| As a new technology combining anaerobic biological treatment and membrane filtration system, anaerobic membrane bioreactor (AnMBR) has many advantages such as lower investment, lower energy consumption, lower sludge production and biogas generation. Equally important, complete biomass retention thoroughly decouples the SRT from the HRT, allowing anaerobic biomass concentrations to increase in the basin, thus higher treatment ability can be achieved. Recently, many researches have been carried out about the use of AnMBR, most of which are focused on the treatment of high-strengrh wastewater, only a few related to low-strength wastewater treatment and membrane fouling in AnMBR, so the widely application of AnMBR was limited due tothe uneven development. Two identical submerged membrane bioreactors (aerobic vs. anaerobic) with hydraulic retention times (HRT) of 14, 10, 6 hours and solids retention time (SRT) of 280 days were studied for their ability and fouling characteristics to treat synthetic domestic wastewater. Comparison of sludge properties and microbial metabolites from the two systems was made to elucidate major factors governing the different membrane fouling characteristics. An assumption was proposed about the dynamic proesss of membrane fouling in AnMBR, which could help to control the fouling effectively.The results showed that: AnMBR has very high COD removal efficiency,which is slightly lower than that for AMBR due to the metabolic features of anaerobic microorganisms. The effluent COD of AnMBR is 49mg/L under the HRT of 10h, achieving the basic requirement of water reuse. The COD mass balance indicated that high biomass concentration can be achieved in AnMBR, which leads to high COD removal efficiency. In addition, high hydraulic shear force could reduce the acticity of microorganisms that, in turn, leads the reduction in biogas production and a large number of organics are intercepted by membrane in AnMBR. The NH4+-N removal of in AnMBR and AMBR was 45% and 98%, respectively. The removal efficiency of PO43--P in AnMBR was about 50%, which is high than that for AMBR. The metabolism utilization of nitrogen and phosphates, phosphates deposition on membrane surface were the main reason for nitrogen and phosphorus removal in AnMBR.The rate of membrane fouling for AnMBR was faster than that for AMBR. The results of membrane resistance analysis showed that the rapid deposition of cakelayer on membrane was the main reason governing the rapid fouling in AnMBR. Further analysis indicated there are more fine particles (<10μm) in AnMBR which could accelerate the formation of cakelayer. In addition, the protein in SMP and EPS has a significantly positive relativity with membrane fouling. It could reduce the porosity of the cakelayer and make it more compact, leading the rapid fouling in AnMBR.An assumption was proposed about the dynamic proesss of membrane fouling in AnMBR: during the start-up period,fine particles preferred to attach on the membrane to form a thin but compact cakelayer. The attachment of fine particles makes the membrane surface more hydrophilic and modifies the surface charge of it, facilitating the deposition of sludge flocs,colloidal materials and solutes on the membrane surface, which increased the membrane resistance rapidly; The turbulence of the fluid around the membrane surface increased with the decline in anaerobic sludge viscosity, which could help to suppress the deposition of such substance on the membrane surface. So, the rate of membrane fouling in AnMBR will be slow during this period. The broken granular sludge will release a large amounts of EPS, which leads to the accumulation of SMP in supernatant. A large amount of protein will deposite into the pores of the cakelayer sludge and compact the cakelayer, which would reduce the permeability of cakelayer and cause severe membrane fouling in AnMBR.The results of this research indicated that the traditional aeration flush goes against anaerobic membrane fouling control. Furthermore, it will exacerbate the treatment efficiency of AnMBR. To achieve a high efficiency and low fouling operation, more researches should be ploughed into the structure of the AnMBR, the position of the membrane and the forecast and simulation of the membrane fouling. |
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