Effect And Mechanism On Control Of Membrane Fouling By Coagulant Addition In Membrane Bioreactor

Membrane fouling is a key problem blocking the membrane bioreactor's wideapplication. In our study, coagulants is added to the mixed liquor in the membranebioreactor in order to enhance the membrane filterability of the mixed liquor and controlmembrane fouling. Batch experiments were carried out to make studies on the affects onthe membrane filterability or the specific filtration resistance of the mixed liquor byadding eight types of coagulants respectively such as polymeric ferric sulfate (PFS),polymeric aluminium chloride (PAC), chitosan etc. Changes of the supernatant TOC,the size of the sludge flocs, and other characteristics of the mixed liquor were determinedand the mechanism of the improvement of membrane filterability was studied further.The effect on membrane filtration and the cost of the coagulants were compared. Inthe coutinuous experiments, the most suitable method including optimal coagulant,dose and addition time was selected, the effect on membrane fouling control wasvalidated, and the influence on the running of the membrane bioreactor was obtained atthe same time.In the batch experiments, all the coagulants selected could enhance themembrane filterability except anionic PAM. Among them, PFS was the mosteffective. As the dose is 100mg/L, the slips of TMP, the slips of increase rate ofTMP, the slips of TMP after coagulant addition and the supernatant TOC were76.5%, 60.4%, 8.4%, 45.4% respectively. The major mechanism was was thatcoagulant addition could reduce the zeta potential of colloids was reduced, acceleratethe aggregation between the colloids and sludge flocs, which enlarged the sludgeflocs and reduced the supernatant organic matter, enhanced the membranefiltration of mixed liquor consequently. Among the coagulants studied, PFS couldmake the sludge specific filtration resistance decrease by 80%, and the costincreased was only about 0.0001RMB/t. PFS was the best one with better effectand lower cost.In the continuous experiments, it was proved that the optimal dose was180mg/L and the suitable time was before the MBR was run for PFS addition. Throughlong-term experiment, the cleaning cycle of the MBR with PFS addition prolongs 6-7times as that without PFS addition. Zeta potential of the colloids was reduced, theaverage supernatant TOC was reduced by 57%, and kept in a low level for a longof time, the size of the sludge flocs was enlarged by almost one time, themembrane filtration was enhanced effectively. The gel layer fouling is 80% off,which made the membrane fouling occurred more slowly. At the same time, PFS did notbring about the serious inorganic fouling, and did not have much blight on viable bacteriaand their activity.