Forward osmosis membrane bioreactor for water reuse

Forward Osmosis Membrane Bioreactor for Water Reuse BY ZHANG JUNYOU (B Eng Wuhan Univ.) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENT The author would like to express his deepest thanks to the following persons for his master degree study: Associated Professor Ng How Yong for the invaluable guidance, patience and understanding throughout the entire research project Mr S.Chandra, Miss Lee Leng Leng and Miss Tan Xiaolan for guidance and helps during rector systems construction, chemical purchase, and sample analysis This research would not have been possible without their kind assistance Mr Tan Chien Hsiang and Dr Duan Wei for the advice and cooperation in the FO-MBR research With their helps, the research work was carried out more easily and efficiently All my family members for their constant support and encouragement from the beginning of the study i Contents ACKNOWLEDGEMENT i SUMMARY vi NOMENCLATURE x LIST OF FIGURES xi LIST OF PLATES xiii LIST OF TABLES xiv CHAPTER ONE INTRODUCTION 1.1 Background 1.2 Objectives and Scope of the Study 1.3 Outline of Thesis 2.1 Membrane separation technology 2.1.1 Introduction 2.1.2 Membrane development history 2.1.3 Membrane types 2.1.4 Membrane fouling issues 13 2.1.5 Membrane market 14 2.2 Forward osmosis 15 2.2.1 Introduction 15 2.2.2 FO key consideration – draw selection 17 2.2.3 FO applications 22 2.3 2.3.1 Membrane Bioreactor 23 Introduction 23 ii 2.3.2 MBR Fouling issues 27 2.3.3 Fouling effects 29 2.3.4 Fouling factors 30 2.3.5 Fouling controls 32 2.4 Forward Osmosis Membrane Bioreactor 35 2.4.1 Introduction 35 2.4.2 Advantages and challenges 36 2.4.3 Current research progress on FO-MBR and needs 39 CHAPTER THREE 43 MATERIALS AND METHODS 43 3.1 Introduction 43 3.2 Experiment part 1: FO membrane module selection 44 3.2.1 Experimental set-up 44 3.2.2 Membrane material 47 3.2.3 Operations 47 3.3 Experiment part 2: Draw selection 48 3.3.1 Draw selection – FO tests 48 3.3.2 Draw selection – NF tests 50 3.4 Experiment part 3: Laboratory-scale FO-MBRs 54 3.4.1 Experimental set-up and operating conditions 54 3.4.2 Membrane material 56 3.5 Measurements and Analysis Methods 58 3.5.1 Reverse salt flux 58 3.5.2 Water flux 59 3.5.3 Conductivity Analysis 60 3.5.4 Salt rejection 61 iii 3.5.5 Soluble microbial product and excellular polymeric substance quantification 62 3.5.6 Suspended solids, volatile suspended solids, total dissolved solids, chemical oxygen demand, measurements 63 3.5.7 Total nitrogen and total organic carbon 63 3.5.8 Scanning electron microscope and energy-dispersive X-ray spectroscopy observation 63 CHAPTER FOUR 64 RESULTS AND DISCUSSIONS 64 4.1 Introduction 64 4.2 Results on FO membrane module selection 65 4.2.1 Comparison of water fluxes of the three module designs 65 4.2.2 Comparison of reverse salt flux of the three module designs 68 4.2.3 Conclusion of FO membrane module selection 70 4.3 Draw selection for FO-MBR 72 4.3.1 Draw selection- FO tests 72 4.3.2 Draw selection- NF tests 80 4.3.3 Draw selection conclusion 86 4.4 Laboratory-scale FO-MBR experiments 87 4.4.1 Phase 1: FO-MBR without backwash schemes (39 days) 87 4.4.2 Phase 2: FO-MBR with a weekly backwash and monthly chemical cleaning schemes (120days)……………………………………………………………………………………………………………………………….…91 4.4.3 Conclusion of the laboratory-scale FO-MBR study 107 CHAPTER FIVE 109 CONCLUSION AND RECOMMENDATIONS 109 5.1 Conclusion 109 5.2 Recommendations 110 iv LIST OF PUBLICATIONS 112 REFERENCES 113 v SUMMARY Water is becoming increasingly important as the population over the world continues to grow, which leads to the increasing demand of portable water In addition, the pollution of water is causing the shortage of fresh water suitable for consumption Thus, it is necessary to reclaim water from used water or wastewater to overcome the water shortage problem through advanced technologies such as reverse osmosis process and membrane bioreactor Currently, membrane technology is preferred for wastewater reclamation because of its high contaminant rejection and water productivity Forward osmosis membrane bioreactor (FO-MBR) is a combination of forward osmosis (FO) and membrane bioreactor (MBR) to treat wastewater It requires lower energy compared to the conventional MBR With a nanofiltration (NF) process as the reconcentration process for the draw solution from the FO-MBR, draw solution can be reused in the FO process while clean water is being produced A suitable draw solution is one of the key factors in determining the efficiency of the process as it affects the water productivity of the system and determines the suitability of the process for producing freshwater from the diluted draw solution and reusing the draw solution in the FO process In addition, the reverse diffusion of the draw solute from the draw solution across the FO membrane back into the mixed liquor also has an impact on the biological process In this study, an optimal FO membrane module design was investigated before a suitable draw solution was selected, as it could maximize the effective membrane area for draw solution to transport water Thereafter, a suitable draw solution was selected with this module design and used in the wastewater treatment process to investigate the performance and fouling mechanism of the FO-MBR vi Three FO membrane modules were chosen to study the water fluxes and reverse salt fluxes in the FO process From the results of water fluxes, it can be concluded that the 6-chamber FO membrane module was able to achieve highest water flux among the three modules The FO membrane module with 4-chamber had a lower water flux which was due to the less effective membrane area because it had a shorter draw travel path as compared with the 6-chamber module The 1-chamber module had a large volume of dead zone, which suggested that there was a large area of membrane not being utilized for water diffusion From the results of the reverse salt flux, it was observed that the reverse salt fluxes generated by the two draw solutions individually with the different FO membrane modules were all lower than g m-2 h-1 This was because the FO membrane was a highly selective membrane which was able to retain almost all the solutes with high rejection rates The 6-chamber FO membrane module was a suitable design for the FO-MBR as it was able to generate a high water flux with a large effective membrane area FO membrane module design plays an important role in transporting water through optimizing the effective membrane area and therefore improving the water productivity Very limited studies were conducted on FO module design Thus, this study provides information on the effect of FO module design on water productivity Draw solution serves as a key role in the FO process owing to the driving force, known as osmotic pressure, it produces With a variety of draw solutions, various osmotic pressures as well as reverse salt fluxes are produced in the FO process Hence, it is necessary to choose a suitable draw solution for FO-MBR Draw solutions were studied through comparing the water fluxes and reverse salt fluxes in the FO process, vii and water fluxes and rejections in the NF process From the results of the FO tests, it was found that MgCl2 and the mixture of Na2SO4 and MgCl2 as draw solutions were able to achieved high water fluxes; while Na2SO4 and the mixture of MgSO4 and MgCl2 as draw solutions had relatively lower water fluxes However, the reverse salt fluxes of the two mixed salts as draw solutions were higher than all the draw solutions with single salt Na2SO4 had a lower reverse flux than MgCl2 The water flux produced by MgSO4 as draw solution was too low, although the reverse flux was also low From the results of the NF tests, MgSO4 was found to have the highest rejection with a fairly high water flux Na2SO4 also had a relatively high salt rejection while the water flux was similar to the mixture of MgSO4 and MgCl2 However, the rejection of the mixed solution of MgSO4 and MgCl2 was lower than that of Na2SO4 Mixed solution of Na2SO4 and MgCl2 as a draw solution achieved the highest water flux yet the rejection was the lowest Therefore, Na2SO4 was considered as the most suitable draw solution for the FO process in terms of high water flux, low reverse salt diffusion and high rejection A number of studies on draw solution selection were conducted by researchers However, there is no best draw solution that is applicable to all the processes Each process requires different draw solution to maximize its performance For this study, the FO process was coupled with NF process as the posttreatment, thus Na2SO4 was the most suitable draw solution for the process in terms of water flux and permeate quality With the 6-chamber FO membrane module and Na2SO4 as the draw solution, a laboratory-scale low fouling FO-MBR system coupled with NF as a reconcentration process was conducted to study the performance and fouling of FO-MBR with different MCRTs The performance results of the two phases’ studies were similar viii Results on the quality of the final product water from the NF system showed that the removal efficiencies of TOC and COD were both above 95% for all three MCRTs Conductivities and total dissolved solids (TDS) of the final product water for the three MCRTs were all lower than 500 us/cm and 400 mg/L, respectively Results also indicated that all the three FO-MBRs under three different MCRTs had very low fouling propensity during the operation The concentration of EPS decreased as the MCRT of the FO-MBR was increased while the concentration of SMP increased with the increase of the MCRT of the FO-MBR Water flux of the 10-d MCRT FO-MBR showed the most significant reduction compared with that of the 3- and 5-d MCRT FO-MBRs The reduction of water flux was mainly due to the reduced effective osmotic driving force across the FO membrane From the results of the SEM and EDX, there might be some scaling phenomenon occurring on the FO membrane surface of the FO-MBR operated at 10-d MCRT as the salt concentration in the mixed liquor was the highest among the MCRTs Results on the backwash study indicated that bi-monthly backwash was an appropriate option for FO membrane cleaning FOMBR shows a great potential in wastewater treatment with a low energy cost and decent performance, however, there are very few studies on FO-MBR The results of performance and fouling of FO-MBR in this study agree with that of the FO wastewater application studies by Achilli et al, Lay et al, Cornelissen et al and Mi and Elimelech, which indicated that FO-MBR had a high contaminant rejection and low fouling propensity Keywords: Forward osmosis membrane bioreactor, draw solute, water flux, rejection, domestic wastewater, nanofiltration, low fouling ix Chapter Four - Results and Discussions 4.4.2.6 Effect of backwash on fouling control Effect of fouling control using backwash was studied The 5-d MCRT FO-MBR was chosen to study the efficiency of fouling control and water flux recovery The 5-d MCRT FO-MBR was run continuously with three different backwash schemes, namely, weekly, bi-monthly and monthly backwashes, conducted sequentially Each backwash scheme lasted around 60 d Normalized water fluxes, which were discussed earlier in Section 4.4.2.3 were calculated for the three backwash schemes and the results are shown in Fig 4.18 The rate of change of the normalized water fluxes reflected the severities of water flux declines caused by fouling As shown in Fig 4.18, the overall normalized water flux of 5-d MCRT FO-MBR showed a decline throughout the 195 d of operation, dropping from 0.17 to 0.11 L m-2 h-1 bar -1 This decrease was largely due to irreversible membrane fouling, and possibly a small extent of scaling which was mentioned earlier in Section 4.4.2.5 Scaling on the membrane might have resulted from salt accumulation in the mixed liquor, which contributed to the reduction of the normalized water flux According to Fig 4.18, an average flux recovery of more than 90 percent was achieved in the 5-d MCRT FO-MBR with weekly and bi-monthly backwash schemes For the case of monthly backwash scheme, a lower average flux recovery of only around 75 percent was observed This was attributed to greater irreversible fouling formed on the membrane, prohibiting the diffusion of water molecules through the membrane, over a one-month of operation without backwash Thus, the normalized water flux of 5-d MCRT FO-MBR with monthly backwash showed greater normalized water flux 106 Chapter Four - Results and Discussions decline compared to those of the weekly and bi-monthly backwash schemes Therefore, a bi-monthly backwash scheme is a relatively more appropriate option of membrane cleaning for the 5-d MCRT FO-MBR when considering the frequency and Normalized water flux (L m-2 h-1 bar-1) efficiency of cleaning 0.19 0.17 0.15 0.13 0.11 0.09 Weekly 0.07 Bi-monthly Monthly 0.05 20 40 60 80 100 120 140 160 180 200 Time (day) Figure 4.18 Normalized water fluxes of 5-d MCRT under different backwash schemes (The arrow showing the day on which backwash was carried out) 4.4.3 Conclusion of the laboratory-scale FO-MBR study Two phases of FO-MBR study, including Phase - 39 d of continuous operation without backwash and Phase - 120 d of operation with three different backwash schemes, were carried out to investigate the performance and the fouling of FOMBRs with three MCRTs, namely 3-, 5- and 10-d The following conclusions can be made: All the three FO-MBRs, in spite of the difference in MCRTs, had high TOC and COD removal efficiencies, above 95 and 97%, respectively In addition, the TDS and conductivities of product water were able to meet the WHO 107 Chapter Four - Results and Discussions drinking water guidelines However, total nitrogen (nitrate-nitrogen) showed a low removal efficiency which suggested that FO-MBR needs to be operated with a pre-anoxic mode to enhance the nitrogen removal efficiency FO-MBRs with three different MCRTs showed significant water fluxes decline and these were mainly due to the reduced effective osmotic pressure difference across the membrane Membrane fouling of the three FO-MBRs had a minimal effect on the water flux decline SMP and EPS showed different trends with the increase of MCRT of the FOMBR SMP increased as MCRT was increased, while EPS decreased as MCRT was increased Regardless of the effect on SMP and EPS, fouling of the three FO-MBRs were similar The SEM and EDX results showed that the foulants on the FO membrane surface of the FO-MBRs operated under 3-d and 5-d MCRTs were generally microorganisms and organics Those from the 0-d MCRT FO-MBR contained not only the microorganism and organics, but also crystal-like substances which probably were CaCO3 according to the EDX results Results of backwash effect on fouling control showed that the weekly and bimonthly backwash schemes for the FO-MBR achieved an average flux recovery of above 90 percent, and monthly backwash achieved a lower flux recovery of around 75 percent 108 Chapter Five- Conclusion and Recommendations CHAPTER FIVE CONCLUSION AND RECOMMENDATIONS 5.1 Conclusion FO-MBR is proposed as an alternative for wastewater treatment because less energy is required when compared to the conventional MBR This study investigated the performance of FO-MBR on treating domestic wastewater as well as the FO membrane fouling propensity It showed the great potential of FO-MBR in domestic wastewater treatment Before the experiments for the FO-MBR with a nanofiltration as post-treatment were conducted, an optimal FO module configuration and a suitable draw solution for the FO process were selected Module selection Comparisons of the results of the water and reverse salt fluxes, the 6-chamber module configuration was the optimal option for FO-MBR among the three configurations tested, as it achieved a high water flux and maintained a low reverse salt flux Draw selection From the results of the FO and NF test, Na2SO4 was identified as the most appropriate draw solution for the FO-MBR, as it generated a fairly high water flux in the FO and NF processes, and a low reverse salt flux, compared to the other draw solutes tested 109 Chapter Five- Conclusion and Recommendations Laboratory-scale FO-MBR study FO-MBR had high TOC, COD and TDS removal efficiencies as a consequence of the non-porous FO membrane However, high total nitrogen (nitrate-nitrogen) in the draw solution and final NF permeate suggested that the FO-MBR has to be operated with a pre-anoxic mode to enhance the nitrogen removal efficiency All the three FO-MBRs showed water flux decline, which was largely attributed to the reduced effective osmotic pressure difference across the membrane Fouling of the FO membrane had a minimal effect on the water flux decline SMP and EPS showed different trends with an increase in the operating MCRT of the FO-MBR SMP increased as the MCRT was increased, while EPS decreased as the MCRT was increased Foulants from the 10-d MCRT FO-MBR contained both organic and microorganisms, and crystal-like substances which probably were CaCO3 according to the EDX results Weekly and bimonthly backwash schemes for the FO-MBR were sufficient for the FO membrane cleaning, providing a higher normalized water flux recovery of above 90 percent, as compared to the monthly backwash scheme Thus bi-monthly backwash scheme is the appropriate option of membrane cleaning for the 5-d MCRT FO-MBR 5.2 Recommendations There are a few recommendations for future research on FO-MBR coupled with a NF post-treatment process, for wastewater treatment In this study, three FO membrane modules including 1-chamber, 4-chamber and 6chamber designs were assessed to determine the optimal configuration For future 110 Chapter Five- Conclusion and Recommendations research, more FO module configurations can be developed to optimize the working efficiency of the draw solution Five kinds of solutes were chosen in this study to determine the most appropriate draw solution for the laboratory-scale FO-MBR-NF system For future draw solution selection studies, a wider variety of draw solution including organic and inorganic, and nanoparticles can be investigated The results of the laboratory-scale FO-MBR experiments obtained in this study showed a low total nitrogen (nitrate-nitrogen) removal efficiency, which indicated that a denitrification process should be considered to be added into the FO-MBR 111 List of Publications LIST OF PUBLICATIONS J Zhang, V Parida, C.H Tan, W Duan, S.L Ong and H.Y Ng, Performance Of A Low Fouling Forward Osmosis Membrane Bioreactor With Different Mean Cell Residence Times, accepted for presentation at the 3rd IWA Asia Pacific Young Water Professionals Conference, 21-23 November 2010, Singapore C.H Tan, J Zhang, L.Y Lee, W Duan, S.L Ong and H.Y Ng, Treatment of domestic wastewater using a hybrid forward osmosis membrane bioreactor (FO-MBR) process for water reclamation, accepted for presentation at the 19th KKNN symposium on environmental engineering, 27-30 June 2010, Kyoto, Japan C.H Tan, J Zhang, L.Y Lee, W Duan, S.L Ong and H.Y Ng, High grade water reclamation using hybrid forward osmosis membrane bioreactor (FO-MBR) with nanofiltration, accepted for presentation at the IWA World Water Congress, 1924 September 2010, Montreal, Canada C.H Tan, J Zhang, L.Y Lee, W Duan, S.L Ong and H.Y Ng, Direct water reclamation from domestic wastewater with a hybrid forward osmosis membrane bioreactor (FO-MBR) with nanofiltration, accepted for presentation at the IWA Regional conference on Membrane Technology and Water Reuse, 18-22 October 2010, Istanbul, Turkey 112 References REFERENCES Achilli A., Cath T.Y., Marchand E.A., Childress A.E., (2009) The forward osmosis membrane bioreactor: A low fouling alternative to MBR processes, Desal., 239, 1021 Achilli A., CathT Y., Childressa A E., (2010) Selection of inorganic-based draw solutions for 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Filtration &Separation, 42, 24-27 Wen G., Ma J., Zhang L., Yu G., (2010) Membrane Bioreactor in Water Treatment, Comprehensive Membrane Science and Engineering, Chapter 4.07, Pages 195-209 Wisniewski.C., (2007) Membrane bioreactor for water reuse, Desal., 203 ,15–19 Wu J., Huang X., (2009) Effect of mixed liquor properties on fouling propensity in membrane bioreactors, J Membr Sci., 342 88–96 120 ... process and membrane bioreactor Currently, membrane technology is preferred for wastewater reclamation because of its high contaminant rejection and water productivity Forward osmosis membrane bioreactor. .. proposed for wastewater treatment and reclamation using an osmoticmembrane bioreactor or forward osmosis membrane bioreactor (FO-MBR) (Cornelissen et al 2008; Achilli et al 2009) 2.3 Membrane Bioreactor. .. surface 2.4 Forward Osmosis Membrane Bioreactor 2.4.1 Introduction Forward osmosis membrane bioreactor (FO-MBR) process is a combination of FO process and MBR process to treat the used water into