3.2
Treatability Evaluation
Gianni Andreottola and Paola Foladori
3.2.1 Introduction
3.2.2 Organic Compounds as Aggregate Parameters
3.2.2.1 Fractions of Total COD in Wastewater and their Treatability
3.2.2.2 Respirometric Approach for COD Fractionation
3.2.2.3 COD Fractionation from Data of Conventional Analytical Monitoring in
WWTPs
3.2.2.4 A Case Study at Regional Level
3.2.3 Organic Micropollutants
3.2.3.1 Categories of Organic Micropollutants
3.2.3.2 Treatability of Organic Micropollutants
3.2.4 Nutrients: Nitrogen and Phosphorus
3.2.4.1 Fractions of Nitrogen and their Treatability
3.2.5 Metallic Compounds
3.2.5.1 Treatability of Metallic Compounds
3.2.6 Final Considerations
References
3.2.1 INTRODUCTION
‘To know treatability is to know the fate of contaminants in WWTPs’.
The pollutants introduced into the sewerage collecting system and reaching munic-
ipal wastewater treatment plants (WWTPs) derive principally from human activities
Wastewater Quality Monitoring and Treatment Edited by P. Quevauviller, O. Thomas and A. van der Beken
C
2006 John Wiley & Sons, Ltd. ISBN: 0-471-49929-3
180 Treatability Evaluation
and in particular from domestic sources, industrial districts and urban run-off rain-
water. A very large amount of different organic and inorganic compounds, estimated
as several thousand, has been detected in raw wastewater. The treatability of these
compounds in the conventional WWTPs can differ significantly depending on each
considered contaminant. The importance of knowing the treatability of the differ-
ent kinds of pollutants present in municipal wastewater is related to the prediction
of the fate of these contaminants in WWTPs before the discharge in the receiving
water bodies. The following principal categories of contaminants in municipal raw
wastewater can be distinguished:
rOrganic compounds as aggregate parameters. The whole amount of organic matter
is generally measured as aggregate organic parameters, such as chemical oxygen
demand (COD), total organic carbon (TOC), or biological oxygen demand (BOD)
in the case of the measurement of only biodegradable compounds. Aggregate or-
ganic constituents are comprised of a number of individual compounds that cannot
be distinguished separately. Eventually the fractionation of COD can be performed
with the aim to discriminate biodegradable and nonbiodegradable fractions of or-
ganic matter:
rOrganic micropollutants. The determination of these organic compounds is done
as individual parameters; some of them are associated with a potential toxic risk
to health and the environment.
rNutrients, such as nitrogen (N) and phosphorus (P). Among the inorganic non-
metallic compounds, N and P in their different ionic or organic forms, represent
the most important pollutants and are also, in most cases, the major nutrients of
importance.
rMetallic compounds. Some, including cadmium, chromium, copper, mercury,
nickel, lead and zinc, are characterized by a potentially toxic action.
The effectiveness of the removal of these categories in WWTPs depends on the
plant configuration and not all WWTPs are able to remove all the pollutants present
in the influent wastewater.
Most WWTPs designed or upgraded in the last decades to European level are
characterized by primary and secondary treatment (adopting activated sludge or
biofilm configurations) able to achieve complete removal of biodegradable COD in
influent wastewater. Furthermore, plants located in areas sensitive to eutrophication
reach high efficiency in nitrification, denitrification and P removal, as directed by the
European Directive promulgated in 1991 (91/271/CEE) that imposed more restrictive
effluent limits for the discharge of treated wastewater in the receiving water bodies
(see Chapter 1.1). In particular, the effluent concentration limit for total nitrogen is
equal to 15 or 10 mg/l for a population equivalent (PE) lower or higher than 100 000,
respectively. Analogously in the same Directive, the effluent limit for phosphorus is
2 and 1 mg/l for plant capacity below or above 100 000 PE, respectively.
Organic Compounds as Aggregate Parameters 181
Plants currently guaranteeing to meet the discharge limits for COD, biological
oxygen demand for 5 days (BOD5) and total suspended solids (TSS), could not
meet the limits for N and P as imposed by 91/271/CEE for sensitive areas, requiring
further upgrading.
Discharge limits are indicated also for other constituents, such as metals or organic
micropollutants; due to their wide heterogeneity and their different treatability not
all the WWTPs are suitable for the complete removal of these contaminants, but
many of them can be removed only partially. For example, organic micropollutants
can be biodegraded only in part, but often are removed physically from water and
accumulated in excess sludge, transferring the pollution problem from water to
sludge. This occurs also in the case of metals.
For evaluating the wastewater treatability, two key aspects have to be considered:
the composition of the influent wastewater; and the treatment capacity in the WWTPs.
In particular, the treatment capacity is related to the physico-chemical processes
performed in the plant and the biodegradation capacity of activated sludge or biofilm
processes in the secondary treatment. The wastewater composition in combination
with the plant treatment capacity constitutes the basis of the ‘treatability’ concept.
The knowledge of these aspects is fundamental in order to evaluate the entity of
pollutants removal in the plant and to predict the quality of the treated effluents aimed
to respect the imposed limits and to reduce the impact in receiving water bodies.
In the following paragraphs the fate through WWTPs of the categories of pol-
lutants cited above are described and the repartition of contaminants in sludge or
effluent water is indicated. In particular, the influence of the various treatment pro-
cesses (physico-chemical primary treatment, biological secondary treatment and
eventually tertiary treatment) is considered for each category of contaminants.
3.2.2 ORGANIC COMPOUNDS AS
AGGREGATE PARAMETERS
The quantification of the total organic matter in wastewater and its characterization
is of primary importance for the correct design, management and optimization of
a WWTP. Carbonaceous substrates are generally quantified by using aggregate pa-
rameters such as BOD5or COD, but only the analysis of COD is able to represent the
whole amount of organic matter, while BOD5is representative of the biodegradable
fraction only.
As far as the BOD5parameter is concerned, it has been widely applied in the
field of receiving water bodies and for wastewater characterization. Due to the
5-day duration of the BOD test (BOD5), the measurement of oxygen consump-
tion (index of biodegradability) is relative to 5 days and therefore very different
from wastewater retention time in WWTPs where the biodegradation occurs. The
problems related to the interpretation of the BOD5test for the measurement of
biodegradable compounds in wastewater and its use in the design and management
182 Treatability Evaluation
of treatment processes gave increasing interest to new characterization proposals.
In particular, interest in biodegradability characterization has been increased from
the simulation models for the activated sludge process that do not use traditional pa-
rameters [for example, the Activated Sludge Model, from ASM No. 1 (Henze et al.,
1987) to ASM No. 3 (Gujer et al., 1999)]. In the literature, proposals for the charac-
terization of the biodegradability of carbonaceous substrates are available, especially
based on respirometry (Henze, 1992; Spanjers and Vanrolleghem, 1995; Orhon et
al., 1997; Spanjers et al., 1999). Respirometry is defined as the measurement and
the interpretation of the rate of oxygen consumption (oxygen uptake rate,OUR) by
activated sludge or wastewater under different load conditions. The consumption of
oxygen is due to two different factors:
(1) Endogenous respiration (OURendo) measured for a biomass in the absence of
external substrate and due to cellular maintenance and oxidation of dead cells.
(2) Exogenous respiration (OURexo) measured during the oxidation of biodegrad-
able COD present in wastewater added to a biomass.
The quantification of biodegradable COD in wastewater can be assessed through
respirometric tests carried out on activated sludge after the addition of an adequate
amount of wastewater. The dynamics of OURexo are monitored for a period of about
10–20 h and the data are interpreted as described in more detail in Section 3.2.2.2.
Alternatively, in the absence of respirometric measurements, a rapid estimation
of COD fractions (less precise than the results obtainable by respirometry) can be
done in existing WWTPs, according to an easy calculation based on BOD5and COD
analyses in influent and effluent wastewater, as indicated in Section 3.2.2.3.
3.2.2.1 Fractions of Total COD in Wastewater and their Treatability
While some organic compounds are easily biodegradable in WWTPs, others are
persistent and refractory and they are found in the treated effluents or in the excess
sludge. The complete fractionation of COD in raw wastewater is shown schematically
in Figure 3.2.1, in which symbols are adopted according to ASM models. The total
COD concentration is subdivided into two biodegradable and nonbiodegradable
fractions and into an active biomass fraction. A soluble part (S) and a particulate
part (X) are distinguished for both biodegradable COD (indicated by subscript S)
and nonbiodegradable COD (indicated by subscript I).
In COD fractionation the following terms are introduced and defined:
(1) Total COD: determined experimentally by chemical analysis without any pre-
treatment of the wastewater (APHA, AWWA and WPCF, 1998).
(2) Soluble COD (S): determined experimentally by means of the chemical anal-
ysis of COD after a pretreatment of wastewater with coagulation, flocculation
Organic Compounds as Aggregate Parameters 183
Total COD
biodegradable nonbiodegradable active biomass
XBH, XBA
Soluble
SS
Particulate
XS
Soluble
SI
Particulate
XI
Figure 3.2.1 Scheme of total COD fractionation in wastewater
and 0.45-μm-filtration, according to the procedure proposed by Mamais et al.
(Mamais et al., 1993). Alternatively, the determination of soluble COD can be
carried out by the direct filtration of wastewater at 0.1 μm, in order to minimize
the occurrence of colloidal solids. The results obtained from the two kinds of
measurements are similar with a difference of about 1 % (Roeleveld and van
Loosdrecht, 2002);
(3) Particulate COD (X): determined as the difference between total COD and
soluble COD.
(4) Soluble biodegradable COD (SS): made up of simple molecules ready to be as-
similated through the cellular membrane (readily biodegradable COD) or easy to
be hydrolysed (rapidly hydrolysable COD); it can be measured by respirometry.
(5) Particulate biodegradable COD (XS): made up of suspended and colloidal solids
and compounds with high molecular weight that require enzymatic hydrolysis
before being metabolized. It is also called ‘slowly biodegradable COD’ and can
be measured by respirometry; the biodegradation rate of XSis about 10 times
smaller than the rate of SS.
(6) Soluble inert COD (SI): made up of dissolved nonbiodegradable molecules. It
is calculated as the difference between S and SS.
(7) Particulate inert COD (XI): made up of nonbiodegradable compounds, both
in suspended and colloidal forms. It is calculated as the difference between
X and XS.
(8) Heterotrophic and autotrophic active biomass (XBH and XBA, respectively):
made up of the cellular active biomass present in wastewater and represents an
inoculum for the biological process in the WWTP. The value of XBH can be
quantified by respirometry, while the amount of XBA is often neglected in the
COD fractionation.
The total COD is given by:
total COD =SS+XS+SI+XI+XBH +XBA