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 [Al2(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.

3.2.4. Discussion on the increased amount of excess sludge Because coagulant addition might result in excess sludge increase, sludge concentration and sludge production of two running MBRs with and without PFS addition was studied further. Fig. 14 shows the change of MLSS in these two MBRs with time. After PFS addition (before running) with a dose of 1.0 mM Fe, MLSS did indeed increase. However, the difference between the MLSS concentrations in the two MBRs was less than 1 g L–1, which would not cause a huge amount From the experiments above, the following conclusions could be drawn: (1) Al and Fe coagulants in the study had a positive effect on improvement of mixed liquor filterability. The hydrolyzed coagulants supplied positive charges and long chain molecules for the flocs and colloids in the mixed liquor, then enhanced the charge neutrality and bridging which resulted in enlarged sludge flocs. (2) Polymeric coagulants could supply more positive charges and longer chain molecules, so that they had a better effect on filterability im- Table 4 Effect on TMP control by four polymeric coagulants Polymeric coagulants PFS PAC PAFC PFS+PAM Slip of initial TMP, % Slip of increase rate of TMP, % Slip of running TMP, % Removal of TOC, % 76.5 60.4 8.4 45.4 68.5 26.3 17.85 38.1 69.5 25.0 6.9 39.1 76.7 65.5 4.8 48.1 provement of the mixed liquor, while excess addition of polymeric coagulant led to “colloidal restabilization”. (3) Among all the coagulants studied, PFS was the most effective for membrane fouling control. The optimal dose was 1.05 mM Fe. (4) In order to have a better effect on membrane fouling, the coagulant should be added before the MBR is run.