Study On Effect And Mechanism Of Flocculants On Mitigating Membrane Fouling In Membrane Bioreactors

Membrane bioreactor has been regarded as one of the most promisingtechnologies for wastewater treatment and reclamation due to manyoutstanding advantages over conventional activated sludge processes.However, membrane fouling has still been a major barrier limiting its wideapplication because fouling could cause high operating costs andinconvenience in application .Recently, application of filter aids or antifoulingagents in MBR to modify biomass characteristics artificially for alleviation ofmembrane fouling has been considered as a new technology of fouling control.this study, the effects of addition of six types of flocculants (aluminiumsulphate, ferric chloride, polyaluminium chloride, polymeric ferric sulfate,Chitosan, polyacrylamide) on mitigation of membrane fouling in membranebioreactors (MBR) were investigated respectively. According to actionmechanism, the flocculants adopted could be classified as monomer, inorganicpolymer and high molecular weight organic flocculants respectively. In membrane fouling process,the rise in TMP is described as a 3-stageprocess. The first stage occurs in a period of afew hours and involves abruptTMP rise due to"conditioning"presumably by pore blockage and closure.Stage 2 is a prolonged period of slow TMP rise,which is ascribed to formationof gel layer. Stage 3 is a sudden rise in TMP due to accumulation of cake layer.The effect of flocculants on membrane pore blocking, gel layer and cake layerresistance were analyzed respectively.Significant improvement of the sustainable filtration was demonstrated in allflocculants added MBRs. The membrane fouling rate of the MBRs operatedunder 20L/m2.h flux was in the order of Control MBR (no filter aid added)<Al₂(SO₄)3 <PAC < FeCl₃< PAM < Chitosan < PFS added MBR. For theChitosan and PFS added MBRs, the sustainable filtration time at optimumdosage were nearly 7 times more than that in the control MBR. Membraneinner fouling due to pore blocking was analyzed by means ofFourier-transform infrared microscope (FTIR). Compared to the control MBR,significantly low protein and carbohydrate concentrations were measured inthe membranes of the flocculants added MBRs, indicating that flocculantscould effectively alleviate membrane pore blocking. Gel PermeationChromatography (GPC) analysis suggested that both the concentration andmolecular weight distribution of the macromolecules in supernatant play animportant role in gel layer formation and loss of membrane porosity. Thereduction of fouling rate in the filter aids added MBRs could be attributed to lower concentration and reduction in molecular weight of macromolecules insupernatant.The biomasses in various MBRs were characterized by morphologicalproperties (mean floc size (dp), fractal dimension (df)), physical parameters(surface charge, relative hydrophobicity (RH), dynamic viscosity) and thebiochemical components of the mixed liquor (concentration of extracellularpolymeric substances (EPS)). Statistical methods such as normalization,nondimensionalization and multiple linear regressions were used to identifythe dominant membrane fouling contributors and to simulate membranefouling rates. The results demonstrated that the key factors affectingmembrane fouling varied in different flocculants added MBRs. Organicpolymeric flocculants have significant effects on biomass morphologicalproperties. Membrane-fouling alleviation was mainly due to the decrease inSMP and df as well as the increase in dp. Inorganic flocculants have strongeffects on SMP, EPS, Zeta potential and RH but weak effects on d p, df andviscosity. For the inorganic flocculants added MBRs, the lower fouling ratecould be mainly attributed to the decrease in SMP and surface charge as wellas the increase in RH. For each type of flocculants, the empirical equations ofsustainable filtration time (Γ45) were simulated to predict membrane foulingrates in different MBRs.