Mechanism And Control Of Membrane Fouling By Flocculant Addition In Membrane Bioreactor

Membrane fouling is the hot topic today in membrane bioreactor (MBR) field. The method that flocculants added into the mixed liquor in order to ameliorate filterability of MBR sludge and mitigate membrane fouling is an effective solution to the problem of"Membrane Fouling". However, the effect and mechanism of flocculants remain largely unkown. In this study, three types of flocculants (aluminium sulphate, polyacrylamide and polymeric ferric sulfate) were selected, and they belong to respectively the three kinds of flocculants (monomer, inorganic polymer and organic polymer). Adopt sludge specific resistance as index of filterability of MBR sludge, and the results showed that the optimum dosages of the three types of flocculants is 400mg aluminium sulphate (AS), 500mg polymeric ferric sulfate (PFS) and 20mg polyacrylamide (PAM) per liter waste sludge.On this basis, three types of flocculants were added into MBR with the optimum dosage, and the influence of flocculants on EPS, SMP, colloid-TOC, RH and on membrane fouling in MBRs was investigated. The experimental results indicate that the period of filtration with constant flux divided into two phases which include slow stage and sudden rise in TMP. The membrane fouling rate (Γ45) of MBRs is dependent on slow stage. The membrane fouling rate of flocculant added MBR is longer than controlled trials noticeably, and different flocculants have different effects on the membrane fouling rate. Three types of flocculants arranged according toΓ₄₅ is PFS > PAM > AS, and the optimum dosages of the three types of flocculants is PFS 600mg/L, PAM 20mg/L and AS 450mg/L.Compared with controlled MBR, the concentration of supernatant DOC (dissolved organic carbon) changed little, the concentration of SMP (Soluble microbial product) and supernantant colloid TOC (total organic carbon) dropped noticeably, and then concentration of EPS (extracellular polymeric substances) and RH (relative hydrophobicity) increased. The removal of SMP is dependent on supermantant colloid TOC, and the ability of SMP and supermantant colloid TOC removals of different flocculants is that PFS is the best one, and PAM is better than AS. Three types of flocculants arranged according to the ability of EPS and RH increases is PFS > AS > PAM.The collected data of EPS, SMP, supermantant colloid TOC, RH andΓ45 were analyzed by multiple regression analysis, and the results show that the key factor of mitigate membrane fouling rate of aluminium sulphate (AS) is EPS, SMP for polymeric ferric sulfate (PFS) and RH for polyacrylamide (PAM). AS largely depend on charge neutralization to remove SMP and supermantant colloid TOC from liquid, and meanwhile, AS force microorganism to produce more EPS. With the growth of EPS, the relative hydrophobicity of sludge increase, and therefore, the membrane fouling rate was dropped. EPS can express AS's capacity for membrane fouling. PAM reduce large molecule from liquid by bridging action. The ability of flocculation of PAM is better than AS. So, PAM can largely improve membrane fouling in MBRs, and SMP can express its capacity for membrane fouling. High valence positive charge of PFS can strongly absorb large molecule in liquid, and transfer to the surface of the sludge. Meanwhile, PFS stimulate microorganism to produce more extracellular polymeric substances, thus the relative hydrophobicity of sludge increased. The ability of mitigate membrane fouling PFS is the best, and relative hydrophobicity (RH) can express its capacity for membrane fouling.