Characteristics Of Sludge Digestion And Membrane Fouling In Anaerobic Membrane Bioreactor

Aerobic membrane bioreactor(MBR) has been widely used in Municipal Wastewater Treatment Plants because of the improved requirement for effluent quality. However, the application of MBR would have a great impact on the properties of the waste activated sludge(WAS). The anaerobic digestibility of the WAS discharged from large-scale MBRs was studied. A cross-flow anaerobic membrane bioreactor(AnMBR) was constructed for the treatment of MBR’s WAS. The membrane fouling control and cleaning methods were studied as well as the fouling mechanisms during long operation of the AnMBR.The WAS from MBRs has worse anaerobic digestibility than that from the parallel conventional activated sludge processes. The long solid retention time(SRT) applied in MBRs is the major reason, which caused significant endogenous respiration and high proportion of dead cells. AnMBR achieved sufficient volatile solid(VS) reduction rate in treating the MBR’s WAS.Soluble microbial products(SMP) is the major substance causing membrane fouling in AnMBR. The formation of inter-molecular crosslinking between organics in the presence of calcium and magnesium ions can lead to increase of the SMP particle size and decline of its filtration flux. It can also enhance the adsorption strength between the foulant and membrane surface. Hydrophilic neutrals(HIN) are major organics in SMP, which contain high proportion of polysaccharide. The fouling layer formed by HIN was hard to be removed thoroughly by backwashing using pure water.Dosing polyaluminum chloride(PAC) in the bulk sludge of AnMBR could mitigate membrane fouling by reducing SMP and improving sludge filterability. The optimal dosage of PAC is 500 mg/L. Dosing PAC at the beginning of operation and re-dosing according to the amount lost during sludge sampling is the recommended way for membrane fouling control. PAC had no significant influence on sludge digestion. The VS reduction rate could meet the requirement of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant for WAS treatment. The offline cleaning for membrane is inevitable when the fouling is severe. The cake layer could be removed by physical washing. In order to remove the gel layer and recover the membrane with its initial filtration flux, the membrane should be cleaned by using citric acid, sodium hypochloride and citric acid again in sequence.The microbial communities in bulk sludge, cake layer and WAS were studied by using high-throughput sequencing. The microbial communities in bulk sludge was affected by the WAS during the startup period, and high microbial diversity was observed. The phylum Cloacimonetes accumulated during the operation, and the microbial diversity declined gradually. Solid concentration, phosphate and COD were the major environmental factors straping the microbial communities. A relatively higher abundance of saccharide fermentation bacteria and filamentous bacteria was found in the cake layer than that the in bulk sludge.