Research On The Relationship Between Microbial Polymers And Membrane Fouling In MBRs

Membrane bioreactors (MBRs) have many advantages, but membrane fouling remains a major obstacle to their wide application. Activated sludge extracellular polymeric substances (EPS) has been reported as an important factor governing membrane fouling. Most studies focused on the influence of EPS quantity on membrane fouling. To investigate the mechanism of membrane fouling deeply, it is necessary to analyze the relationship between sludge composition and membrane fouling.Thus, in the dissertation, two parameters, aeration rates and influent composition, are examined for their influence on the quantity and composition of microbial polymers attached on membrane surface as well as in sludge mixed liquor, and on related characteristics of sludge and the consequences for membrane fouling in submerged MBRs run for a long period, respectively. Microbial polymers mentioned above include soluble EPS, bound EPS and total polymers in sludge flocs, and their quantity and composition are expressed as the sum of the quantity of protein and that of carbohydrate and the ratio of the quantity of protein to that of carbohydrate (called p/c for short), respectively.In Chapter 3, aeration rate was examined for its influence on the quantity and composition of microbial polymers and on membrane fouling in MBRs. Three MBRs with aeration rates of 120, 80 and 40 L/h were operated in parallel for 170 d at a constant permeate flux of 8 L/(m~2- h) with other parameters the same. The main experimental results and conclusions are as follows.(1) Aeration rates have significant effects on treatment performance of MBRs. With increasing aeration rates, supernatant and membrane effluent COD increase and decrease, respectively, while supernatant and membrane effluent NH3-N concentrations both decrease with increased aeration rates.(2) Aeration rates have significant influence on both quantity and composition of soluble EPS, bound EPS and total polymers in flocs (the sum of bound EPS and polymers in microbial cells) in sludge mixed liquor.(a) In the case of the same aeration rate and different time, the results indicate that, soluble EPS undergoes the initial accumulation and subsequent degradation with time. The amounts of bound EPS and total polymers in flocs fluctuate slightly with time. And the p/c ratios of microbial polymers are relatively stable and decrease slightly with time in general.