Long-term Study On Operating Characteristics Of Submerged Flat-sheet Membrane Bioreactor

embrane bioreactor（MBR）process has emerged as a new and efficient technology for wastewater treatment.Compared with conventional activated sludge （CAS）system,MBR process has many advantages including superior effluent quality, higher sludge concentration,higher volumetric organic load（VOL）and a smaller footprint.Therefore,MBR has been increasingly popular for the treatment of municipal and industrial wastewater in recent years.In international MBR market, flat-sheet membrane has been intensively studied and widely used due to its high flux, anti-fouling capability and easily-controlled hydraulic conditions.According to related statistics,flat-sheet MBRs accounted for 68%of the total full-scale MBRs throughout the world by the end of 2006;however,the research on flat-sheet MBR in China obviously lagged behind.Thus,intensive efforts must be dedicated to the research of flat-sheet MBR in our country in order to better promote the development of MBR technology in China.The objective of this thesis is to study the characteristics of submerged flat-sheet membrane bioreactor（SMBR）for the treatment of classic municipal wastewater by employing a pilot-scale MBR under long-term operation and several lab-scale MBRs. The filtration properties,membrane cleaning,sludge yield coefficients together with organic material removal such as COD,phosphorous,etc.under long-term operation were studied.Furthermore,the relationship between sludge characteristics including sludge concentration,sludge physiology properties,sludge composition and membrane fouling was critically investigated in order to determine the effects of sludge characteristics on MBR long-term performance.Based on the research mentioned above,the membrane fouling mechanisms of flat-sheet MBR under long-term operation were proposed and the results were expected to provide a sound understanding of the design and operation of full-scale flat-sheet MBRs.The variable in terms of membrane permeability was adopted in this study,and the variations of.the membrane permeability in the pilot-scale submerged MBR during the operation were critically investigated.The variable could be used to comprehensively demonstrate the separation and filtration characteristics of flat-sheet membrane during long-term operation.Test results showed that the net membrane flux could be maintained at 20.0～25.0 L/（m² h）except that of operation in winter and the highest net flux could reach 30.6 L/（m² h）during the first year operation of the pilot-scale MBR.It was observed that the net membrane flux decreased in the second year and averaged at 17.0 L/（m² h）.During the early stage（the first year）,membrane cleaning procedure would be carried out per 3～4 months;however,the cleaning procedure was needed per 1～2 months as the operation time increased.Maintenance cleaning-in-place （CIP_M）,to some extent,could recover membrane permeability and intensified cleaning-in-place（CIP₁）was needed if CEP_M could not effectively recover membrane permeability.Sometimes,it was also necessary to use offline cleaning method to efficiently recover membrane permeability after a long-term operation.It was also found that the MBR could bear the shocking load and effluent COD 30 mg/L and 95%removal efficiency on average could be achieved though the influent COD fluctuated greatly.In this study,it was observed that DO concentration in the MBR affected the removal of ammonia,and effluent ammonium concentration was less than 8 mg/L if the DO concentration of the top areas of the two down-comers could be maintained at above 0.3 mg/L.Good simultaneous nitrification and denitrification （SND）could be achieved under high sludge concentration in the reactor.13-19 mg/L of the effluent TN（i.e.removal efficiency 60-70%）could be reached under DO concentration 0.2-0.3 mg/L and influent TN concentration 39-70 mg/L.The effluent phosphor was less than 1.0 mg/L and average concentration less than 0.5 mg/L by addition of polymeric ferric sulfate（PFS）under the proper agent 14 mg ferric ions/（L-influent water）.The study on sludge yield coefficient of the MBR showed that the observed yield coefficient(Y_obs)under SRT 10d,20d,30d and 40d was about 0.247,0.151,0.128 and 0.113 kgMLVSS/kgCOD,respectively.The net yield coefficient（Y）and endogeneous decay coefficient（K_d）of the pilot-scale MBR for municipal wastewater treatment was about 0.402 kgMLVSS/kgCOD and 0.068 d^-1, respectively In this study,it was observed that sludge concentration had effects on sludge rheology.The mixed liquor belonged to Newtonian fluid with MLSS less than 10 g/L while it demonstrated non-Newtonian fluid behavior（Thixotropic fluid）as MLSS increased to above 10 g/L.Sludge concentration was also found to have negative effects on oxygen transfer coefficient（a）,and a value decreased from 0.565 to 0.155 with the increase of sludge concentration from 4.5 g/L to 21.5 g/L.Cross flow velocity（CFV）was also affected by sludge concentration,and CFV tended to decrease as sludge concentration went up under the same aeration intensity.The study on sludge properties indicated that capillary suction time（CST）, soluble chemical oxygen demand（SCOD）,extracellular polymeric substances（EPS）, carbohydrates in the supernatant of mixed liquor,proteins in the supernatant of mixed liquor,viscosity and MLSS had positive influences on membrane fouling.It was also found that temperature of the mixed liquor had negative effects on membrane fouling. Compared with tight bound part of EPS,Loose bound（LB）was observed to have more significant influences on membrane fouling.Critical flux was also affected by sludge properties,and test results showed that critical flux declined as the CST or SCOD of the mixed liquor increased.Proper operational flux of the MBR was found to be 56%of the critical flux.The contribution of three sludge compositions to membrane fouling under various SRT was also studied,and test results showed that the effects of suspended solids（SS）tended to increase as the SRT prolonged.The resistance caused by the three sludge compositions under abnormal conditions including low-temperature operation,foaming sludge and sludge after membrane chemical cleaning was much larger than that of sludge during stable-operation.The resistance caused by colloids or soluble products was much higher than that of SS under low-temperature operation in the winter.Based on the long-term study of the operational characteristics of the MBR together with the research on membrane fouling properties,it could be drawn that fine particles are prone to block membrane pores or to deposit onto membrane surfaces while large particles are not easy to form membrane fouling.In submerged MBR under constant flux operation,the membrane fouling can be classified into two stages. In the first stage,the membrane fouling is mainly caused by EPS,SMP and other large molecular organic materials and the fouling velocity is rather slow.Sludge cake layer is not deposited on the membrane surfaces during this stage,which is totally different from the conventional fouling understandings.Besides,the first stage is also the main part of the operational period（one cleaning period）.In the second stage, dramatic membrane fouling was observed and membrane resistance increased rapidly to the terminal pressure point,which indicated that membrane cleaning procedure should be carried out.It is also necessary to realize that sludge cake layer and gel layer are different from each other in the submerged MBR operation.Sludge cake layer can be formed onto the membrane surfaces by sludge flocs deposition,which is mainly attributed to improper design or unreasonable operation.The formation of sludge cake layer can be controlled by scientific design and optimized operation.Gel layer is mainly caused by EPS,SMP,carbohydrates and proteins which have large molecular weight but fine particles compared with sludge flocs.The gel layer fouling can not be prevented even under sub-critical flux operation mode in submerged MBRs.Economical analysis of submerged flat-sheet membrane technology for wastewater treatment and reuse was carried out in the last section of the thesis,and the comparison of economy variables between MBR and conventional wastewater reclaiming technology was also conducted.It indicated that MBR technology was fairly competitive when used for wastewater treatment and reuse.