Using inorganic coagulants to control membrane fouling in a submerged membrane bioreactor

Changes of properties (specific filtration resistance, floc size, supernatant TOC and zeta potential) of activated sludge mixed liquor after monomeric and polymeric coagulants [Al-2(SO4)3, FeCl3, PAC (polymeric aluminum chloride) and PFS (polymeric ferric sulfate)] addition were studied in this paper. It was found that polymeric coagulants had a better effect on filterability enhancement of mixed liquor than monomeric coagulants. Further research found that the reason was that polymeric coagulants could supply more positive charges for organic particles and sludge flocs, and enhance the functions of charge neutralization, which resulted in higher removal of supernatant organic matter and enlargement of sludge floc size. Thus, polymeric coagulants (PAC, PFS and PAFC) were added in a submerged membrane bioreactor (MBR) and the effect on membrane fouling control was investigated. It was found that polymeric coagulants could control membrane fouling by reducing the initial TMP and the TMP increase rate. Three functions of polymeric coagulants on membrane fouling control were supposed: restraining the formation of gel layer, decelerating the development of foulants and removing stable foulants from the membrane surface. Among the selected coagulants, PFS was the most effective in controlling membrane fouling and the optimal dose was 1.05 mM Fe. If coagulant was added before the MBR was run, a better effect could be obtained.