Summary of Environmental engineering doctoral thesis: Research and development of the internal circulation (IC) high rate anaerobic treatment technology

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This thesis focuses on simulating the internal circulation process (gas pulling water) to determine: The amount of water (QN) was pulled up by the amount of gas (QK); and mixing ability of generated gas and circulating water flow, from which to calculate the periodic structure in the IC system.

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Nội dung Text: Summary of Environmental engineering doctoral thesis: Research and development of the internal circulation (IC) high rate anaerobic treatment technology

MINISTRY OF VIETNAM ACADEMY OF EDUCATION AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY Tran Manh Hai RESEARCH AND DEVELOPMENT OF THE INTERNAL CIRCULATION (IC) HIGH RATE ANAEROBIC TREATMENT TECHNOLOGY Major: Environmental Engineering Code: 9.52.03.20 SUMMARY OF ENVIRONMENTAL ENGINEERING DOCTORAL THESIS Hanoi - 2019
This thesis done at: - Department for Technology Application and Tranfer - Institute of Environmental Technology, Vietnam Academy of Science and Technology - Department of Enviromental Technology - Center for Environmental Technology & Sustainable Development (CETASD), Hanoi University of Science (HUS). - Graduate University of Science and Technology - Vietnam Academy of Science and Technology. Supervisor 1: Assoc.Prof., Dr. Cao The Ha Supervisor 2: Assoc.Prof., Dr. Nguyen Hoai Chau The dissertation will be defended at Graduate University of Science and Technology - Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet street, Hanoi. Time: .............,.............., 2019 This thesis could be found at: - National Library of Vietnam - Library of Graduate University of Science and Technology.
1 INTRODUCTION 1. Rationale In the history of development, there are 4 types of anaerobic treatment techniques that have been applied: (1) The continuous stirred-tank reactor - CSTR, (2) Anaerobic contact - AC, (3) Up-flow Anaerobic Sludge Blanket – UASB, and (4) Expanded Granular Sludge Bed - EGSB and Internal Circulation - IC. In the past 40 years, many high-rate anaerobic techniques have been applied with different designs [2]. The references show that IC and EGSB system are the most advanced and efficient one, the IC technology has the highest loading rate and productivity in current techniques [3]. Therefore the growth rate of IC systems is very high. In 1997 the world had 32 IC systems, up to 2007 there were 2266 all over the world and by 2015 the estimated number of installed high-load anaerobic systems exceeded 4.000 [4]. An important feature of microbiological treatment techniques is mixing, increasing mixing make to increase of contact and reaction ability of microorganisms with pollutants, so increase the efficiency of metabolism. The UASB and EGSB techniques use the energy of generated gas and circulating flow (using circulating pump) to increase mixing ability. IC technology uses gas generated to circulate water. This is the advantage that makes ICs highly efficient while consuming less energy (because of no use of circulating pump). In order to study the fabrication of IC systems, the key point is to study the fabrication of re-circulating structures that requires no external energy (circulating pumps), using the process of drawing water with self-generated gas to circulate mixed mud and sewage.
2 Currently, UASB design guidelines are relatively common while water circulation techniques such as ICs are rarely published. For example, the latest monograph on waste water biotechnological treatment "Handbook of Biological Wastewater Treatment, 2012" [5] there are 14 examples of the UASB system design with detailed explanation of the UASB without any examples of advanced system of EGSB and IC types. Therefore, in order to promote IC system application in Vietnam, it is necessary to have researches to determine basic parameters for equipment design and manufacture. This thesis focuses on simulating the internal circulation process (gas pulling water) to determine: (i) the amount of water (QN) was pulled up by the amount of gas (QK); and (ii) mixing ability of generated gas and circulating water flow, from which to calculate the periodic structure in the IC system. This thesis also presents the results of fabrication experiment of IC model system at lab scale to determine the processing capacity of IC system when operating the system with pig-farming wastewater. 2. Research targets Study on application of the internal circulation anaerobic treatment system (IC) in the treatment of organic-rich wastewater, specifically: (i) Establishs the relationships between commonly used parameters (load, surge rate) and IC system design parameters (size of the riser, reaction area height). (ii) Determines the correlation between the load and processing capacity of the IC anaerobic system in case of swine wastewater.
3 (iii) Determines the mixing ability by generated gas in the system. (iv) Determine value parameters (equipment height, reaction area height, inside diameter, size of the riser) to serve the anaerobic IC design. 3. Research content Content 1: Experiment to determine the K ratio equal to the amount of water (QN) which was pulled up by the amount of gas (QK) depends on the riser cross section (S), submergence levels (H1), water discharge heigh (H2) at the constant viscosity and density solution Content 2: Create equations to represent the relationship of K with parameters: riser cross section (S), submergence depth (H1), water discharge height (H2) at the constant viscosity and density. Content 3: Fabrication and operation of the IC system with swine wastewater to determine the processing capacity of the IC system. Content 4: Calculation of mixing ability of generated gas and circulating water to determine IC system design parameters. CHAPTE I: REVIEW In the field of microbiology technology for wastewater treatment, kinetics of anaerobic decomposition reaction in particular and microbiological reactions in general follow Monod equation [12]: 𝑘.𝑆 𝑟𝑠𝑢 = 𝐾 𝑋 (1.1) 𝑆 +𝑆 Where: rsu = substrate utilization rate per unit of reactor volume, g/m3.d
4 k = maximum specific substrate utilization rate, g substrate/g microorganisms.d X = biomass (microorganism) concentration, g/m3 S = growth-limiting substrate concentration in solution, g/m3 Ks = half-velocity constant, substrate concentration at one-half the maximum specific substrate utilization rate, g/m3 According to the equation 1, the reaction rate (treatment) is proportional to maximum specific substrate utilization rate - k (in anaerobic treatment process is a Specific Methanogenic Activity (SMA) and microbial concentration (X) in the system. To decrease the volume of equipment, X must be increased. New reactive techniques that have high productivity must simultaneously solve the requirements: increase X but not lose microbiological and microorganisms with wastewater must be in good contact. New high- rate devices (AC, AF, UASB, FB, EGSB, IC, ...) will solve these problems. The Internal Circulation (IC) technique The IC technique started with a patent of Vellinga (1986) [54]. The basic composition of the IC system includes: water inlet distributor, internal circulation device, high-load reaction area at the bottom and low-load reaction area (deep cleaning) on the side above (Figure 1.4). The IC-type reactor is in the form of two overlapping UASB tanks, usually cylindrical with about 20 m of height, the ratio between height and diameter within the range of 2.3 – 8.
5 The structure of the input distributor has a conical shape, cyclic flow from top to down, entering the cone in a tangential way to form a vortex and mixing with the influent in the conical structure. Above the distributor is High-load reaction area with expanded bed of granular sludge. The high-load reaction area extends to the first gas separator, about 55-65% of the height of the water column and the bed of granular sludge can also occupy Figure 1. 4. Diagram of IC its full height (the first gas system principle [48] separator has about 2m of height). Most of the organic matter is metabolized in this chamber, the biogas produced is collected by the first phase separator and up through the tube 1 (riser). When the gas flows up, it pulls water and sludge from the high-load decomposition zone to the third liquid-gas separation chamber, the gas separates, microorganisms and water are brought back into the distributor. The effluent from the high-load area only contains a small amount of pollutants into the polishing area (upper chamber), this chamber acts as a secondary treatment system and the organic convert process be continue, gas generated is also collected by a second phase separator and also performs gas-lift process, like the first chamber At the first chamber of the IC, water raising velocity can reach over 20 m/h, when the granular sludge and wastewater contact
6 with the three phase separation at high speed, the gas separation efficiency will be higher. The highlight of the IC technique is at very high microbiological density, the flow rate is very high due to the contribution of internal circulation, combined with the high gas flow (due to high conversion rate), which enhances the mixing ability of the system. The internal circulation flow is self-regulating: high substrate concentration produces a high amount of gas, which entails high flow of the circulating flow, increases mixing ability and dilutes the influent. Thus, to enhance the mixing, different from the cases of AF, FB and EGSB (they must use a circulating pump), the stirring motivation in the IC system includes: (1) influent, (2) biogas produced by anaerobic digestion, (3) circulating water flow by airlift (when starting, it must use pump and then we have (4) liquid flow due to circulating pump. In the IC system, biogas generated by anaerobic digestion process (COD) is an important mixing motivation but the level of biogas generation depends on volumetric loading rate (VLR) and conversion ability of microbial [19] [55]. Water pulling mechanism by air flow is the operating principle of airlift reactor (ALR). ALR consists of a riser, a gas supply pipe into the water rise pipe and a down-comer. The generated gas is collected to the bottom of the tube submerged in water, when moving upward, it expands (due to reduced pressure and specific gravity) and causes the same effect on the water in the pipe, resulting in water flowing back up and pulling the surrounding water flow, like an airlift pump (airlift).
7 CHAPTER II: SUBJECTS, SCOPE AND RESEARCH METHODOLOGY 2.1. Subjects and scope of the research Research subjects Internal circulation techniques (IC), airlift models and swine wastewater. Research scope - Airlift model with submerge depth of the riser pipe of 2.85 m and water discharge height of 50 cm. - Anaerobic treatment of wastewater from wet-type pig farms with IC systems of a useful volume of 30 liters. Laboratory scale. 2.2. Research Methods 2.2.1. Experiment for K determination Figure 2: 1. Diagram of the experimental system of determining K The experiment was carried out by applying the determined quantity of air (QK) into bottom of the riser (T1), changing the factors:
8 (i) Submergence height - H1, (ii) water discharge height - H2, and (iii) inner diameter of the riser - d. The gas-water mixture is directed into the downcomer (T2) with a diameter of d' (d'> d), the exit gas along the open end above the T2, amount of water (QN) was pulled up by the gas in over a specified period of time which is collected in the water collection tank and quantified by the weighing method. 2.2.2. Set up the equation to represent the relationship between the design parameters of airlift system and the operating parameters - water flow and gas / water ratio This equation is established based on the data obtained from the experiment, combined with the equations and published data in computer documents and programs. First of all, based on conservation of energy law, in order to be able to carry out the process of pulling water, the energy of the gas (which is the potential energy of the gas - Etk) must be sufficient to carry out the following processes: - Push the water up to the H2 flushing height at the specified upward section (potential energy of water at altitude H2- Etn). - Create flow of gas-water mixture (kinetic energy of gas- water mixture in the riser pipe-Ed). - Energy loss due to friction of the flow with the upward pipe at the kinetic energy of the mixture - Ett. (Etn + Ed + Ett) = η.Etk (2.3) In which, E is represented by J/s. From the expressions for calculating Etn, Ed, Ett and Etk (with K = QN / QK) combine with the above equations and use the Excel program to set up the equation and find the value of K.
9 2.2.3. Method to find experimental models of statistics Through analysis of experimental conditions for determining K, we see that K is a function of QK, H1, H2 and d parameters. Gas velocity (vk) is determined by taking the flow of gas (Qk) divided by cross section pipe (S): Qk Qk Qk vk    (2.4)  .R  .( d 2 2 S / 4) Then, replace the two quantities QK and d with the gas velocity (vK), we have the K representing function as follows: K  f (vK , H 1, H 2 ) (2.5) Use the FORTRAN programming language to find experimental models. 2.2.4. Experimental treatment of pig-farming waste water Figure 2.2. Diagram of Experimental IC system for livestock wastewater treatment Experimental objectives: Determine the processing capacity of the IC system with NTCNL and the amount of gas generated when converting each substrate (in COD).
10 2.2.5. Calculate the capacity and mixing intensity in the reaction area from biogas generated 𝑝𝑏𝑖𝑜𝑔𝑎𝑠,ℎ𝑖 𝑃𝑏𝑖𝑜𝑔𝑎𝑠 = 𝑝𝑎′ 𝑉𝑏𝑖𝑜𝑔𝑎𝑠 𝑙𝑛 ( 𝑝𝑎′ ) (2.16 b) hi = H + a – (i – i/2) (2.17) P calculation diagram is as follows: Figure 2.5. Mixing components in IC system
11 RESULTS AND DISCUSSION 1. Result of determination K 1.1. Experimental results Accordingly, the amount of water (QN) collected by the amount of air (QK) put-in and K value (ratio of QN/QK) depends on the gas velocity (v) and submergence level (H1) and water discharge height (H2). Figure 3.6. Trend of changing values of K and QN (at H1 = 285 and 185 cm) - K increases when (i) gas velocity (vk = QK / S) decreases and/or (ii) H’ (ratio of H1/H2) increases (submergence level H1 increases or water discharge height H2 decreases); In this experiments, maximum value of K (Kmax = 7,2) achieved at H1 = 285 cm; H2 = 10 cm (H’= 28,5) and v = 291 m/h (Figure 3.6), minimum value of K (Kmin = 1,66) achieved at H1 = 185 cm, H2 = 50 cm (H’= 3,7) and v = 1456 m/h (Figure 3.6).
12 - The amount of water collected (QN) increases when (i) gas velocity increases; and/or (ii) H1 increases (or H2 decreases) - H' = H1 / H2 increases. - vk increases when QK increases, though K decreases but QN still increases, K decreases due to QK but the decrease speed is slower following the hyperbolic function. 1.2. The equation represents the relationship between the design parameters of airlift system and the operating parameters - water flow and gas/water ratio. Based on the energy conservation law to represent the relationship of the amount of water pulled up by the gas, depends on: (i) across section (S) of the upward pipe (T1); (ii) submergence depth H1; and (iii) water discharged height (H2) – function K(H1,H2,d1). Energy conservation equation: Etn + Ed + Ett - ηEtk = 0 (3.1) Etk is the potential energy of gas [88]:  gH E tk  Q k P a ln( 1  1 ) (3.2) Pa Etn is the potential energy of water: E tn  Q N  gH 2 (3.3) Ed is the kinetic energy of gas-water mixture in the pipe: Q N (Q N  Q K )  2 Ed  2 (3.5) 2S Ett is energy loss due to flow resistance [89]: H1  H E tt  ( 0 , 7   2 )Ed (3.6) d - 0,7 is the coefficient of resistance.
13 0 , 3164   0 , 25 (3.7) Re Re- Reynolds number [89]: Re = ρ.v.dtl/μ (3.8) dtl = 2.(S/π)0,5.(1-1/(K+1)0,5) (3.9) Replace the corresponding expressions above into equation (3.1). 2 Qk  gH  gH 2  0 , 85 2 K ( K  1 )   Pa ln( 1  1 ) S Pa (3.10) 0 , 75 H1  H K 1 1  0 ,1535    0 0 , 75 0 , 25 2 1 , 875 QK K( ) d S 4 K 1 1 Solving equation (3.10) is to find the hidden number K. The hidden number K is easy to find by graph method or using computer programs. Here, use the optimal analytical method (What-if Analysis function) by shifting the result (Goal-seek). The degree of confidence (suitability) of equation (3.10) is assessed by calculating the error percentage (K) between K value obtained from experiment (Ktn) and K value calculated from equation (3.10) at the same experimental condition: K tinh  K tn K  . 100 % (3.11) K tinh 1.3. Evaluate the relevance of the proposed equation Statistical results show that there are 254/270 experimental data with errors less than 10%. Thus, it can be seen that the proposed equation is consistent with the experimental results and is the basis for calculating K value depending on the gas velocity (v) the submergence level (H1), water discharge height (H2) and riser cross sections (S).
14 Comparing Ktinh with KTN (Table 3.18), we see that Ktinh is usually smaller than KTN, which proves that the real efficiency is higher (about 80%), so the design with 80% efficiency is relatively safe. 1.4. Evaluate the change of K when increasing the pipe cross-section In order to clarify the effect of the riser cross section, the experiment was conducted with the cross section of the T1 tube of 4.45 cm2 (2,16 times greater) and 15.76 cm2 (7,65 times greater) at H1 = 285 cm and H2 = 50 cm. Table 3. 1. The K value with the riser cross section S = 4.45 cm2 QK (L/min) 2 2.5 3 3.5 4 4.5 5 vk (m/h) 270 337 405 472 539 607 674 Ktn 3.60 3.60 3.70 3.66 3.40 3.29 3.12 Ktính 3.70 3.577 3.451 3.327 3.207 3.093 2.985 (K (%) 2.85 -0.63 -6.73 -9.02 -5.67 -5.95 -4.34 Table 3. 2. The K value with the riser cross section is S = 15,76 cm2 QK (L/min) 5 6 vk (m/h) 190 228 Ktn 4.40 4.00 Ktính 4.03 3.98 (K (%) -5.95 -4.34 These results show that it is possible to use equation (3.10) to calculate K for the cases of cross section increase. 1.5. Changing of K according to viscosity and density a. Changing of K when viscosity increases
15 Types of wastewater which are considered to be organic-rich is only a few tens of thousands of mg/L. The fact that the largest organic ingredient is carbohydrates. The assumption of wastewater is the sucrose solution. The content of 30 g/L corresponds to COD = (30*384)/342 = 33,7 g/L (33.700 mg/L). Calculation results show that the K value decreased insignificantly, such as K at H1 = 285 and H2 = 50 cm, K285-50 decreases almost linearly when the viscosity increases, the reduction is about 1; 1.8 and 3.8% corresponding to a viscosity increase of 8,2; 14,4 and 33,3% (or 30, 50 and 100 g sucrose). Figure 3: 7. The change of K Figure 3: 8. The decrease of K value at different viscosities value at different viscosities 3.1.4.2. Changing of K when the density of the solution increases In fact, in high rate anaerobic treatment tanks the density of sludge commonly 20-40 kg/m3 (maximum of 80 kg VSS/m3) and the density of active anaerobic sludge is at 1,00-1,05 g/mL [4], the maximum density of the anaerobic sludge and wastewater mixture calculated at 25oC about 999 kg/m3, compared to 997 kg/m3 of water, the difference is only 0,2%. Thus, the terms in equation (3.10) related to density are only about  0,2% difference.
16 With the density ρ = 1050, the K decrease range from 0,4 to 0,02% corresponding to vk increased from 291 to 1456 m/h. This deviation is not remarkable, it can be ignored and it is possible to use water instead of the reaction mixture in the next calculation. Figure 3: 9. The change of K Figure 3: 10. The decrease of K value when density increases value when density increases 1.6. Selection of performance models Use fitting technique in Excel to represent the trend of changing the value of K depending on vk, H1 and H2. Calculating K according to the representation functions, calculating the error between K obtained with Ktn, the result is the most suitable quadratic function. With the option of a quadratic function and using "FORTRAN PROGRAMMING LANGUAGE" to find "MATHEMATICAL MODEL" we obtain the following equation: K = 7,889 – 5,534.vk + 0,4.H1 – 13,597.H2 – 0,604.vk.H1 + 6,478.vk.H2 + 0,711.H1.H2 + 1,613.vk2 + 0,08.H12 + 3,786.H22. (3.19) The result of calculating K by equation (3.19) and the error level compared to experimental K (see appendix 3) shows that the average error for 253 results is 3.95% with R2 = 0.977. This result
17 shows that equation (3.19) can use for calculating the K in cases of vk, H1, H2 and the cross section pipe change. 2. The operating results of the IC system with swine wastewater 2.1. The relationship between productivity, efficiency and organic loading A synthesis of the experimental results for the relationship between productivity, efficiency and OLR was shown in Figure 3.16. and Figure 3.17. Figure 3: 16. CODin, CODout loading and treatment efficiency Figure 3: 17. The relationship between productivity and OLR
18 The results show that the system stable in range of total organic loading rate (OLRtotal) is 7 to 10,12 kg/m3/day, the highest efficiency reaches 82%, average at the stable stage reaches 75%. When the load exceeds 10 kg/m3/day, the efficiency and productivity values are reduced. The more OLR is the more dispersed of efficiency value and the regression value between OLR and efficiency is weaker, regression coefficient is quite low. 2.2. Flow of biogas in IC system The gas production efficiency increases gradually when the OLR increases, the maximum value is 64,6%, in stable stage the average value reaches 55%. The methane content in biogas also increased gradually when LOR increased, the average value of methane in biogas was 59%. Figure 3.23: Evolution of biogas flow generated in IC system 3. Calculation of mixing ability due to biogas Suppose the IC system has cross section 1m2; Case 1: equipment height 12 m; reaction area height of 8 m; Case 2: equipment height 20 m; reaction area height of 14 m;