Asian Institute of Technology School of Environment, Resources and Development Bangkok, Thailand August, 1996

Transcription

1 APPLICATION OF THE AIR BACKFLUSHING TECHNIQUE IN A MEMBRANE BIOREACTOR FOR SEPTIC WASTEWATER TREATMENT by Sudtida Pliankarom A thesis submitted in partial fulfillment of the requirements for degree of Master of Engineering. Examination committee Dr. C. Visvanathan (Chairman) Mrs. Samorn Muttamara Dr. Byung-Soo Yang Nationality Previous Degree Scholarship Donor Thai B.Sc. (Environmental Health Science) Mahidol University Bangkok, Thailand Government of Netherlands Asian Institute of Technology School of Environment, Resources and Development Bangkok, Thailand August, 1996

2 Acknowledgements The author wishes to expressed her profound gratitude, great appreciation and indeptness to her advisor Dr. C. Visvanathan for his valuable guidance, encouragement, support and sharing the knowledge throughout the research. Special thanks are extended to Mrs. Samorn Muttamara (Co-chairman) and Dr. Byung-Soo Yang for their valuable suggestions and guidance given for this study as members of the examination committee. The author also thankful to the staff and the friend of the Environmental Engineering Program for their prompt assistance and cooperation during this study. A very grateful acknowledgment is extended to the Government of Netherlands for providing the author with a scholarship to study the master program at AIT. The author wishes to express her sincere gratitude to Prof. R. Ben Aim of Department of Chemical Engineering, University de Technologic de Compiegne, France and Mr. Robert T. Wale of Memtec Ltd., Australia for providing membrane and giving valuable technical information. Finally the author express her profound gratitude to her parents for their strong encouragement and inspiration given to her.

3 Abstract In this study the possibility of application of air backflushing technique through hollow fiber microfiltration was investigated. The process employed direct solid-liquid separation by immersed two membrane modules with pore size of 0.2 µm directly in the activated sludge aeration tank of 80 L volume. This study was conducted with high concentration of activated sludge and divided into short-term and long-term experiments. In short term experimental runs, the optimum air backflushing and filtration cycle was investigated. 15 minutes filtration and 15 minutes air backflushing provided the best result in term of flux improvement and stability. Due to the membrane module stability limitation, the applied compressed air pressure of 1 bar was not sufficient to remove the clogging completely. However this cyclic operation provided higher flux stability compared to operation without air diffusion. In long-term experiments, the initial sludge concentration was 13,000 mg/l. Three different hydraulic retention times (HRT) of 26, 18 and 10.5 hour which corresponds to the permeate flux of 3.08, 4.44 and 7.62 L/m 2. h were investigated. Here, it was noted that the filtration pressure related to the MLSS concentration. Whereas the stable operation could be obtained at 26 and 18 hours. All experimental runs provided more than 90% removal of COD, BOD and TKN with final MLSS of 40,000 mg/l in the reactor. Although the operation with daily sludge draining (1.6 L/d), the MLVSS/MLSS values seem slightly decreased. However, such conditions could not effect significantly to the process performance in term of physical, chemical, biological and bacteriological qualities of membrane bioreactor effluent.

4 List of Abbreviations AS ASP BOD COD CST DO EA ED PHF Eff F-BOD F-COD F / M HRT Inf J k kd K L a K s MBR MF MLSS MLVSS NF NH 3 -N NO 2 -N NO 3 -N NTU Rd R g R g Rm Rm o Rm 1 Rm 2 RO R su SRT SS TKN T-N TP TS - Activated Sludge - Activated Sludge Process - Biochemical Oxygen Demand - Chemical Oxygen Demand - Capillary suction time - Dissolved Oxygen - Extended aeration - Electrodialysis - Polyethylene hollow fiber - Effluent - Filtered biochemical oxygen demand - Filtered chemical oxygen demand - Food / Microorganism ratio - Hydraulic retention time - Influent - Permeate flux - Maximum rate of substrate utilization per unit mass of microorganism - Endogenous decay coefficient - Overall gas transfer coefficientµ - Half- velocity constant (saturation constant) - Membrane bioreactor - Microfiltration - Mixed liquor suspended solids - Mixed liquor volatile suspended solids - Nanofiltration - Ammonia nitrogen - Nitrite nitrogen - Nitrate nitrogen - Naphelometric turbidity unit - Membrane resistance due to the deposition of solids - Rate of bacterial growth - Net rate of bacterial growth - Apparent membrane resistance - Initial membrane resistance - Membrane resistance after first cleaning - Membrane resistance after second cleaning - Reverse osmosis - Substrate utilization rate - Solids retention time - Suspended solids - Total kjedahl nitrogen - Total nitrogen - Total phosphate - Total solids

5 List of Abbreviations TVS - Total volatile solids UF - Ultrafiltration VSS - Volatile suspended solids Y - Sludge growth coefficient µ - Dynamic viscosity µ m - Maximum specific growth rate 15:15-15 minutes of filtration then 15 minutes of air diffusion 15:15* - 15 minutes of filtration then stop without sending air for 15 minutes

6 Table of Contents Chapter Title Page Title Page Acknowledgements Abstract Table of Contents List of Tables List of Figures Abbreviations i ii iii iv vi vii ix 1. Introduction 1 2. Literature Review 2.1 Fundamentals of Activated Sludge Fundamentals of Microfiltration Biological Nitrification Denitrification Phosphorrus Removal from Wastewater Microfiltration Membrane in Domestic Wastewater Treatment Application of Membrane Bioreactors in Domestic Wastewater Treatment Clogging Mechanisms in Microfiltration DecloggingTechniques Fundamentals of Gas transfer Applications of Gas Diffusion through Membrane Experimental Set Up 3.1 Measurement of Gas Transfer Efficiency of Aeration Units Measurment of Initial Membrane Resistance Short Term Experiments Long Term Experiments Analytical Methods Results and Discussion 4.1 Gas Transfer Efficiency of Aeration Units and Initial Membrane Resistance Short Term Experiments Long Term Experiments 38

7 Table of Contents Chapter Title Page 5. Conclusion and Recommendations 5.1 Conclusions Recomendations for Future Work 65 References 66 Appendix A 68 Gas Transfer Efficiency and Membrane Resistance Appendix B 76 Detail Results of Short Term Experiments Appendix C 87 Detail Results of Long Term Experiments Appendix D 98 Detail Results of Typical Coefficients of Activated Sludge Process for Domestic Wastewater Treatment Appendix E 110 Screening and Toxicity Methodololy for Measurement of Biomass Activity Appendix F 112 Detail Results of Nutrient Removal Appendix G 116 Detail Results of Activated Sludge Characteristics Appendix H 121 Detail Results of Membrane Cleaning Efficiency

8 List of Tables Table Title Page 3.1 Characteristics of Raw Septic wastewater Parameters Analyzed ( long tem experiments) Comparison of Performance of Different Suction Pressure with 15:15* Operation Mode and with Air Diffusion Comparison of Performance of Different Suction Pressure with 15:15 Operation Mode and 1 bar Compressed Air Comparison Performance of Different Mode of Operation with Suction Pressure at 7 kpa and 1 bar Compressed Air Comparison Performance of Different Compressed Air Diffusion with Suction Pressure at 7 kpa and 15:15 Operation Mode Kinetics of Acclimatized Sludge Growth Transmembrane Pressure of each Experimental Runs Average Value of Biological Solids Concentration of each Experimental Runs a Nitrogen Mass Balance at Steady State b Calculated Data for Nitrogen Mass Balance a Phosphate Mass Balance at Steady State b Calculated Data for Phosphate Mass Balance c Characteristic of total Phosphate in Sludge from Bioreactor Characteristics of Sludge from Conventional Activated Sludge and Membrane Bioreactor Process Microbiological Quality of Permeate and Effluent from Conventional Activated Sludge 4.12 Dynamics of Activated Sludge at Steady State for each Experimental Runs 58

9 List of Figures Figure Title Page 2.1 Dead-End Filtration and Crossflow Filtration Schematic of Pilot - Scale Membrane Bioreactor Module Backflushing with Gas Effect of Gas Backflushing during Wine Filtration Schematics of Two-Film Theory of Gas Transfer Membrane Module Used for All Experiments Experimental Set Up for Gas Transfer Efficiency a Schematic of Membrane Bioreactor Set Up b Actual Membrane Bioreactor Set Up Variation of Oxygen Concentration with Time when Using Ambient Air (stone diffusers) Variation of Oxygen Concentration with Time when Using Pure Oxygen (stone diffusers) Plot of Cs - Ct with Time when Using Ambient Air (stone diffusers) Plot of Cs - Ct with Time when Using Pure Oxygen (stone diffusers) Comparison of K L a (20) o C at Different Flow Rates for Ambient Air and Pure Oxygen (stone diffusers) K L a (20) o C at Different Pressure (membrane diffusers) Relationship between Permeate Flux of Clean Water and Transmembrane Pressure (membrane module 1) Relationship between Permeate Flux of Clean Water and Transmembrane Pressure (membrane module 2) Variation of Average Permeate Flux with Time at Each Hour for Different Transmembrane Pressure with 15:15* Operation Mode (* = without sending air) Comparison of Permeate Flux with Time for 40 kpa Suction Pressure between Operation with and without Air Diffusion Comparison of Flux with Different Suction Pressure Variation of Permeate Flux with Time for Long - Run Observation when using Transmembrane Pressure of 7 kpa Comparison of Permeate Flux with Time between 10:10 and 15:15 Operation Modes for 7 kpa Suction pressure and 1 bar Compressed Air Comparison of Permeate Flux with Time between 20:20, 25:25 and 30:30 Operation Modes for 7 kpa Suction pressure and 1 bar Compressed Air 36

10 List of Figures Figure Title Page 4.15 Variation of MLVSS and Effluent Filtered-COD with Time after Sludge Acclimatization a Variation of Transmembrane Pressure (suction pressure) with Time for Different Experimental Runs b Variation of Permeate Flux with Time for Different Experimental Runs Variation of Effluent Turbidity and DO with Time for Different Experimental Runs Variation of COD Concentration with Time for Different Experimental Runs a Variation of Biological Solids Concentration with Time for Different Experimental Runs b Accumulation of Inert Materials with Time for Different Experimental Runs Variation of F/M-ratio and BOD Concentration with Time for Different Experimental Runs Visual Characteristic of Raw Influent, Sludge in Bioreactor and Effluent Variation of TKN Concentration with Time for Different Experimental Runs Variation of NO 3 -N Concentration with Time for Different Experimental Runs Viation of Total Phosphate concentration with Time for Different Experimental Runs a Biological Flocs with Safanin-O at steady state of RUN 1, 200x b Biological Flocs with Safanin-O at steady state of RUN 2, 200x c Biological Flocs with Crystal Violet at steady state of RUN 3, 200x a Relationship between Flux of Clean Water and Transmembrane Pressure (membrane module 1) b Relationship between Flux of Clean Water and Transmembrane Pressure (membrane module 2) 63

11 Chapter 1 Introduction 1.1 General The most common and classical wastewater treatment process which has been used to treat domestic wastewater is the activated sludge process (ASP). In this system the organic and inorganic matters present in the suspended solid, colloidal and soluble forms can be removed up to 95%. However, there are some limitations in classical ASP when a high quality of effluent is required. In such situations, large secondary sedimentation tank is required to provide sufficient retention time. Moreover, there are various factors that must be concerned to reach good settling characteristic. Therefore, various types of combination between ASP and membrane unit have been studied and adapted to overcome these problems and to obtain good effluent quality. Membrane separation technology in water and wastewater treatment can be categorized into four classes according to the membrane, namely, reverse osmosis(ro), ultrafiltration (UF), microfiltration (MF) and electrodialysis (ED). UF and MF techniques are useful in removing macromolecule, colloids and suspended solids. Membrane separation technology has been introduced for solid/liquid separation in biological treatment system. The advantages of employing membrane separation are minimum sludge wastage by maintaining low F/M ratio, reducing plant size by maintaining higher biomass concentration in the reactor, and solid free effluent could be obtained. Complete retention condition could be maintained by operation without sludge wastage since the solid/liquid separation could be done regardless of sludge settleability. For domestic wastewater treatment, combined activated sludge/membrane filtration can provide a high degree of treatment in terms of organic oxidation and nitrogen removal. However, the power/energy consumption that has been reported is much higher than the value for conventional ASP. (Yamamoto,1989) As Yamamoto (1989) indicated, the process is not cost effective. The main reason for the high cost is due to the recirculation pump, which connects the main reactor with a membrane unit and maintain high crossflow velocity on the membrane surface to keep the flux undeclined. The solution for this has been investigated by direct membrane separation using hollow fiber in an activated sludge aeration tank which still give a stable operation and good quality of effluent. Considering the process performance, direct membrane separation in ASP with continuous suction operation caused severe clogging of the membrane module whenever transmembrane pressure is increased. Using the intermittent suction

12 operation enabled a stable flux to be maintained for suitable, particular conditions.(yamamoto et al., 1989) Cyclic operation with air diffusion has been investigated by Chiemchaisri (1990). Air backflushing technique was applied to achieved the recovery of permeate flux and net cumulative volume. However, increasing the pressure applied for air backflushing to achieve complete membrane cleaning may damage the membrane. 1.2 Objectives of the study 1. To investigate the possibility of using 0.2 µm-membrane pore size for effluent filtration and air diffusion purposes in alternative cycle. 2. To compare the effect of produced gas bubbles by using ambient air and pure oxygen by consideration of gas transfer coefficient. 3. To study the effect of operation cycle ( effluent filtration and air diffusion ) in membrane bioreactor to prolong the operational life of membrane bioreactor. 4. To find out the optimum operating condition for 0.2 µm-pore size of membrane. 5. To study the treatment efficiency and operational stability of the membrane bioreactor. 1.3 Scope of the study 1. This study was carried out in laboratory-scale. 2. Polyethylene hollow fiber (PHF) membranes of 0.2 µm pore size were used in this study. 3. Actual septic wastewater collected from septic tank of public apartment in the area of Pathumthani province was used as feed substrate. 4. COD, MLSS, MLVSS, Turbidity, DO, ph, Temperature, TKN, NO 3 - -N, NO 2 - -N and total phosphate were monitored regularly to observe the reactor temporal performance. In addition, the permeate flux and transmembrane pressure were also monitored to assess the reactor performance.

13 Chapter 2 Literature Review 2.1 Fundamentals of Activated Sludge Process The activated sludge process make use of the suspended biomass to stabilize, biochemically, organic waste in wastewater with the presence of oxygen. The aerobic condition is achieved by the use of diffused or mechanical aeration, which also serves to maintain the mixture called mixed liquor in completely mixed regime. After a specific period of time, the conversion of organic wastes to the more stabilized substances take place and provide the desired quality of water. Extended aeration is similar to the conventional activated sludge process with the exception of the operation in endogenous respiration of growth curve. The process operations prefer the low organic loading, long aeration time and low F/M ratios. Due to the stated operations, the sludge problems can be overcome in view of small amount of waste sludge produced which need to be carried and good sludge characteristic for dewatering unit. 2.2 Fundamentals of Microfiltration Microfiltration membranes are applied for separation of particles within the range of µm. High pressure driven force allows the passage of water through the membrane at the feed side and the tangential liquid flow promote the membrane cleaning at the inverse direction. Microfiltration membrane process is widely used in water and wastewater treatment, which the present pollutants contain diverse particle sizes of colloids and suspended solids. Due to larger membrane pore size, higher flux is obtained in microfiltration system compare to the RO and UF. However, often MF membrane process faces possible internal and external pore clogging due to colloidal fraction, which lead to significant flux reduction. This problem can be overcome by selecting appropriate membrane pore size and by using appropriate pretreatment techniques. The operational mode of microfiltration can be classified into two types as shown in Figure Dead-end or conventional filtration (Vigneswaran et al., 1991) 2. Crossflow filtration

14 Figure 2.1 Dead- End Filtration and Crossflow Filtration (Vigneswaran et al., 1991) In conventional filtration the flow direction perpendicular to the filter medium, while the crossflow filtration has tangential flow to the membrane, the feed is along the membrane surface and the permeate is perpendicular to the feed. Thus this system is known as cross-flow filtration. 2.3 Biological Nitrification Nitrification is the conversion of ammonia nitrogen (NH 4 + -N) and some organic nitrogen form to nitrate nitrogen (NO 3 - -N) with nitrite (NO 2 - -N) formation as an intermediate and is performed by either heterotrophic bacteria or autotrophic bacteria. However, the major nitrifying bacteria are the autotrophic species, Nitrosomonas and nitrobacter which are common in soil and aquatic ecosystems. They derive energy for growth from the oxidation of inorganic nitrogen compounds instead of oxidation of organic matter. The stoichiometric reaction of nitrification and assimilation become 55 NH CO O C 5 H 7 O 2 N + 54 NO H H 2 O (2.1) 400 NO O CO 2 + NH H 2 O C 5 H 7 O 2 N NO H + (2.2) It is seen that approximately 3.22 mg O 2 will be required for each mg of NH 4 + -N oxidized to NO 2 - -N, and 1.11 mg O 2 will be need for each mg of NO 2 - -N oxidized to of NO 3 - -N for a total of 4.33 mg O 2 mg of NH 4 + -N oxidized all the way to NO 3 - -N. It is generally accepted that the specific growth rate of Nitrobacter is higher than the growth rate of Nitrosomonas and hence there is no accumulation of nitrite in the process

15 and the growth rate of Nitrosomonas will control the overall reaction. (Medcalf & eddy, 1991) 2.4 Denitrification Denitrification is a biochemical reaction which involves the reduction of nitrate or nitrite, present in water, to gaseous nitrogen compounds such as nitrogen gas, nitrous and nitric oxides and is carried out by facultative heterotrophic bacteria under anoxic conditions. There are also certain autotrophic bacteria that denitrify using an inorganic energy source. The principal genera are Pseudomonas, Micrococcus, Achromabacter and Bacillus, which were reported as abundant in sewage. Denitrification offers a mechanism of not only removing nitrogen in a non-polluting form, but also oxidizing organic matters in the process. Thus the oxygen which has been supplied in nitrification can, in principle, be effectively recovered and reused in denitrification. Nitrate readily replaces oxygen as electron acceptor because the pathway for the transfer of electrons from the organic substrate to the final electron acceptor is similar, but the presence of dissolved oxygen acts as a strong inhibitor on denitrification as it prevents the formation of the enzyme necessary for the final electron transfer to nitrate. There are four conditions that are necessary for denitrification : (Medcalf & eddy, 1991) 1. Presence of nitrate 2. Absence of dissolved oxygen 3. Bacterial mass that can accept nitrate and oxygen as electron acceptor 4. Presence of a suitable electron donor ( energy source ) 2.5 Phosphorus Removal from Wastewater Chemical Phosphorus Removal Chemical P removal can be achieved by addition of cation, which will cause precipitation of phosphorus holding wastewater. Lime is less used now because it produces large quantity of sludge and alkaline effluent. Alum has also been used, however its active ph condition is slightly acidic. The optimum ph for activated sludge is neutral range so ph adjustment is then required prior to precipitate by alum. Metal precipitant, FeCl 3 or Fe(OH) 3, or coagulant was then selected on the performance of phosphorus removal in wastewater treatment. Jar test have to be conducted in order to determine which chemical and at what dosing level give optimum results. The stoichiometric of ferric salt was shown in the following equations. FeCl 3 Fe Cl - (2.4)

16 Fe +3 + PO4-3 FePO 4 (2.5) Phosphorus can be removed by chemical dosing at: 1. Primary settling tank 2. Activated-sludge aeration tank 3. Prior to secondary sedimentation tank 4. After secondary sedimentation tank Coagulation upstream from primary settling tank results substantially reduction in organic loads on secondary treatment unit because considerable proportion of solids and colloids together with some soluble material were removed. However, greater chemical usage is required and primary sludge production is increased significantly. It has been found that organically-bound phosphorus is not easily precipitated and therefore complete phosphorus removal might not be achieved. Chemical addition as a tertiary treatment is probably a valuable alternatives when high quality of effluent is required to achieved reliable total Phosphorus standards below 0.5 mg-p/l. However, addition of solids polishing facilities are then required in order to reduce solids loss to the effluent. Dosing directly into an aeration tank or prior to a secondary sedimentation tank is likely to be preferred in situations as: (Cooper et at., 1994) 1. It need less chemical 2. Organically-bound phosphorus is oxidized and precipitated in the aeration tank 3. Less excess sludge is produced Biological Phosphorus Removal The basic mechanisms is to create the alternative conditions of anaerobic and aerobic or oxic stages. Under the anaerobic conditions the growth of particular strains of bacteria such as Acinetobacter is selected. Energy uptake under these condition is gained by hydrolysis of polyphosphates stored in the cells. The hydrolyzed polyphosphates are then released out from the cell into the liquid as Orthophosphates. During the aerobic stage the soluble phosphorus is taken up and stored as polyphosphate in order to produce energy for their cells. The unit processes of biological phosphorus removal can be applied in different ways. Some of the processes include the anaerobic stage within the existing activated sludge process. This is called water-line phosphorus removal process. Other processes, particularly for PhoStrip, create an anaerobic stage outside the existing activated sludge plant where some part of recycled activated sludge is stripped of its phosphorus and then returned to the aerobic activated sludge plant to take up more

17 phosphorus. This is called the sludge-line phosphorus removal process. (Cooper et at., 1994) 2.6 Microfiltration Membrane in Domestic Wastewater Treatment Many researchers have studied the application of membrane technology in domestic wastewater treatment. In conventional treatment, membranes can be inserted at three locations; namely: (Vigneswaran, 1991) 1. After the primary sedimentation 2. In the activated sludge tank 3. After the secondary sedimentation, in the tertiary treatment with or without pretreatment. Combination of membrane together with the activated sludge process is used for separating of liquid from solids. This process performance provide attractive results such as: 1. SS are totally eliminated through membrane separation 2. Settleability of the sludge has no effect on the quality of the treated water 3. Adequate sludge retention time (SRT) which allow the proliferation of low growth rate microorganisms such as nitrifying bacteria 4. Maintain of high concentrations which over all activities level can be raised due to high concentration of the dispersed microorganism maintaining in the bioreactor as long as possible, high concentrations create a favorable environment for endogenous thereby ensuring high treatment efficiency 5. This method can produce bacteria and virus free treated water. Because of all solids are retained in the bioreactor and long SRT, dissolved organic substances with low molecular weights can be taken up, broken down and gasified by microorganisms or converted to polymers as bacterial cell. Thereby, improving the quality of treated water could be achieved. The retained polymeric substances can be biodegraded, then provide less accumulation of substances within the treatment process. (Yamamoto, 1994) 2.7 Application of Membrane Bioreactors in Domestic Wastewater Treatment Talat (1988) investigated hollow fiber microfiltration for solid-liquid separation from the aeration tank of an activated sludge process. The variation of 3 parameters of pore size (0.1, 0.2 and 0.45µm), MLSS in the reactor (5000, 10,000 and 20,000 mg/l) and

18 suction pressure (1.36, 2.72, and 7.5 m head of water) were conducted during a short term experiment in order to find out the suitable mode of operation for long term experiment. The short term result shown that at 10:10 intermittent operation provided the best condition for the stable flux. In long term experiments, membrane modules were regulated at constant flux of 1.5, 2.5 and 3.5 L/m 2.h and the corresponding increase in suction pressure was recorded. Volumetric organic loading of 3 kg COD/m 3.d shown critical condition toward the separation process. However, loading of 2 kg COD/m 3.d appeared to provide most suitable condition since the COD removal efficiency was upto 95-97%. Nitrification and denitrification was achieved 100% and 30-40% respectively. Under similar operating conditions, the removal efficiency were independent of the membrane pore size. The 0.45 µm membranes which operated at lower suction pressure than the 0.1 µm membrane under similar operating conditions can provide the highest flux (3.5 L/m 2.h or m 3 /m 2.d) and similar in clogging characteristic to others. Low value of Y, k d and F/M ratio showed very small sludge production. The 100% removal of fecal coliform can be achieved by using 0.1 and 0.45µm membrane filters. Yamamoto et al. (1989) investigated direct membrane separation by using 0.1µm pore size hollow fibers which was immersed in the aeration tank, regardless of using secondary sedimentation tank for solid/liquid separation. The treated water was filtered by suction with various operation modes. Continuous suction exhibited dramatic flux decrease as well as high MLSS together with high pressure difference. Intermittent suction at low pressure (13 kpa) provided good result in order to prevent the unrecoverable clogging and to prolong the operating time without cleaning. COD removal can be achieved higher than 95% while the nitrogen removal can be reached up to 60% by investigating the intermittent aeration mode. Chiemchaisri (1990) investigated an activated sludge using 0.1µm hollow fiber membrane modules for solid liquid separation. This study was conducted to treat low strength wastewater from AIT domestic wastewater. Comparison of the membrane bioreactor under different operating conditions, such as non-aerated and aerated, with different initial hydraulic retention time (HRT) of 1, 3 and 6 h which provided corresponding permeate flux of 4.17, 1.38 and 0.7 L/m 2.h was studied. The process was operated at 10:10 intermittent time. From the experiment, it can be seen that the non-aerated bioreactor has an advantage over the aerated condition at an initial HRT of 3 and 6 h, since lower energy consumption was required while giving similar effluent quality and process stability. However, at lower HRT of 1 hour (or higher permeate flux, 4.17 L/m 2.h) aeration is required in order to prevent membrane clogging. This highest flux of 4.17 L/m 2.h seem to be a critical value since creating severe clogging condition. At lower flux, no clogging was observed under non-aerated and aerated conditions. The quality of permeate in term of COD was independent of the low volumetric organic loading at the range of kg COD/m 3.d. Because of the long solid retention time (SRT), the process was stable and steady, COD removal efficiency was similar in every experimental conditions.

19 The performance of 0.03 µm pore size with 9 m 2 surface area of hollow fiber membrane was also investigated in pilot-scale unit. Two hollow fiber membranes module was immersed in an aeration tank which feed with diurnally AIT domestic wastewater. The suction pump was used at 10:10 minute intermittent operation to extract the permeate through the membrane as shown in Figure 2.2. For jet aeration, the effect of jet aeration period (1/2 and 1 h) and jet aeration pattern 15 minutes for two times a day and 30 minutes for once a day was investigated. The jet aeration flow rate used was 20 L/min. In the bioreactor consist of 2 zone: aerobic and anaerobic on top and at bottom of the reactor respectively. consequently, low MLSS in aerobic zone of which it could reduce the clogging problem of membrane. Figure 2.2 Schematic of Pilot-Scale Membrane Bioreactor (Chiemchaisri, 1990) The mean hydraulic retention time (HRT) was determined after the permeate flux reached steady state. At the flux 4.17 L/m 2.h has reached, average HRT of 1 day was obtained under diurnal varied loading. Diurnal variation in loading play a minor role in the nitrification process since more than 80% nitrification can be observed throughout the experiment. The MLSS in the bioreactor was affected by the aeration flow rate and optimum air flow rate in this experiment was taken as 7.5 L./min. which provided sufficient oxygen for the microorganisms and maintain low MLSS in the aerobic zone. Maythanukhraw (1995) applied 0.1 µm hollow fiber membrane directly in the reactor for solid-liquid separation to treat domestic wastewater from AIT campus, Bangkok. For short term experiments, the effect of transmembrane pressures, intermittent mode of operation and duration of air diffusion were investigated to find out the optimum conditions which corresponding to high and constant flux obtained. Variation of transmembrane pressures were studied with values of 13.3, 21.3, 32.0 and 41.0 kpa. It was found that using 13 kpa transmembrane pressure was a limiting pressure for all experiments. The different of

20 operation modes were studied by varied the duration of effluent filtration and air diffusion: 5:5, 10:10, 15:15, 30:30, 60:60 and 15:15* (15* = 15 minute without sending air). The results shown that at the operational mode of 15:15 provided the best results. Although cyclic operation with air diffusion could not completely remove the clogging, air backflush technique in this mode of experiment could improve the flux by up to 371 % compared to the continuous operation. At 15:15 operation mode, further experimental run was continued to find out the optimum air diffusion duration on that mode. By varied the values of 15:5, 15:10 and 15:15, the best performance can be observed under 15:10 mode. Considering both recovery of permeate flux and net cumulative flux. To study the process performance of long term experiment under the optimum conditions obtained from short term experiment, the effects of HRT at 12, 6 and 3 hr. attributed to variation of volumetric organic loading. At 3 hr.- HRT provided fluctuation of volumetric organic loading in range of kg.cod/m 3 -d. Transmembrane pressure was increased according to the cake formation on the membrane. So, after the long period of experimental runs the steep increasing in transmembrane pressure can be observed even operated under air backflushing. The periodically chemical membrane cleaning was needed in order to recover the permeate flux. The permeate flux contains good quality in term of the very low SS. Since the infinite SRT was operated, more than 90% of COD with effluent concentration below 20 mg/l was achieved in all runs. The TKN removal was more than 90 %, and total phosphate removal around 50 % in all experimental runs. The MLVSS/MLSS in the bioreactor was in the order of 20-30%. Inorganic mass balance calculation indicated a steady accumulation within the reactor. The lower fraction of active microorganisms in the bioreactor did not show any significant effects on the process efficiency. Nevertheless, it is anticipated that in longer run it might affect the process, thus it is advisable to have periodic sludge draining. To use the membrane as an air diffusers, the compressed air pressure should be high enough to produce steady stream of micro-air bubbles according to the bubble point concept. One way to overcome this problem was to use relatively large pore size membranes. The membrane cleaning process which was adopted in this study was found to be adequate to remove mainly the external membrane resistance. It is necessary to have chemical cleaning procedure for complete elimination of internal and external resistances, which mainly caused by the macromolecular adsorption. Longer air diffusion will improve the recovery of permeate flux. However, by considering both the recovery of permeate flux and net cumulative flux 15:10 operational mode gave better results than the 15: Clogging Mechanisms in Microfiltration The following four different types of clogging mechanism are observed in a filtration system: 1. Complete blocking occurs when particles plug the capillaries;

21 2. Cake filtration involves the formation of a porous layer on the membrane surface which poses an additional resistance; 3. Standard blocking occurs when solids adhere to the walls of the capillaries which reducing their internal diameter. Standard blocking was observed to be the most common in the absence of cake filtration. 4. The forth mechanisms was called intermediate blocking because of the rate of blocking falls between cake filtration and standard blocking. 2.9 Declogging Techniques In microfiltration system, particle deposition and internal clogging cause major operational problems in membrane filtration. These depositions cause permeate flux reduction in addition to decrease membrane life span. There are two simple ideas of declogging technique which are described as followed : 1. prevent particles reaching the membrane surface or 2. flush the deposited out Backflushing technique is used in order to achieved higher membrane process efficiency. The rate of permeate flux would be increased by backflushing. A high pressure of air is applied from the permeate side in order to removed the deposits out of the membrane surface. For gas backflushing, gas is brought to pressure in the lumens from permeate side and then explodes through the membrane wall whereby the boundary layer is released and can easily be transported away as shown in Figure 2.3. This results in a very efficient cleaning of the membrane. The effects of backflushing with gas in permeate flux is shown in Figure 2.4

22 Figure 2.3 Module Backflushing with Gas ( Peters & Pederson, 1990) Figure 2.4 Effect of Gas Backflushing during Wine Filtration (Vigneswaran et al., 1991) 2.10 Fundamentals of Gas Transfer Gas transfer is defined as the process by which gas is transferred from gas phase to liquid phase. Oxygen transfer in the biological treatment of wastewater is the most common application in field of wastewater treatment. Due to the low solubility of oxygen, normal surface air-water interfaces can not provide sufficient oxygen. To satisfy the requirement of aerobic waste treatment, aeration devices are used to create additional gasliquid interface. The rate of molecular diffusion of a dissolved gas in the liquid depends on the characteristics of the gas and the liquid, temperature, concentration gradient and the crosssectional area across which diffusion occurs. Equation (2.5) is used to explain Figure 2.5. r m = Kg.A (Cs-C) (2.5) where r m = rate of mass transfer ( mg/s.) K g = coefficient of diffusion for gas (L/m 2.s.) A = area through which gas is diffusing (m 2 ) Cs = saturation concentration of gas in solution (mg/l)

23 C = concentration of gas in solution. (mg/l) Under the conditions of mass transfer encountered in the field r m = V.dC/dt (2.6) So, Equation (2.6) can be written as : r c = dc/dt = K g (A/V) (Cs-C) (2.7) where K g (A/V) = K L a (2.8) Therefore Equation (2.7) can be written as : r c = dc/dt = K L a (Cs-C) (2.9) where r c = change in concentration, mg/l.s K L a = overall mass-transfer coefficient, s -1 Cs = saturation concentration of gas in solution, mg/l C = concentration of gas in solution, mg/l V = volume of gas (L) Equation (2.9) could be modified to the practical form which is presented as follow : Log (Cs-C L ) = Log (Cs-C 0 ) - (K L a/2.3)* t (2.10) where, C 0 = initial concentration of gas in liquid phase C L = concentration of gas in liquid phase For a given volume of water being aerated, oxygen-transfer can be evaluated on the basis of the quantity of oxygen transferred per unit of air, which is introduced to the water with equivalent conditions. (temperature and chemical composition of the water, depth at which the air is introduced, etc.)

24 To measure oxygen transfer in clean water, the accepted testing procedure involves the removal of oxygen (DO) from a known volume of water by addition of sodium sulfite followed by reoxygenation near the saturation level belonging to the water temperature. The DO of the water volume is monitored during the reaeration period by measuring DO concentration at several different points. The data obtained were then analyzed to estimate the apparent volumetric mass-transfer coefficient, K L a. These estimates of various point of DO are adjusted to standard conditions. Figure 2.5 Schematic of Two-Film Theory of Gas Transfer ( Metcalf & eddy, 1991) 2.11 Applications of Gas Diffusion through Membrane Semmems et al. (1991) developed a bubbleless hollow-fiber membrane aerator and tested for oxygenation of water. The aerator houses a bundle of sealed, hollow, gas permeable fibers that are filled with pure oxygen under pressure. By considering Equation (2.9) for mass transfer; raising the oxygen transfer efficiency could be accomplished by increasing K L a and Cs values. Eventhough, practically the external surface area of membrane is fixed. Therefore large K L a can be achieved with the action of elongated stationary bubbles of oxygen. Very high value of Cs can also achieved if specially coated membrane are used, of which it can be operated at pressures up to 60 psi with pure oxygen. At temperature of 10 o C and a flow rate of 75 gpm. through the pipe, dissolved oxygen of water is ranged from mg/l Pierre et al. (1988) investigated bench-scale experiments using both dense polymer membrane and porous membrane as an aeration units. Bubble-free aeration using membranes has potential applications for wastewater treatment when conventional bubble aeration gives unsatisfactory results, such as toxic volatile organic compound stripping or foam production. In order to find out the design parameters of membrane aerators, both types of membrane were immersed directly in well-mixed biological reactor.

25 Three systems which were studied for these experiments based on different settings of design parameters such as: 1. Specific oxygenation capacity 2. Mass transfer characteristics of the membranes 3. Type of gas and 4. Operating gas pressure Three operation modes consist of A, B and C were conducted in order to define the essential design parameters for membrane aerator. Cases A and B used 5 and 9 bar. Case C was evaluated by using pressurized industrial oxygen. The mass transfer analysis indicated that the high oxygen flux were based on surface area of the membrane. The optimal value of 38.3 m 2 -membrane per m 3 -aeration basin was suggested to meet the aeration requirements of conventional bioreactor (100 g. O 2 / m 3.h) while the high- rate bioreactor required 213 m 2 / m 3. The fraction of oxygen transferred is a design choice. The value of 0.8 was much higher than that possible with conventional diffused or mechanical aeration. As the fraction of oxygen transferred is increased, however, the average driving force decreases. Limitations of membrane aerator were due to the high capital cost according to the membranes. Furthermore, the membrane themselves seem to represent as an additional resistance to oxygen transfer, which could be translated into high energy cost. However, it is suggested that applications should be developed with industrial oxygen in order to reduce the requirement of membranes. Tariq & Semmems (1992) studied the mass transfer in a various pore diameter of hollow fiber membrane aerator. Individually-sealed hollow fibers were filled with oxygen and immersed in a flowing stream of water. Three experiments have been checked by measuring the pressure drop, gas flow velocity and gas composition along the length of the fibers. Pressure drop was measured according to the difference between inlet and outlet. The pressure drop due to friction across the 362 cm length was negligible. By calculating, the optimum operating pressure was below 1-3 psi. Gas flow velocity inside the fiber depend on the mass transfer coefficient. The gas flow velocity was ranged from cm/sec to 0.03 cm/sec. The decrease of oxygen partial pressure inside the fiber was observed along the fiber length. Better oxygen transfer could be achieved by pumping oxygen continuously through the hollow fibers in addition to removing accumulated nitrogen due to the back diffusion from outside to inside the fibers. Feeding of pure oxygen encourages nitrogen enter the fiber. Since partial pressure of nitrogen within the fiber is less than external partial pressure of approximately 0.79 atm, back diffusion along the fiber occurred. However, the nitrogen back diffusion rate dose not increase significantly when operated with high water flow velocity outside the fiber and high oxygen feed flow rate inside the fiber in order to decrease the nitrogen concentration gradient and increase oxygen concentration gradient between outside and inside the fiber respectively.

26 Chapter 3 Experimental Set Up The experiments that were carried out in this study can be classified into three parts as follows: 1. Gas transfer efficiency of aeration units and measurement of initial membrane resistance 2. Short term experiments 3. Long term experiments 3.1 Measurement of Gas Transfer Efficiency of Aeration units Materials Used - Ordinary ceramic porous diffusers - Micro filtration Membrane For this study, a microporous hollow fiber membrane was used. It is produced from high density polyethylene with the pore size of 0.2 µm. Each membrane module was assembled in plastic air tight cap to ensure the capability of operation under vacuum pressure as shown in Figure Clean water Tap water was prefiltered by using cartridge ultra-filter to produce clean water for this test. - Chemicals : i) Sodium sulfite ( Na 2 SO 3 ) : 60 mg/l ii) Cobaltous chloride ( CoCl 2 ) : 0.2 mg/l - Dissolved oxygen probe One probe was used for measuring dissolved oxygen concentration. The sampling points were at the head, middle and at the end of reactor. The position of each point was at the middle of water depth. - Turbine mixer One turbine mixer was used for promoting turbulence and homogenous mixture. Speed with value of 50 rpm. was used for this test Experimental Set up

27 Figure 3.2 presents the experimental set-up. The porous diffusers and membrane are immersed separately in acrylic-rectangular reactors which held a working volume of 80 L. The reactor was filled with clean water, and the increase in oxygen concentration was measured by dissolved oxygen probe. Turbine mixer was used to promote turbulence. Figure 3.1 Membrane Module Used for All Experiments Figure 3.2 Experimental Set up for Gas Transfer Efficiency Process Operation

28 The effect of pressure was studied in batch operation in order to define the appropriate K L a. To compare the transfer efficiency of two aeration devices, the membrane modules and ordinary air diffusers, were operated as shown in Figure 3.1. The testing procedure began with the removal of oxygen from water by addition of 60 mg/l sodium sulfite (Na 2 SO 3 ) with cobalt (CoCl 2 ) as a catalyst. (Pierre, 1989) The increase in oxygen concentration was measured during aeration under specified pressure, and the overall transfer coefficient was calculated from Equation. (2.10) log (C S -C L ) = log (C S - C 0 ) - (K L a /2.3)* t (2.10) when, C 0 = initial concentration of gas in liquid phase C L = concentration of gas in liquid phase The gas, which was sent to the testing unit, come from purified ambient air and pure oxygen. Dissolved oxygen was measured by using dissolved oxygen probe. Three sampling points are at the middle of water depth along the tank; head, middle and end. DO and water temperature were recorded every three minute intervals until the dissolved oxygen reached a constant level. The pressure which were applied in this study were 0.2, 0.4, 0.6, 0.8 and 1.0 bar. The maximum K L a were then indicated the appropriate operating pressure for air diffusion which was used for short term and long term experiments. The effect of gas flow velocity which was sent to aeration devices was studied. By varying the velocity of 9, 14 and 18 L/min., the calculated maximum K L a then indicated the optimum velocity which was used for short term and long term experiments. 3.2 Measurement of Initial Membrane Resistance Material Used - Membrane Two microporous hollow fiber membranes produced from high density polyethylene with the pore size of 0.2 µm was used in this study. Each membrane module is assembled in plastic air tight cap to ensure the capability of operation under vacuum pressure Experimental Set up

29 The measurement of initial membrane resistance was conducted by immersing membranes in the rectangular reactor. Clean water was fed to the reactor. The speed controlled roller pump was used for extracting the permeate. The transmembrane pressure was measured by mercury-filled manometer and the filtered water was recorded and returned to the reactor to keep the volume constant during the experiment Process Operation The relationship between the flux and transmembrane pressure is given in the following equation : J = δp / µr m (3.1) where, J = flux (L/m 2 -h.) δ = transmembrane pressure (kn./m 2 ) µ = viscosity (kn..s /m 2 ) R m = apparent membrane resistance = Rm o + R d where, Rm o R d = initial membrane resistance = membrane resistance due to the deposition of solids The modified equation to find the initial membrane resistance, when clean water is used, is: δp = µ.rm o.j + δp o (3.2) effect. δp o = initial transmembrane pressure required to over come the air blocking So, by modifying the transmembrane pressure and measuring the permeate flux, the value of Rm o could be determined. 3.3 Short Term Experiments Materials Used

30 - Membranes Microporous hollow fiber membranes produced from high density polyethylene with the pore size of 0.2 µm were used in this study. The properties of the membrane was similar to that used in measurement of membrane resistance. Each membrane module is assembled in plastic air tight cap to ensure the capability of operation under vacuum pressure application. - Feed substrate Glucose solution and tap water were used for concentrating or diluting the wastewater respectively to the desired COD concentration. - Biomass culture The seed microorganisms which was used in these experiments was obtained from the acclimatization of biomass which initially exist in septic wastewater Experimental Set-up - Acclimatization unit Acclimatization procedure of these biomass to raw septic wastewater was properly conducted by daily fill and draw operation before being placed in the reactor. Glucose solution was fed in order to concentrate biomass mixture in the acclimatization unit. By considering the initial characteristics of raw influent septic wastewater which presents in Table 3.1, glucose solution was prepared and fed daily in proportion to maintain COD concentration around 5,000 mg/l. Stock solution of 98.2 g-glucose/l is equivalent to 100 g- COD/L. Table 3.1 Characteristics of raw septic wastewater Parameters Values Units 1. Settled solid 2. Suspended solids 3. COD 4. Filtered COD 5. BOD 6. Filtered BOD 7. Total nitrogen 8. Total phosphate 9. ph 700 2,200-3,500 3,500-5, ml/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l - Membrane bioreactor The schematic diagram of the experimental set up was similar as presented in Figure 3.2. Membranes were immersed directly in the reactor of activated sludge system.