the liquid phase to sulphide in the air phase and diffuse into the atmosphere. H2S concentration during this period was shown in Table 4.6b. For this stage, two gas detectors were installed on both sides of the sewer pipe: inlet and outlet. However, logging data showed that on the outlet pipe, H2S concentration was always below 0.1 ppm. The cause of this phenomenon may be as follows. When the pump was fully open, the flow rate was 56.6 L/min and laboratory sewer pipe was half-full, with a water depth of 77.5 mm. As the flow rate decreased to 1 L/min, the water depth was 22 mm. When the whole system was set up, the 25 mm diameter tube that connected to the inlet of the sewer pipe was installed at the centre while the outlet pipe was installed close to the bottom of the sewer pipe. Moreover, when the experimental work moved into the final stage, the sewer pipe was set at a slope of 1%. As a result, the inlet pipe to the pipe had a higher turbulence flow conditions than at the outlet and higher turbulence could improve the generation of H2S. One factor that affects the emission of H2S is turbulence (Liss and Slater, 1974). Therefore, the conditions at the pipe’s outlet may not meet the requirement for H2S to diffuse to the atmosphere. Moreover, this may also be a reason why sulphide concentration in the liquid 56 phase at the outlet was usually higher than at the inlet. Parameter Maximum concentration (ppm) 1.1 (Time = 47.5 hours) H2S The sulphide concentration in the water phase decreased 0.026–0.005 = 0.021 mg/L with a theoretical equilibrium of 15 ppm H2S. This concentration was much larger than the measured value. Thus, it could be concluded that in our project, under this experimental condition, the 57 loss of sulphide in the water phase was due to the oxidation and precipitation of sulphide. Pipe Inlet Pipe Outlet Tank Time Sulphate Sulphide Sulphate Sulphide Sulphate Sulphide CODs CODs CODs (hour) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 0 - 29.5 0.024 242 22.6 175 22.3 0.337 209 17.2 0.167 178 19.4 0.22 29.5 139 11 0.364 180 17 0.217 149 15.5 0.224 45.9 56 9.2 0.187 54 10.4 0.145 48 10 0.166 71.4 41 10.7 0.077 10.8 0.075 43 13.6 0.08 144.1 51 8 0.011 42
364(#) 11.8 0.018 32 6 0.051 58 Time (hour) A 22.6 0.453 22.6 0.540 22.6 0.458 29.5 0.412 29.5 0.544 29.5 0.451 45.9 0.202 45.9 0.191 45.9 0.167 71.4 0.137 71.4 0.145 71.4 0.143 144.1 0.135 144.1 0.954 144.1 0.091 During this experimental period, the synthetic sewage travelled 0.034 m/s*3600 s/ hr *144.1 hr = 17.64 km in flow conditions that were almost same as Table 4.6. Table 4.7b shows the H2S concentration during this period. Parameter Maximum concentration (ppm) 0.5 H2S The initial concentration of sulphate was 29.5 mg/L, 11.3 mg/L larger than sulphate concentration in Table 4.6. Therefore, sulphide concentration in the water phase was still increasing at 29.5 hours in Table 4.7 while sulphide concentration decreased at 26.5 hours in 59 Table 4.6. During this experimental period, the sulphide concentration in water increased from 0.024 mg/L to 0.224 mg/L. Table 4.7c shows the comparison of sulphate, sulphide and H2S concentrations. ∆Sulphate ∆Sulphide Initial Sulphate Run duration Experiment H2S (ppm) (mg/L) (mg/L) (h) (mg/L) Table 4.6 18.2 2 22.5 0.135 1.1 Table 4.7 29.5 14 29.5 0.20 0.5 According to Table 4.7c, Table 4.6 had a lower sulphate concentration and sulphide increased 8 times. However, in Table 4.7, which had a higher sulphate concentration, sulphate concentration decreased by 47% and sulphide increased 9 times. In addition, the original COD concentrations for Tables 4.6 and 4.7 were 211 and 242 mg/L, respectively, and temperature values were close. Consequently, synthetic toilet sewage with a higher sulphate concentration could generate more sulphide but this may not affect the emission of H2S into the pipe atmosphere. Moreover, synthetic toilet sewage with a higher sulphate concentration needs 60 more time to be reduced to sulphide. Pipe Inlet Pipe Outlet Tank Time Sulphate Sulphide Sulphate Sulphide Sulphate Sulphide CODs CODs CODs (hour) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 0 29 0.015 297 - 6.3 383 29.6 0.108 328 28.6 0.052 24 109 18.4 0.206 107 16.5 0.28 - 30.5 45 15.3 0.207 66 16.4 0.221 51.7 36 13.7 0.063 73 13.6 0.114 145.3 12 22.7 0.004 10 23.2 0.006 61 Time (hour) A 6.3 1.474 6.3 1.251 24 0.397 24 0.391 30.5 0.174 30.5 0.252 51.7 0.138 51.7 0.285 145.3 0.044 145.3 0.037 Synthetic sewage travelled 0.034 m/s*3600 s/hr*145.3 hr = 17.78 km. When using a high concentration synthetic sewage, concentrations of sulphate and CODs were increased (29 mg/L and 383 mg/L compared 18.2 mg/L and 211 mg/L in Table 4.8, respectively). H2S concentration was summarised in Table 4.8b. Parameter Maximum concentration (ppm) 0.5 H2S Tables 4.8 and 4.8b reveal that during the first 24 hours, sulphate concentration decreased 12.5 mg/L, CODs concentration dropped by 71.5% and sulphide concentration went up from 0.265 mg/L to 0.28 mg/L. At the same time, the gas detector logging H2S concentration was over 0 ppm and the highest concentration was 0.5 ppm. Further, at 145.3 hours, sulphate concentrations were closer to the initial concentration compared to the former results. As H2S concentration was 0.5 ppm, most part of total sulphide was lost either as oxidation to sulphate 62 or through being settled. Pipe Inlet Pipe Outlet Tank Time Sulphate Sulphide Sulphate Sulphide Sulphate Sulphide CODs CODs CODs (hour) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 0 30.5 258 - 0.026 18.8 113 16 0.279 16.9 0.382 142 25.8 64 14.8 0.221 14 0.261 80 - 43.3 59 12.3 0.04 11.6 0.058 47 140.2 34 20.1 0.002 19.7 0.001 25 63 For this experimental period, synthetic toilet sewage travelled 0.238 m/s*3600 s/hr *140.2 hr = 120.12 km. As the flow rate changed to 7 L/min, the water depth was 40 mm; therefore, the
cross-sectional area was Acs = 3.8585*10-3 m2, the hydraulic mean depth was dm = 0.028 m and the hydraulic radius was r = 0.0128 m. Sulphate concentration decreased 47.5% during the first 18.8 hours and sulphide built up at the rate of 0.013 mg/(L·hr). CODs declined to less than half of the original concentration. Time (hour) A 18.8 0.227 18.8 0.283 25.8 0.137 25.8 0.169 43.3 0.121 43.3 0.098 140.2 0.068 140.2 0.049 For this period of experiment, the concentration of H2S of the inlet is shown in Table 4.9b. Parameter Maximum concentration (ppm) 0 H2S The result of coefficient A for Table 4.6 to 4.9 is summarised in Table 4.10 and the sulphide 3 T 20 − ) 3 − 1
− 1
− 8 build-up rate equation in our project is as follows: )( (
0.325 ) [
CODs ](
07.1 64 r su d .0 073 10 = × − dS
dt Average Number of samples coefficient Std dev value 48 A 0.3 0.325 According to Table 4.9b, there was no H2S recorded during this experimental period. This 1.6 1.4 1.2 1 may be due to the higher DO concentration, which is shown in Figure 4.14. Inlet DO (mg/L) 0.8 Outlet DO (mg/L) 0.6 0.4 0.2 0 50 100 150 0 Figure 4.14 shows that the dissolved oxygen of synthetic sewage samples was almost above 1 mg/L and, according to the literature, when DO concentration is above 1 mg/L, it could prevent the generation of H2S in water (Pomeroy and Boon, 1990;USEPA, 1991). As shown in Tables 4.9 and 4.9b, when the flow rate of the laboratory sewer pipe is increased, it could raise turbulence to some extent. Conversely, it could also increase the DO 65 concentration, which prevents the generation of H2S. Based on the results in the biological growth stage, when the flow rate of the system was 56.6 L/min, the concentration of H2S was above 0 ppm, which was 0.3–0.4 ppm. It could be concluded that when other conditions were same, a sewer pipe with a lower flow rate (i.e. during a drought or water conservation condition), may create more opportunity for formation of H2S. However, there was no strong evidence from our project to support the idea that an increased flow rate could fully prevent the generation of H2S. Therefore, more research is required to establish the relationship between the flow rate and DO in the laboratory sewer pipe set-up. According to Eq. 2.1, 2.2 and 2.3, mass balances of sulphate and sulphide concentrations for − − Tables 4.6, 4.7, 4.8 and 4.9 could be calculated as shown in Table 4.11. 2
S
+→ 2
4 2 2 − SO organic matter anaerobic bacteria CO (2.1) + + + OH
2 +
2 → + − HS pH S H (2.2) + SH
2 , 04.7 H
+↔ ≈ SH aq
2 ( ) (2.3) Calculated Measured Sulphide Sulphate H2S(g) Sulphide concentration (in concentrations concentration Experiment concentration water phase) (mg/L) (mg/L) (mg/L) (mg/L) 0.67 Table 4.6 2.0 0.135 0.00165 4.67 Table 4.7 14 0.20 0.00075 4.17 Table 4.8 12.5 0.265 0.00075 66 4.83 Table 4.9 14.5 0.253 0 The pH of experimental results were around 7.0 and, according to (USEPA, 1974), the proportions of H2S(aq) of total sulphide at pH 7 is 45 to 50%; therefore, proportions were listed in Table 4.12. Calculated H2S(aq) H2S/ concentration (USEPA, Experiment H2S(aq) 1974) (%) Table 4.6 0.068 2.4 Table 4.7 0.100 0.75 Table 4.8 0.133 0.56 Table 4.9 0.127 - Table 4.12 shows that the proportion of H2S in the air phase of the calculated value was below 10%, which was in agreement with Nielsen et al. (2008a). The relationship between initial sulphate, initial CODs, measured sulphide/calculate sulphide 67 in the water phase and H2S/H2S(aq) are shown in Figures 4.15 and 4.16. 18 450 16 400 14 350 12 300 10 250 8 200 6 150 4 100 Original Sulphate 2 50 Original CODs 0 0 Table 4.6 Table 4.7 Table 4.8 Measured Sulphide/calculate
Sulphide (in water phase) (%) 3 450 400 2.5 350 2 300 250 1.5 200 1 150 100 0.5 50 Original Sulphate 0 0 Original CODs Table 4.6 Table 4.7 Table 4.8 H2S/H2S(aq) (%) Figures 4.14 and 4.15 show that for 12.8% increase in COD concentration, the proportion of measured sulphide to calculated sulphide decreased by 54.5% and the H2S/H2S(aq) reduced 68 from 2.4 (Table 4.7) to 0.49 (Table 4.6), almost 80% reduction. In Table 4.8, when the COD was much higher (141 mg/L larger than in Table 4.7), measured sulphide was much less than the calculated value but H2S(aq) transferred 0.14% more than shown in Table 4.7. Therefore, it could be concluded that sulphide generation and H2S emission condition in our system occurred at the conditions shown in Table 4.6, which had a slope of 1%, a flow rate of 1 L/min, a low concentration of synthetic sewage with sulphate concentration around 23 mg/L and COD concentration around 240 mg/L. From Tables 4.6 to 4.9, it can be seen that there were several parameters that could affect the generation of sulphide in the liquid phase and the emission of H2S. According to Hvitved- Jacobsen et al. (2000), Nielsen et al. (1992) and Tomar and Abdullah (1994), velocity, re- aeration condition and temperature could affect the generation of sulphide in sewage. Yongsiri et al. (2004a,2004b,2005) found sewage constituents, temperature, pH and hydraulic conditions in the water phase and ventilation in the air phase to have an effect on the emission of H2S. As the synthetic toilet sewage had a high concentration constituent in the experiments shown in Tables 4.8 and 4.9, as the flow rates were 0.034 m/s and 0.238 m/s, respectively, these results show how velocity affected sulphide and H2S emissions. When slow velocity applied, the environment was more suitable for bacteria to grow which could enhance the generation of sulphide. Figure 4.17 shows other parameters of these two groups of data. It shows that there were not significant differences between these two groups of data; therefore, we can assume that other parameters were the same in the experiments demonstrated in Tables 4.8 and 4.9. Figure 4.18 shows the sulphide build-up time and concentration under different 69 velocities. 350 300 250 200 Velocity=0.034m/s Velocity=0.238m/s 150 100 50 0 Average pH Origninal
Sulphate (mg/L) Original CODs
(mg/L) Average
Temperature
(°C) 0.3 0.25 0.2 Velocity=0.034m/s 0.15 Velocity=0.238m/s 0.1 0.05 0 0 6.3 18.8 24 25.8 30.5 It could be concluded from Figure 4.17 that if the velocity of the sewage increased by 85.7%, the sulphide build-up rate would be 15.4% more than under the velocity of 0.034 m/s. The 70 concentrations of H2S are shown in Figure 4.19. 0.6 0.5 0.4 Velocity=0.034 m/s 0.3 Velocity=0.238 m/s 0.2 0.1 0 9 1 4 6 9 1 4 6 9 1 4 6 9 1 4 0 . . . . . . . . . . . . . . . 9
1 0
2 0
2 0
2 0
2 1
2 1
2 1
2 1
2 2
2 2
2 2
2 2
2 3
2 3
2 Figure 4.19 shows that the velocity has a certain effect on the emission of H2S. When the velocity decreased by 85.7%, the concentration of H2S increased to 0.5 ppm, which may be due to the DO level in water. However, according to previous studies (Liss and Slater, 1974), increased turbulence could increase the H2S concentration in the air phase. The results of our project show that increased turbulence could also increase the DO level in the water (see 71 Figure 4.20). 1.6 1.4 1.2 1 Velocity=0.034m/s 0.8 Velocity=0.238m/s 0.6 0.4 0.2 0 1 2 3 4 5 6 7 8 9 Other parameters of results that are summarised in Table 4.8 and 4.9 are presented in Table 4.13, which shows the similar value or concentration. Lower sulphate Higher sulphate concentration concentration Average pH 7.3 6.9 Average Temperature 20.0 22.0 (°C) Average DO (mg/L) 0.5 0.6 72 Original CODs 242 211 (mg/L) The initial sulphate concentration shown in Table 4.6 was 18.2 mg/L and 29.5 mg/L in Table 4.7. Figure 4.21 shows the sulphide concentration using synthetic toilet sewage with a 0.25 different initial sulphate concentration. 0.2 0.15 Lower sulphate concentration Higher Sulphate concentration 0.1 0.05 0 0 3.5 22.5 22.6 26.5 29.5 The initial sulphate concentration of Table 4.7 was 11.3 mg/L larger than the sulphate concentration in Table 4.6 and, according to Figure 4.21, after 22.5 hours, sewage that had higher sulphate concentration could generate more sulphide (0.2 mg/L in Table 4.7 and 0.135 in Table 4.6). In addition, sewage with a higher sulphate concentration need more time to reduce sulphate to sulphide. Therefore, it is shown in Figure 4.21 that when sulphide concentration in sewage with a lower sulphate concentration started to decrease after 22.5 hours, sulphate-reducing bacteria still reduced sulphate to sulphide in the system with a higher 73 sulphate concentration. Sulphide in the air phase was recorded in Figure 4.22. 1.2 1 0.8 Lower sulphate
concentration 0.6 Higher sulphate
concentration 0.4 0.2 0 0
.
0 1
.
1
2 8
.
2
2 5
.
4
2 3
.
6
2 0
.
8
2 7
.
9
2 5
.
1
3 2
.
3
3 9
.
4
3 7
.
6
3 4
.
8
3 1
.
0
4 9
.
1
4 6
.
3
4 3
.
5
4 1
.
7
4 8
.
8
4 Figure 4.22 shows that for higher sulphate concentration sewage, H2S was detected between 21.1 to 33.2 hours, while H2S reached the highest concentration around 34.9 hours. The highest concentrations of H2S achieved were 1.1 ppm and 0.5 ppm, respectively (Tables 4.6 and 4.7). It can be concluded that higher sulphide concentration could be attributed to increased sulphate concentration if sulphate-reducing bacteria present at adequate levels. However, at the conditions tested in the laboratory sewer system, it was observed that higher sulphate concentration may or may not lead to the generation of more H2S depending on the how other parameters interact. Therefore, further research is required to assess, with the help of statistical analysis such as NOVA analysis, how the parameters affect H2S formation. According to Tables 4.7 and 4.8, the parameters could be summarised in Figure 4.23. The pH, temperature, concentration of DO and original sulphate were very close so the results could be 74 compared. 35 30 25 20 Lower CODs Higher CODs 15 10 5 0 Average pH Average
Temperature (°C) Average DO
(mg/L) Original Sulphate
(mg/L) 0.3 0.25 0.2 Lower CODs 0.15 Higher CODs 0.1 0.05 0 0 6.3 22.6 24 29.5 30.5 Figure 4.24: Sulphide Concentration under Synthetic Sewage with Different CODs Figure 4.24 shows that using higher concentration of synthetic sewage, e.g. an increase of 75 36.8% in COD, the concentration of sulphide in the sewage could increase by 21.4%. this confirms that COD is another factor affecting the generation of sulphide in the liquid phase. While for sewage with lower CODs, sulphate reduced 34.2% compared to 43% under higher 0.6 0.5 0.4 CODs. The patterns of CODs and emission of H2S are shown in Figure 4.25. Lower CODs 0.3 Higher CODs 0.2 0.1 0 0 4
.
0
2 1
.
1
2 9
.
1
2 6
.
2
2 4
.
3
2 2
.
4
2 9
.
4
2 7
.
5
2 4
.
6
2 2
.
7
2 9
.
7
2 7
.
8
2 4
.
9
2 2
.
0
3 9
.
0
3 7
.
1
3 4
.
2
3 It could be concluded from Figures 4.24 and 4.25 that sulphide concentration (in both the liquid phase and the air phase) of sewage with higher CODs were transferred during the first 24 hours. The concentration of CODs did not affect the concentration of H2S but may have some effect on the emission rate. Based on experimental data, we developed the original two-phase model (see chapter 2) using
MATLAB® software. The model was calibrated using experimental data and used to predict sulphide concentration in the liquid phase and H2S in the air phase in the sewer at different conditions. The concentrations predicted were compared with experimental results and data 76 collected through field monitoring. The two-phase model was developed by Yongsiri et al. (2004) for a stretch sewer pipe in close networks. It could predict sulphide concentration in the water phase (see Eq. 2.19) and air phase (see Eq. 2.20). C dC 20 − 1, 1, (
T fK C = − + − − α V
W CQ
WW CQ
WW VR
Wf a
SHL W )
V
W 1, 0, 1, 1, 1, 1, 2 A
fH W
dt
θ
e
C (2.19) 5.0 −20 (
T ) R In which 50 03.1 = − (
COD
001.0 ) f S K S K
O
+ A
V
W O O 5.0 20 − (
T ) (2.14) (
COD
001 ) S O = .0 50 03.1 − 05.0 05.0
S
+ A
V
W ( ) (
R SHL OS O uw )uf
R 2 R k 10 = = uw K S K S S
O
+ S
O
+ O O O O 5.0 5.0 input output aK S S = − + − − + dS
O
dt O O S (2.15) = 17.0= R
uf Sk
21 A
V
W A
V
W pKa − f = 1
pH
10 1 + dC C 20 − 1, 1, (
T V fK = − + − α )
V
W A CQ
A A CQ
A A a
SHL C
W 1, 0, 1, 1, 1, 2 A
dt A
fH
θ
e
C (2.20) In this research project, DO was monitored and the concentration showed that there was no significant change during each run (see Figure 4.26). Therefore, we assumed that DO 77 concentration was constant during the duration of the experiment. 250 25 200 20 pH DO 150 15 Temperature sulphide 100 10 sulphate CODs 50 5 0 0 0 3.5 22.5 26.5 50.5 145.5 The sulphide concentration measured inside the laboratory sewer pipe at the different and reported in Tables 4.6 to 4.9 were compared with sulphide concentrations predicted using the two-phase model. In the following sections, predictions and experimental results will be analysed for each set of data. Figures 4.27 and 4.28 compare experimental results reported in 78 Table 4.6 with predicted sulphide concentrations at the same conditions. 0.18 0.16 0.14 0.12 0.1 Sulphide (mg/L) Predict Sulphide (mg/L) 0.08 0.06 0.04 0.02 0 0 2.5 4.5 7 9.5 12 14.5 17 19.5 22 1.2 1 0.8 Hydrogen sulphide (ppm) 0.6 Predict Hydrogen sulphide
(ppm) 0.4 0.2 0 19 20 21 22 23 24 25 26 27 28 29 30 79 Figures 4.29 and 4.30 show a comparison between predicted and experimental H2S 0.4 0.35 0.3 concentrations reported in Table 4.7. 0.25 Sulphide (mg/L) 0.2 Predict Sulphide (mg/L) 0.15 0.1 0.05 0 0 3 6 9 12 15 18 21 23.5 26.5 29.5 0.6 0.5 0.4 Hydrogen sulphide (ppm) 0.3 Predict Hydrogen sulphide
(ppm) 0.2 0.1 0
23.5 24 24.5 25 25.5 26 26.5 27 27.5 28 80 Figures 4.31 and 4.32 show the comparison between predicted sulphide concentrations and 0.3 0.25 0.2 sulphide concentrations obtained experimentally (Table 4.8). Sulphide (mg/L) 0.15 Predict Sulphide (mg/L) 0.1 0.05 0 0 2.5 5 7 9.5 12 14.5 17 19.5 22 3 2.5 2 Hydrogen sulphide (ppm) 1.5 Predict Hydrogen sulphide
(ppm) 1 0.5 0 18 19 20 21 22 23 24 25 81 Figures 4.33 and 4.34 show the comparison between predicted sulphide concentrations and 0.45 0.4 0.35 0.3 sulphide concentrations obtained experimentally (Table 4.9). 0.25 Sulphide (mg/L) Predict Sulphide (mg/L) 0.2 0.15 0.1 0.05 0 0 2 4 6 8 10 12 14 16 18 19 0.8 0.7 0.6 0.5 Hydrogen sulphide (ppm) 0.4 Predict Hydrogen sulphide
(ppm) 0.3 0.2 0.1 0 19 20 21 22 23 24 25 26 27 28 29 82 Figures 4.27 to 4.34 showed that although the sulphide build-up rate was a little higher than the experimental results, the two-phase model could predict the sulphide concentration in the liquid. However, when sewage characteristics changed (higher original sulphate concentration or higher CODs level), the prediction for H2S was not well matched (2–6 times higher). For the sewage condition depicted in Table 4.6, the results obtained from the two-phase model were close to the experimental data. It also proved that the laboratory sewer pipe in this project could obtain better performance under a lower sewage concentration. It could be concluded from the results in Tables 4.6, 4.7, 4.8 and 4.9 that velocity, sulphate and CODs were the major factors that affected the formation of sulphide in the aqueous phase and transfer of H2S to the pipe atmosphere. As sulphate concentration was not a parameter in the two-phase model, we assumed that under these conditions, the initial concentrations of 0.45 0.4 0.35 0.3 sulphide were different. v= 0.238 m/s 0.25 0.2 v =0.034 m/s 0.15 0.1 0.05 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 83 0.08 0.07 0.06 0.05 v=0.238 m/s 0.04 0.03 0.02 v=0.034 m/s 0.01 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Figures 4.35 to 4.36 show that sulphide concentration increase with the increase in velocity in both in the liquid phase and the air phase. At 19 hours, the sulphide build-up rate at a velocity of 0.238 m/s was 0.013 mg/(L·hr), which was the same as the experimental data. At the same time, under a velocity of 0.034 m/s, the build-up rate was 0.009 mg/(L·hr), which is slightly lower than the experimental results (0.011 mg/(L·hr)). H2S concentration increased 65.8% 84 when velocity went up at 22 hours, which was higher than the experimental results. 0.25 0.2 Higher original
sulphide 0.15 Lower original
sulphide 0.1 0.05 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0.025 0.02 0.015 Lower original
sulphide 0.01 Higher original
sulphide 0.005 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Figures 4.37 to 4.38 show that when the initial sulphate concentration increased, sulphide generated more (20.2% at 22 hours) in the liquid phase but H2S went down (5.6% at 26 hours). 85 The trend was the same as the experimental data. 0.3 0.25 0.2 Higher CODs Lower CODs 0.15 0.1 0.05 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0.03 0.025 0.02 Higher CODs 0.015 0.01 Lower CODs 0.005 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Figure 4.40: Predicted H2S Concentrations in the Air Phase under Initial CODs 86 0.3 0.25 Higher CODs 0.2 Lower CODs 0.15 0.1 0.05 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0.03 0.025 0.02 Higher CODs 0.015 Lower CODs 0.01 0.005 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Figure 4.42: Predicted H2S Concentrations in the Air Phase under Initial CODs 87 Figures 4.39 to 4.42 show that CODs had a positive effect on the sulphide concentration in sewer pipes, which is in agreement with the experimental data. When under a different original sulphide concentration, it could obtain 12.8% more sulphide in the liquid phase at 24 hours (21.4% for experimental results). For H2S at the same time, higher CODs obtained 31.5% more than lower CODs. If using the same concentration of original sulphide, the difference was larger (14% for sulphide in the liquid phase and 33.5% for H2S). When under a different slope, pH and temperature, the model showed the sulphide concentration in the water and the air phase in Figures 4.43 to 4.48. The flow rate was 1 L/min, original sulphide concentration was 0.019 mg/L, and original COD was 211 mg/L. We assume the slope was 0.004, 0.008 and 0.01, temperature was 10 °C, 20 °C and 30 °C and pH 0.2 0.18 s= 0.008 0.16 0.14 s= 0.004 s= 0.01 was 7, 7.5 and 8, respectively. 0.12 0.1 0.08 0.06 0.04 0.02 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 88 0.025 0.02 0.015 s= 0.004 s= 0.01 0.01 s= 0.008 0.005 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0.2 0.18 0.16 0.14 T= 20°C T= 30°C 0.12 T= 10°C 0.1 0.08 0.06 0.04 0.02 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 89 0.035 0.03 0.025 T= 30°C 0.02 0.015 T= 20°C 0.01 T= 10°C 0.005 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0.35 0.3 0.25 pH= 8 0.2 pH= 7 0.15 pH= 6.75 0.1 0.05 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 90 0.025 0.02 0.015 pH= 6.75 pH= 7 0.01 0.005 pH= 8 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Figure 4.48: Predicted H2S Concentrations in the Air Phase at pH 6.75, 7 and 8 When slope increased, sulphide in the liquid phase decreased while H2S increased. These results were in agreement with the literature (Yongsiri et al., 2003). pH has a positive effect on sulphide concentration in the liquid phase but H2S concentration decreased when pH increased. However, temperature has a positive effect on sulphide concentration in both the liquid phase and the air phase in sewer pipes. According to Nielsen et al. (2003, 2004d, 2004e), the chemical sulphide oxidation rate is as − (
T )20 k S S shown below. 07.1 = S s 1.0
O co
, ∏− ∏− ( )
, ( ) Chemical sulphide oxidation rate (2.23) pH 04.0 5.0 + ) k = S co
, ∏− ( )
, K 1 + (
K
101
a
1
(
) pH
101 a
1 In which 91 Where
Ks(-п),o,c = rate constant for chemical oxidation, (g/m3)-0.1h-1;
Ss(-п) = dissolved sulphide concentration, g/m3; So = DO, g/m3; T = Temperature, °C; Ka1 = the fist dissociation constant for H2S (-), the value of Ka1 at 25 °C is reported at
8.913*10-8 (Lide, 2003). Therefore, the sulphide chemical oxidation rate in the water phase could be calculated and 0.14 0.12 compared with the sulphide oxidation rate in our system shown in Figure 4.49. 0.1 0.08 Chemical Sulphide oxidation
rate 0.06 Sulphide oxidation rate in our
system 0.04 0.02 0 0 1 2 3 4 5 Figure 4.49 shows that both the predicted rate and the real rate have the same trend; however, the rate that was calculated using Nielsen’s equation was almost 11 to 16 times more than our experimental results. This may be due to the lower sulphide concentration of synthetic sewage as compared to real sewage. According to Pomeroy’s equation, the corrosion rate was: )A (
sw 1 92 c (2.27) = k
5.11 φ Where c= average rate of corrosion of pipe walls (mm/year) k = factor representing the proportion of acid reacting, taken as 0.8 after Pomeroy A= constant representing the alkalinity of pipe material, taken as 0.8 for concrete with limestone aggregate 83 sv j In which 7.0 ( ) ](
[
PbDS )' sw =φ (2.26) swφ = average flux of H2S at the air/wall interface (g/m2.h) Where [DS] = average concentration of dissolved sulphide (S1+S2)/2 (mg/L) b = surface width of stream P’ = exposed perimeter (π-P/D)D, P- wetted perimeter j = pH dependent factor for the proportion of H2S unionised and is calculated after the WRc method (1995) where: 2 adj adj 1
−
10 8 − 1 j = k 1 + + k
×
pH
2 k
1
pH
− 10 1 + − 12
− k 10 = 1
×= 2 + k 94.7 10 = × = In which
[
]
SH
2
][
[
−
HS
H adj pH = )5− f Where pH = measured pH of sewage f = a constant depending on the level of hydraulic turbulence, taken as 1.05 by interpolation between figures given by WRc for a pond and for a highly turbulent stream. 93 The results calculated are presented in Table 4.14. Φsw j c pHadj Table 4.6 6.197585109 0.888775976 0.002207 0.025378 Table 4.7 6.319251954 0.857923591 0.002431 0.027956 Table 4.8 6.22215648 0.883058851 0.001902 0.021878 Table 4.9 6.166702457 0.895613523 0.003721 0.042792 WIL54A 6.426230852 0.825176648 0.04709 0.541534 NWI1 6.963028805 0.578298379 0.014749 0.169613 According to Table 4.14, the maximum corrosion rate in experiment was 0.04 mm per year and this result was due to the much lower concentration of synthetic sewage used. However, the calculation of real manholes show the maximum corrosion rate was 0.5mm per year, which was lower than shown in the literature (the average rate was 3 mm per year) (Davis et al., 1998;Ismail et al., 1993;Morton et al., 1991;Mori et al., 1992). Figures 4.50 and 4.51 show the predicted H2S concentration in real sewer pipes using the two- 94 phase model. 1.8 1.6 1.4 1.2 Hydrogen sulphide (ppm) 1 0.8 Predict Hydrogen sulphde
(ppm) 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 The sulphide concentration in the water phase was 0.288 g/m3. The maximum value was 1.4 ppm, which was detected by the gas detector. The highest concentration predicted by the two- 95 phase model was 1.6 ppm. 2.5 2 1.5 Hydrogen sulphide (ppm) Predict hydrogen sulphide
(ppm) 1 0.5 0
11.5 13.0 14.5 16.0 17.5 19.0 20.5 22.0 23.5 ppm, which was detected by the gas detector. The highest concentration that was predicted by the two-phase model was 1.9 ppm. Figures 4.50 and 4.51 demonstrate that the data that was predicted by the two-phase model were close to the data logged by the gas detector. However, in this project, due to a lack of data regarding soluble COD, the model could not accurately predict the H2S at every hour and the real conditions were quite complicated. The two-phase model could be further improved to fully predict the H2S in real sewer 96 networks if it could be coupled with an established online monitoring system. 22 21 20 19 Lower Temperature Higher Temperature 18 17 16 15 0.00 3.00 6.00 9.00 12.00 15.00 18.00 21.00 1.2 1 0.8 H2S under Lower
Temperature 0.6 H2S under Higher
Temperature 0.4 0.2 0 0.00 3.00 6.00 9.00 12.00 15.00 18.00 21.00 Figures 4.52a and 4.52b show that when temperatures for these two days were almost same, 97 the concentration of H2S was not very different. When the temperature increased (see Figure 4.52a after 12pm), the concentration of H2S was higher, with a maximum concentration of 1 ppm at 8pm. It could be estimated that temperature may have a positive effect on the emission of H2S gas. Therefore, if the temperature rises, it may increase the possibility for H2S gas diffuses from the water to the air phase, which is in agreement with the literature (Yongsiri et al., 2003). The H2S concentrations of manhole WIL54A and NWI1 were compared in Figure 4.53 and 4.54. Other parameters were listed in Table 4.15. Parameters WIL54A NWI1 pH 7.13 8.1 88 79 CODs (mg/L) Temperature (°C) 17.4 25.7 Sulphide (mg/L) 0.288 0.433 98 Velocity (m/s) 0.5 0.22 2 1.8 1.6 1.4 1.2 WIL54A 1 NWI1 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 0.007 0.006 0.005 0.004 Figure 4.53: Measured H2S Concentrations of Manholes WIL54A and NWI1 WIL54A NWI1 0.003 0.002 0.001 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Figures 4.53 and 4.54 show that both of the measured and predicted concentrations of H2S in manhole WIL54A were higher than in manhole NWI1. The measured H2S concentration of WIL54A was almost 3 times than NWI for first seven hours and 10 times than the predicted 99 value. Although sulphide concentration of NWI1 was higher than WIL54A and NWI1 had a higher temperature, the pH value of NWI1 was much higher than WIL54A and the velocity of NWI1 was slower than WIL54A. Therefore, pH and flow rate were shown to be major factors affecting H2S emission rate, which is in agreement with the literature (Liss and Slater, 100 1974;Yongsiri et al., 2004). 1) For our experimental set-up, the bacteria was more active in synthetic sewage (Van Manh and Anh, 2005) than Postgate’s grow medium (Hauser and Holder, 1986) in terms of capacity sulphide generation. This could be attributed to the high strength of the Postgate’s medium (i.e. high concentration of glucose). 2) During the stage where the laboratory sewer pipe was fed by synthetic sewage, the highest sulphate reduction rate was 4.17 mg/(L·hr) and H2S reached 18.5 ppm. The results were in agreement with Mudryk et al. (2000), in which the maximum sulphate reduction rate was 4.21 mg/(L·hr) for spring in Gulf of Gdansk. 3) The concentration of H2S measured after the sewage was allowed to remain stagnant (i.e. pump was stopped) were almost 18 times more than H2S concentrations measure under a constant flow rate of sewage circulation. This indicates that sulphate-reducing bacteria was active during that period. This could be explained in terms of the presence of anaerobic conditions, possibly in the bottom part of the pipe contents and result into transfer of h2S into the atmosphere, once it has reached the saturation level. It could be concluded that the sewer system could have more H2S emissions into the atmosphere when receiving high flow rates after a very slow sewage flow periods. 3 T 20 − ) 3 − 1
− 1
− 8 1) According to Pomeroy’s equation, the sulphide build-up rate for our system was: )( (
0.325 ) [
CODs ](
07.1 r su d .0 073 10 = × − dS
dt 2) The coefficient was 0.325 in the experiment, compared to the conservation coefficient 0.64 and reasonable expectation coefficient 0.96 from the literature (Pomeroy and Boon, 1990). 3) Sulphide concentrations at the outlet of laboratory sewer pipe was higher than at 101 the inlet, which is similar to the real sewer system. 4) The maximum concentrations of H2S shown in Tables 4.6, 4.7 and 4.8 were 0.00165 mg/L, 0.00075 mg/L and 0.00075 mg/L, respectively. However, sulphide concentrations were 0.17 mg/L, 0.364 mg/L and 0.28 mg/L in the water phase, which corresponded to equilibrium concentrations of H2S of 0.068 mg/L, 0.153 mg/L and 0.119 mg/L, respectively. The results agree with previous studies that H2S formation is less than 10% of the theoretical equilibrium concentrations. 5) Increasing the sewage velocity from 0.034 m/s to 0.238 m/s, 85.7% increase resulted in a 15.4% increase in sulphide build-up rate. The slower flow rate also could achieve more H2S concentration. It could be concluded from this experiment that when we encounter drought or during water restriction periods, there is a potential for sulphide to build up in the sewer system hence increase the possibility of H2S emissions. 6) Increased sulphate concentration could increase sulphide concentration (11.3 mg/L more sulphates had 0.065 mg/L more sulphide) under the same concentration of COD. Moreover, it took a further seven hours to reduce more sulphate for the experimental set-up. Higher sulphate concentration contributes to more H2S concentration if sulphate-reducing bacteria can obtain more organic carbon to consume. 7) Synthetic sewage with higher initial CODs affected the generation of sulphide concentration in the liquid phase, 21.4% more sulphide was observed for 36.8% increased in CODs. There was no strong evidence to show that higher CODs could obtain a higher H2S concentration, but it may affect the H2S emission rate. 1) The results that were predicted using the two-phase model were in agreement in terms of trend and reasonably in agreement in terms of values with the measured results under certain experimental conditions tested (e.g. slope= 1%, flow rate of 1 L/min, initial sulphate concentration of 18.2 mg/L and initial CODs of 211 mg/L). 2) The results predicted the two-phase model under different velocity, initial sulphate concentration and initial CODs were in agreement with the experimental results. Increasing the velocity from 0.034 m/s to 0.238 m/s, sulphide concentration 102 increased by 38% and H2S concentration went up by 65.8%. Higher sulphate concentration led to more sulphide concentration (14% more) but less H2S (5.6% less). Higher CODs contributed to more sulphide concentrations, 14% more for aqueous sulphide and 33% for H2S, respectively. 3) Increasing the sewer slope, sulphide in the liquid phase decreased whereas H2S went up. This was in agreement with Yongsiri et al. (2003). A proportional relationship was observed between pH and temperature and sulphide concentration in the liquid phase, but an inverse relationship was observed between H2S and pH. 4) Chemical sulphide oxidation rates in our experiment were 11–16 times smaller than results calculated using Nielsen’s equation. It also could prove that our system could obtain the best results under a small load of sewage. 5) Field results show that higher temperature can results in increased rates of H2S emission, which was the same as the experimental and predicted results. The comparison results for two manholes proved that pH and turbulence were major 103 factors affecting H2S emission rates. 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Yongsiri, C., Vollertsen, J. & Hvitved-Jacobsen, T. 2004a, 'Effect of temperature on air-water transfer of hydrogen sulfide'. Journal of Environmental Engineering, 130 (1), 104-109. Yongsiri, C., Vollertsen, J. & Hvitved-Jacobsen, T. 2004b, 'Hydrogen sulfide emission in sewer networks: a two-phase modeling approach to the sulfur cycle'. Water Science & Technology, 50 (4), 161-168. Yongsiri, C., Vollertsen, J. & Hvitved-Jacobsen, T. 2005, 'Influence of wastewater constituents on hydrogen sulfide emission in sewer networks'. Journal of Environmental Engineering, 131 (12), 1676. Yongsiri, C., Vollertsen, J., Rasmussen, M. & Havitved-Jacobsen, T. 2004, 'Air-Water Transfer of Hydrogen Sulfide: An Approach for Application in Sewer Networks'. Water Environment Research, 76 (1), 81-88. Zhang, L., De Schryver, P., De Gusseme, B., De Muynck, W., Boon, N. & Verstraete, W. 2008, 'Chemical and biological technologies for hydrogen sulfide emission control in 111 sewer systems: A review'. Water Research, 42 (1-2), 1-12. Date Time Temperature Humid H2S O2 SO2 CH4 (dd/mm/yy) (hh:mm:ss) (ppm) (ppm) (ppm) (°C) ity (%) 16.6 40 0.4 19.9 0 0 26/08/09 11:06:25 16.6 40 0.3 19.9 0 0 26/08/09 11:07:25 16.5 40 0.3 19.9 0 0 26/08/09 11:08:25 16.5 39 0.3 19.9 0 0 26/08/09 11:09:25 16.5 39 0.4 19.9 0 0 26/08/09 11:10:25 16.5 39 0.3 19.9 0 0 26/08/09 11:11:25 16.6 39 0 19.9 0 0 26/08/09 11:12:25 16.6 39 0 19.9 0 0 26/08/09 11:13:25 16.5 39 0.3 20 0 0 26/08/09 11:14:25 112 16.5 38 0.4 19.9 0 0 26/08/09 11:15:25 Date/Time CO(ppm) H2S(ppm) SPE(%) LEL(%) 08/09/2009 10:32 0 0 20.9 0 08/09/2009 10:33 0 0 20.9 0 08/09/2009 10:34 0 0 20.9 0 08/09/2009 10:35 0 0 20.9 0 08/09/2009 10:36 0 0 20.9 0 08/09/2009 10:37 0 0 20.9 0 08/09/2009 10:38 0 0 20.9 0 08/09/2009 10:39 0 0 20.9 0 08/09/2009 10:40 0 0 20.9 0 08/09/2009 10:41 0 0 20.9 0 08/09/2009 10:42 0 0 20.9 0 08/09/2009 10:43 0 0 20.9 0 08/09/2009 10:44 0 0 20.9 0 08/09/2009 10:45 0 0 20.9 0 08/09/2009 10:46 0 0 20.9 0 113 08/09/2009 10:47 0 0 20.9 0 The program below was used for defining the equations of predict sulphide concentrations both in the sewer water phase and atmosphere. Function dCdt=sulphide (t,C) Global Co n D=0.155; % experimental monitoring, the diameter for sewer was 155mm L=2754; % the length of sewer Ln=L/n; % length of compartment % condition Qw=0.06; % flow rate in the water phase, the flow rate of the set-up was 1 L/min,
1L/min=0.06m3/h velratio=0.2; % velocity ratio(air phase/water phase) QA=velratio*Qw; % flow rate in sewer atmosphere Vw=0.001676*Ln; % volume of water phase of CFSTR compartment n VA=0.0172*Ln; % volume of air phase of CFSTR compartment n pH=7.13; pKa=7; T=20; % average of temperature % Model parameters alpha=0.6; % ratio of KL aH2S for domestic wastewater to that for clean water tetar=1.034; % temperature correction factor for re aeration Hc=0.3616; % Henry's law constant of H2S 114 teta=1.034; % temperature correction factor for H2S emission f=1/(1+10^(pH-pKa)); %ratio of H2S(aq) to total sulphide s=0.01; % slope of sewers u=0.034; % velocity in water phase, mean flow velocity dm=0.01477; % mean hydraulic depth g=9.81; % gravitational acceleration Fr=u/((g*dm)^0.5); %Froude number %calculate overall mass transfer coefficient of H2S KLaO2=0.86*(1+0.2*Fr^2)*((s*u)^(3/8))*(dm^(-1))*(tetar^(T-20)); KLaH2S= (1.736-0.196*pH)*KLaO2; ratioAVw=64.44; % interfacial area to volumes ratio CODs= -3.360*L/(u*3600)+243.7; % soluble chemical oxygen demand equation Ko=0.05; % half saturation concentration for dissolved oxygen %calculate rate of sulphide formation Rf=0.001*(CODs-50)^0.5*1.03^(T-20)*(ratioAVw)*(Ko/(Ko+0.76)); dCdt=[Qw*Co(1)/Vw-Qw*C(1)/Vw+Rf-alpha*f*KLaH2S*(C(1)-(C(2)/(f*Hc)))*teta^(T-20) QA*Co(2)/VA-QA*C(2)/VA+alpha*f*KLaH2S*(C(1)-(C(2)/(f*Hc)))*(teta^(T- 20))*(Vw/VA)]; End clc clear Global Co n 115 Cend= [0.019 0]; % initial value of concentration sulphide in the water phase and air phase n=input ('how many compartments? = '); %number of compartment For s=1: n Co=Cend(1,:); % input concentration for compartment n trange=[0:1:24]; nint=length(trange); [t,C]=ode23('sulfide',trange,Co); C Cend(1,:)=C(nint,:); Cout(s,:)=Co; End Cout(s+1,:)=Cend; figure(1) plot(t(:,1),C(:,1),t(:,1),C(:,2)) xlabel('t(hour)') ylabel('concentration (g/m3)') legend('Sulfide in water phase','Sulfide in air phase') clc clear global Co nr % for one compartment nr=1; Co=[0.019 0]; trange=[0:1:24]; [t,C]=ode23('sulfide',trange,Co); C plot(t(:,1),C(:,1),t(:,1),C(:,2)) xlabel ('t(hour)') ylabel ('concentration') 116 legend ('Sulphide in water phase', 'Sulphide in air phase')2.5.1 Models for Sulphide Build-up
2.5.2 Hydrogen Sulphide Emission Prediction
(
K
)
)
(
KK
s
1
2.5.3 Sulphide Oxidation Rate
2.5.4 Concrete Corrosion Rate
]
]
]
[
HS
][
[
2
−
HS
pH
(
pH
Chapter Three: Materials and Methods
3.1 Experimental Set-up
3.1.1 Preliminary Experimental Set-up
Table 3.1: Summary of Experimental Program
Figure 3.1: Schematic Diagram of the Preliminary Experimental Set-up
3.1.2 Final Experimental Set-up
Figure 3.2: Schematic Diagram of the Final Experimental Set-up
Figure 3.3: Final Experimental Set-up Picture
3.2 Materials
3.3 Methods
3.3.1 Development of Biological Growth
Table 3.2: Postgate’s Grow Medium Constituents (Hauser and Holder, 1986)
Table 3.3: Composition of the Synthetic Sewage Used to Feed the Laboratory Sewer Pipe,
with a Ratio of COD: N: P=100:5:1 (Weidhaas et al., 2007)
3.3.2 Laboratory Sewer Pipe Runs
Table 3.4: Synthetic Sewage Constituents (Van Manh and Anh, 2005)
3.4 Analysis
3.5 Field Monitoring
3.5.1 Monitoring Locations
Figure 3.4: Monitoring Location that Shows on Google® Maps
Figure 3.5: Location of Manhole WIL54A. Located in Field, Kororoit Creek road
Figure 3.6: Location of Manhole NWI1. Located Opposite 4–14 Orange Street
Table 3.5: Parameters of Manhole WIL54A and NWI 1
Figure 3.7: Sampling Cabinet and Testing Auto-sampler Located in Manhole NWI1
3.5.2 Sampling Procedure
Chapter Four: Results and Discussion
4.1 Introduction
4.2 The Development of Biological Growth in the Laboratory Sewer Pipe
4.2.1 Experimental Results in the Initial Stage
Table 4.1: Parameters of Gas Samples M
Figure 4.1: Sewer Pipe at the End of the Initial Stage Showing Bacteria Growth on the
Inside Walls of the Laboratory Sewer Pipe
4.2.2 Experimental Results during Postgate’s Grow Medium
Table 4.2: Sulphide Concentration during the Second Stage when Using Postgate’s Grow
Medium
)
)
%
(
2
O
C
°
(
e
r
u
t
a
r
e
p
m
e
T
Time (hour)
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.3: H2S Concentration during Test A Period
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.4: Time at which Gas Detector recorded H2S Concentration above 0.0 ppm
(Test B)
4.2.3 Experimental Results when Using Synthetic Sewage
Table 4.3: Characteristics of Constituent of Postgate’s Grow Medium and Synthetic
Sewage
)
L
)
L
/
/
g
m
g
m
(
e
d
i
(
e
t
a
h
p
h
p
l
l
u
S
u
S
Time (hour)
Figure 4.5: Measured Sulphide and Sulphate Concentration when Using Synthetic
Sewage
)
m
p
p
(
S
2
H
Time (minute)
of Biological Growth Development
4.3 Performance of the Laboratory Sewer Pipe
4.3.1 Sulphur Processes in the Laboratory Sewer pipe
)
³
m
/
g
(
e
d
i
h
p
l
u
S
Time (hour)
4.3.2 Sulphate Calibration
Table 4.4: Sulphate Stock Solutions
)
L
/
g
m
(
e
u
l
a
v
l
d
e
t
a
l
u
c
l
a
c
measured value (mg/L)
Figure 4.10: Sulphate Calibration Curve
Figure 4.11: Measurement of the Sulphate Concentration. The Machine Shown in the
Picture is HACH DR 4000
Table 4.5: Initial Sulphate Concentration in the Different Solutions Used in Preparing
Synthetic Toilet Sewage
4.3.3 Sulphate Reduction and Sulphide Build-up
)
C
°
(
e
r
u
t
a
r
e
p
m
e
T
,
)
L
/
(
g
m
O
D
,
H
p
Time (hour)
Figure 4.12: Temperature, pH and DO Concentration of Sewage Samples
Table 4.6: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and the Feed/Effluent Tank (flow rate=1 L/min,
slope=1%, initial sulphate concentration =18.2 mg/L)
199
131(*)
90
179(#)
Figure 4.13: Calculation of the Parameters for Laboratory Sewer Pipe
Table 4.6a: Calculation of Coefficient A for Table 4.6
Table 4.6b: H2S Concentration at the Inlet (Table 4.6)
Table 4.7: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and Feed/Effluent Tank (flow rate=1 L/min,
slope=1%, initial sulphate concentration = 29.5 mg/L, initial CODs = 242 mg/L)
Table 4.7a: Calculation of Coefficient A for Table 4.7
Table 4.7b: H2S Concentration at the Inlet (Table 4.7)
Table 4.7c: Comparison of Sulphate, Sulphide and H2S Concentration
Table 4.8: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and Feed/Effluent (flow rate=1 L/min, slope=1%,
initial sulphate concentration = 29 mg/L, initial CODs = 297 mg/L)
Table 4.8a: Calculation of Coefficient A for Table 4.8
Table 4.8b: H2S Concentration of the Inlet (Table 4.8)
Table 4.9: Sewage Concentrations of Sulphate, Sulphide and CODs at the Inlet, Outlet and Feed/Effluent Tank (flow rate=7 L/min,
slope=1%, initial CODs = 258 mg/L).
Table 4.9a: Calculation of Coefficient A for Table 4.9
Table 4.9b: H2S Concentration at the Inlet during this Period
Table 4.10: Coefficient A for Sulphide Build-up Equation for the Different Experimental
Results
)
L
/
g
m
(
O
D
Time (hour)
Figure 4.14: DO Concentration during the Experimental Period
4.3.4 Mass Balances of Sulphate and Sulphide Concentration
Table 4.11: Mass Balances of Sulphate and Sulphide Concentrations for Tables 4.6, 4.7,
4.8 and 4.9
Table 4.12: Proportion of Measured Sulphide Concentration, Calculated Sulphide and
H2S of Calculated H2S (aq)
)
%
/
/
)
L
g
m
(
n
o
i
t
r
o
p
o
r
p
)
L
g
m
(
s
D
O
C
l
(
e
t
a
h
p
u
S
Experiment
Figure 4.15: Initial Sulphate and CODs Concentrations on Measured and Calculated
Sulphide in the Liquid Phase
)
%
/
)
L
g
m
/
(
n
o
i
t
r
o
p
o
r
p
)
L
g
m
(
s
D
O
C
l
(
e
t
a
h
p
u
s
Experiment
Figure 4.16: Effect of Initial Sulphate and CODs Concentration on H2S/H2S(aq)
4.4 Factors Affecting the Concentration of Sulphide in the Liquid Phase
and H2S in the Air Phase
4.4.1 Effect of Velocity on the Generation of Sulphide in Sewage and H2S in the
Atmosphere
l
e
u
a
v
Paramters
Figure 4.17: Comparison of pH, Temperature, Concentrations of Initial Sulphate and
CODs of Experiments with Velocity of 0.034 m/s and 0.238 m/s
)
L
/
g
m
(
n
o
i
t
a
r
t
n
e
c
n
o
c
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.18: Sulphide Concentrations at Different Velocities
)
m
p
p
(
n
o
i
t
a
r
t
n
e
c
n
o
c
S
2
H
Time (hour)
Figure 4.19: H2S Concentration at Different Velocities
)
L
/
g
m
(
O
D
Sample number
Figure 4.20: DO Concentration under Different Velocity
4.4.2 Effect of Sulphate Concentration on the Generation of Sulphide in Sewage and H2S
in the Atmosphere
Table 4.13: Parameters of Experimental Results under Synthetic Sewage with Different
Original Sulphate Concentration
)
L
/
g
m
(
n
o
i
t
a
r
t
n
e
c
n
o
c
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.21: Sulphide Concentration under Synthetic Sewage with Different Initial
Sulphate Concentration
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.22: H2S Concentration under Synthetic Sewage with Different Original
Sulphate Concentration
4.4.3 Effect of COD on the Generation of Sulphide in Sewage and H2S in the
Atmosphere
e
u
l
a
V
Parameters
Figure 4.23: Comparison of pH, Temperature, Concentrations of DO and Original
Sulphate of Experiments with CODs of 242 mg/L and 383 mg/L
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Concentration
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.25: H2S Concentration Using Synthetic Sewage for Different CODs
4.5 Model to Predict Sulphide in Sewer System and Predicted Sewer Pipe
Corrosion
4.5.1 Two-phase Model
Sulphide in the Water Phase:
Sulphide in the Air Phase:
,
)
)
L
/
g
m
)
³
/
(
s
D
O
C
,
)
³
m
g
(
e
d
C
°
(
e
r
u
t
a
r
e
p
m
e
T
i
/
h
p
,
)
L
l
/
u
S
(
m
g
(
e
t
a
h
p
l
g
m
O
D
,
u
S
H
p
time (hour)
Figure 4.26: pH, DO, Temperature, Sulphide, Sulphate and CODs of the Liquid Phase in
the Laboratory Sewer Pipe
4.5.2 Comparison of Prediction and Experimental Results
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.27: Comparison of Sulphide Concentrations in the Liquid Phase Predicted
using the Two-phase Model and Experimental Data Reported in Table 4.6.
)
m
p
p
(
S
2
H
Tim e (hour)
Figure 4.28: Comparison of H2S in the Air Phase Predicted using the Two-phase Model
and Experimental Data Reported in Table 4.6
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.29: Comparison of Sulphide Concentration in the Liquid Phase Predicted
Using the Two-phase Model and Experimental Data Reported in Table 4.7
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.30: Comparison of H2S in the Air Phase Predicted Using the Two-phase Model
and Experimental Data Reported in Table 4.7
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.31: Comparison of Sulphide Concentration in the Liquid Phase Predicted
Using the Two-phase Model and Experimental Data Reported in Table 4.8
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.32: Comparison of H2S in the Air Phase Predicted Using the Two-phase Model
and Experimental Data Reported in Table 4.8
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.33: Comparison of Sulphide Concentration in the Liquid Phase Predicted
Using the Two-phase Model and Experimental Data Reported in Table 4.9
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.34: Comparison of H2S in the Air Phase Predicted Using the Two-phase Model
and Experimental Data Reported in Table 4.9
4.5.3 Comparison of Sulphide Concentration Using the Two-Phase Model under
Different Sewer Conditions
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.35: Predicted Sulphide Concentrations in the Liquid Phase at Sewage Velocity
of 0.034 m/s and 0.238 m/s
/
)
3
m
g
(
S
2
H
Time (hour)
Figure 4.36: Predicted H2S Concentrations in the Air Phase under Velocity 0.034 m/s
and 0.238 m/s
/
)
L
g
m
i
l
(
e
d
h
p
u
S
Time (hour)
Figure 4.37: Predicted Sulphide Concentrations in the Liquid Phase under Initial
Sulphate Concentrations 18.2 mg/L and 29.5 mg/L
/
)
3
m
g
(
S
2
H
Time (hour)
Figure 4.38: Predicted H2S Concentrations in the Air Phase under Initial Sulphate
Concentrations 18.2 mg/L and 29.5 mg/L
/
)
L
g
m
i
l
(
e
d
h
p
u
S
Time (hour)
Figure 4.39: Predicted Sulphide Concentrations in the Liquid Phase under Initial CODs
Concentrations 242 mg/L and 383 mg/L (Initial Sulphide Concentrations 0.024 mg/L
and 0.015 mg/L, Respectively)
/
)
3
m
g
(
S
2
H
Time (hour)
Concentrations 242 mg/L and 383 mg/L (Initial Sulphide Concentrations 0.024 mg/L
and 0.015 mg/, Respectively)
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.41: Predicted Sulphide Concentrations in the Liquid Phase under Initial CODs
Concentrations 242 mg/L and 383 mg/L (Same Initial Sulphide Concentrations 0.024
mg/L)
/
)
3
m
g
(
S
2
H
Time (hour)
Concentrations 242 mg/L and 383 mg/L (Same Initial Sulphide Concentrations 0.024
mg/L)
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.43: Predicted Sulphide Concentrations in the Liquid Phase under Sewer Slope
of 0.004, 0.008 and 0.01
/
)
3
m
g
(
S
2
H
Time (hour)
Figure 4.44: Predicted H2S Concentrations in the Air Phase under Sewer Slope of 0.004,
0.008 and 0.01
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.45: Predicted Sulphide Concentrations in the Liquid Phase under
Temperatures of 10°C, 20°C and 30°C
/
)
3
m
g
(
S
2
H
Time (hour)
Figure 4.46: Predicted H2S Concentrations in the Air Phase under Temperatures of
10°C, 20°C and 30°C
)
L
/
g
m
(
e
d
i
h
p
l
u
S
Time (hour)
Figure 4.47: Predicted Sulphide Concentrations in the Liquid Phase at pH 6.75, 7 and 8
/
)
3
m
g
(
S
2
H
Time (hour)
4.5.4 Sulphide Oxidation Rate in the Liquid Phase
)
r
h
.
L
/
g
m
(
e
t
a
r
n
o
i
t
a
d
i
x
o
e
d
i
h
p
l
u
S
Experiment
Figure 4.49: Chemical Sulphide Oxidation Rate Calculated Compared with the Rate in
our System for Results Reported in Tables 4.6 to 4.9
4.5.5 Predict Concrete Sewer Pipe Corrosion Rate
]
]
]
[
HS
][
[
2
−
HS
pH
(
pH
Table 4.14: Concrete Sewer Pipe Corrosion Rate under Results Reported in Table 4.6 to
4.9 and Field Study
4.6 Field Monitoring Results
4.6.1 Two-phase Model to Predict H2S in Sewer Pipes
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.50: Comparison of H2S Concentrations in the Air Phase using the Two-phase
Model with Real Data for Manhole WIL54A
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.51: Comparison of H2S Concentrations in the Air Phase using the Two-phase
Model with Real Data for Manhole NWI1
The sulphide concentration in the water phase was 0.433 g/m3. The maximum value was 1.8
4.6.2 Effect of Temperature on H2S Concentration
)
C
°
(
e
r
u
t
a
r
e
p
m
e
T
Time (hour)
Figure 4.52a: Temperature Value for Two Different Days in Same Manhole WIL54A
)
m
p
p
(
S
2
H
Time (hour)
Figure 4.52b: Comparison of H2S Concentrations under Different Temperatures in the
Same Manhole WIL54A
4.6.3 Effect of Sewage Constituent on H2S Concentration
Table 4.15: Temperature, pH, CODs and Sulphide Concentration of Manhole WIL54A
and NWI1
)
m
p
p
(
S
2
H
Time (hour)
/
)
3
m
g
(
S
2
H
Time (hour)
Figure 4.54: Predicted H2S Concentrations of Manholes WIL54A and NWI1
Chapter Five: Conclusions
5.1 Biological Growth of Sulphate Reducing Bacteria
5.2 Laboratory Performance of Sewer Pipe
5.3 Model for Prediction of Sulphide Concentrations
References
41 (6), 107-115.
130, 1382.
42 (1), 105-117.
37 (1), 105-113.
Appendix
Example of YESAIR Gas Detector
Example of QRAEII Gas Detector
MATLAB Program for Two-phase Model
Basic Function
Graph Function 1
Graph Function 2
