| Membrane bioreactor(MBR), as a new efficient and pratical technology for wastewater treatment, combines membrane filter and wastewater biological treatment. But the widespread application of the membrane bioreactor is constrained by membrane fouling. Membrane fouling could lead to the rapid decline of filtration flux, which results in the increase of operation cost. Thus, settling the problem is the main aspect of research in this field.In this paper, effect of dosing powdered activated carbon (PAC) on the characteristics of the sludge mixed liquor in MBR was investigated through parallel tests. And the reason why PAC addition mitigated membrane fouling was explored. The results showed:1) COD, NH4+-N and turbidity could be removed effectively by both MBR and PAC-MBR.Both of the averaged removal efficiency were over 95%. COD, NH4+-N and turbidity of the effluent were respectively below 15mg/L, 1.0mg/L and 2NTU. PAC-MBR exhibited better results, which illustrated PAC could improve treatment efficiency of membrane bioreactor.2) Mixed liquor suspended solids was higher, since the growing environment was improved when there was PAC in reactor. Dosing PAC made sludge particles resist aeration shear force to a certain extent. The sludge particle size increased by 9.21%,and the sludge viscosity minished by 15.92%, which prevented the forming of the cake layer and made it loose and well- penetrated. And the increasing of the transmembrane pressure was weaken, which extended the cycle of the membrane assembly.3) PAC removed effectively organic matter in the supernatant, and exhibited better results in removal of organic matter with small relative molecular mass (removal efficiency increased by 5.5%). It was observed that PAC also played a role in improving the water quality from the relative molecular mass distribution characteristics of the reactor effluent. While membrane assembly appeared good results in removal of large relative molecular mass organic matter, and ensured the stability of the reactor effluent.4) Membrane resistance was analyzed during the cycle of membrane assembly. The result showed total resistance (Rt), cake resistance (Rc) and pore blocking resistance (Rp) increased with SRT; Rc/Rt (26.32%~63.16%) was always greater than Rp/Rt (7.89%~35.32%) which suggested the Rc was the main factor in membrane fouling. Moreover, it was also found that controlling of dosing PAC on Rc was better than that on Rp. Controlling the pollution of the cake layer could mitigate membrane fouling effectively. 5) Mixture extracellular polymeric substances (EPS) content and specific resistance to filtration took on a good correlation. The relation formula of MBR and PAC-MBR were respectively following as: y=630496x-2E+07 and y=268792x+945830, which showed that dosing PAC could decrease EPS concentration and specific resistance to filtration. Rc increased exponentially with increasing of EPS. The relation formula of MBR and PAC-MBR were respectively following as y=0.1972e0.653x and y=1.0381e0.2125x. The lower specific resistance to filtration was, the smaller increment speed of Rc was. And membrane fouling was controlled.6) It could be concluded the relationship formula of MBR and PAC-MBR was y=0.1424e0.3336x and y=0.1129e0.337x, by fitting the relationship between soluble microbial products (SMP) concentration and Rp. The higher SMP concentration in the supernatant was, the larger increment speed of Rp was. PAC addition reduced the SMP concentration and slowed down the growth rate of Rp, which was helpful to mitigating membrane fouling.
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