| Membrane bioreactor (MBR) is a high efficient wastewater treatment andreclamation process, which is a combination of conventional activated sludge process(CASP) with membrane separation technology. MBR has a promising perspective interms of technology progress and application, due to its outstanding advantages,including high sludge concentration, superior effluent quality, small footprint, separatecontrol of hydraulic retention time (HRT) and sludge retention time (SRT), etc.However, membrane fouling is the main factor limiting the more widespreadapplication of MBRs. On one hand, due to membrane fouling issue it is inevitable toapply frequent physical and chemical cleaning of membrane modules which willincrease the operating and maintenance costs. On the other hand, membrane foulingwill shorten the life time of membrane modules and cause the increase of membranereplacement cost. So far, most of the studies on membrane fouling were conducted inlab/pilot scale setup, little work was done to investigate membrane fouling in full-scaleMBRs. Therefore, it is essential to study membrane fouling behaviors in real MBRplants, in order to establish theoretical and technical fundamentals for membranefouling control by regulating operational conditions.This work was supported by the National Natural Science Foundation of China(No.50838005). This research work was conducted in the wastewater treatment plant(WWTP) of Xi’an Siyuan University, which adopted A2O-MBR process for domesticwastewater treatment and reuse with a treatment capability of2000m3per day. Theevolution of trans-membrane pressure (TMP) and membrane permeability during9-month operation was evaluated. The fouling properties of major foulants were characterized by sample analysis in field and batch filtration tests. Moreover, thefouling mechanism of A2O-MBR system under constant flux operation mode wasanalyzed. Main results were summarized as follows.(1) In this part, by continuous sampling and detecting, process properties,pollutants removal and membrane filtration performance of the A2O-MBR systemwere evaluated. The system consists of two treatment units, anaerobic-anoxic-aerobic(A2O) unit for bio-chemical treatment followed by a submerged MBR unit usinghollow fiber membrane for solid/liquid separation. A2O unit employed a one by onepattern of sludge backflow, which not only made a proper utilization of dissolvedoxygen (DO) but also effectively maintain the activated sludge in suspended condition.While in MBR unit, several fouling control methods (such as intermittent operation ofsuction pump and periodical maintenance cleaning) were designed to mitigate fouling.During long-term operation, the system showed a preferable treatment performancewith the effluent quality superior to the requirements of water quality standard forurban miscellaneous water consumption and for scenic environment use. Furthermore,under constant flux mode (16L/m2h), TMP changed in a narrow range between12.4-21.5kPa, indicating a stable operation of the A2O-MBR system under low TMP.(2) By collecting mixed liquor samples in A2O-MBR system, characteristics ofsludge mixed liquor, dissolved organic matter (DOM) and components content ofextracellular polymeric substances (EPS) were measured, meanwhile the transfermechanism of membrane foulants was discussed in this chapter. Several parameters,such as dissolved oxygen (DO) and oxidation reduction potential (ORP), sludgeconcentration (MLSS and MLVSS), viscosity were detected along the A2O-MBRprocess, and the experimental results revealed that anaerobic tank, anoxic tank, oxictank and membrane tank were in well bio-chemical status. As a group of representativefoulants, the properties of extracellular polymeric substances (EPS) were analyzed.The results showed that EPS concentrations and components (proteins andpolysaccharides) decreased obviously along the A2O-MBR treatment array, while withthe increase of operational time no accumulation of EPS occurred in MBR tank,indicating that A2O-MBR treatment process was able to prevent large amount of EPSfrom entering the MBR tank. The EEM analysis of DOM showed that proteins-like substances were biodegradable and could be retained by the membrane, meanwhile thedecrease of their concentrations along treatment units was in favor of membranefouling control. Moreover, humic-like substances showed a poor biodegradability andno obvious decrease of their concentrations was detected, which would cause littleeffect on membrane fouling because they could penetrate membrane pores as a groupof low molecular weight (MW) substances.(3) The characteristics of activated sludge, DOM and EPS were systematicallyinvestigated in this part. Comparing the properties of DOM in raw wastewater, mixedliquor and MBR effluent showed that different DOM components (proteins,polysaccharides and humics) demonstrated various biodegradation rates and membranerejection rates, and it was found that polysaccharides had much greater impacts onmembrane fouling than proteins and humics due to their low biodegradation rates andhigh membrane rejection rates. EPS in mixed liquor of MBR tank were further dividedinto SEPS, loosely bound EPS (LBEPS) and tightly bound EPS (TBEPS) and theirfiltration behaviors were study by using batch filtration tests. It was revealed thatLBEPS having a lowest content showed highest components membrane rejection rateand fouling potential with specific modified fouling index (SMFI) as two times and17times high as those of SEPS and BEPS, and LBEPS located at the outside of sludgeflocs moving freely and had a relatively low molecular weight, which could cause poreblocking and decrease the porosity of cake layer.(4) Samples of bulk sludge in MBR tank and cake sludge on membrane surfacewere collected and the filtration properties of them were compared by using batchfiltration tests, and also the mechanism of cake layer formation and foulantsaccumulation was studied. The main results were showed as follow.①The modifiedfouling index (MFI) of cake sludge was3.3times higher than bulk sludge, and cakesludge had a total filtration resistance (Rt) of3times as that of bulk sludge, however,cake layer resistance (Rc) contributed90%of the increase of Rtand pore blockingresistance showed little increase. It revealed that cake sludge had a higher foulingpotential and mainly caused significant rise in Rcbut which was a easily removableresistance.②Compared to bulk sludge, cake sludge showed a broader and greaterparticle size and a lower MLVSS/MLSS ratio, indicating that the content of inorganic components in cake sludge improved and thus this compact structure caused asignificant increase in filtration resistance.③Compared to bulk sludge, theconcentration of BEPS in cake sludge showed no obvious increase but theconcentrations of proteins to those of polysaccharides ratio (PN/PS) had a significantdecrease, indicating that BEPS in cake sludge was in a dynamic variation between theaccumulation of foulants and the consumption as a substrate by microorganisms incake layer. During long-term operation, the change of BEPS components could have acertain effect on membrane fouling.④Using series of analytical methods, the mainfoulants in cake sludge and the morphology of fouled membrane were detected and thefoulants were found to be a gelatinous mass uneven distributed on membrane surface.It was thought that cations (such as Fe, Mg and Al) in mixed liquor would be couplingwith biopolymers (proteins and polysaccharides) attached on membrane surface,causing the accumulation of certain amount of cations, promoting the growth of cakelayer, improving the compactness of cake layer and lowering the porosity of cake layer,which would be the main factor resulting in the increase in filtration resistance of cakesludge. Also cake layer played a role in inhibiting the rise of pore blocking fouling.(5) Using membrane filtration model, filtration resistance was divided into threeparts, membrane intrinsic resistance (Rm), removable resistance by EFM cleaning (Rr),irremovable resistance by EFM cleaning (Rir). The data of TMP variation from theearly stage and latter stage of long-term A2O-MBR system operation with EFMcleaning were used for simulating and calculating. The results indicated that EFMcleaning removed Rreffectively and Rirwas controlled at a constant low level.Compared to the early stage of operation, at the latter stage the linear increase rate ofTMP in one EFM cycle showed no obvious rise and the increase rate of TMP kept atabout0.01kPa/h. Above-analysis indicated that this A2O-MBR system showed asuperior property of membrane fouling control. |
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