EPS Distribution Characteristics And Membrane Fouling Control Mechanism Of A Hybrid Membrane Bioreactor

This work was supported by the National Natural Sciences Foundation of China (50138020) and the National Natural sciences Foundation of China(50621140001).A hybrid membrane bioreactor(HMBR) which was developed by introducing the biofilm carriers into a conventional membrane bioreactor(CMBR) was exploited in the treatment of municipal wastewater and attention was mainly focused on its extracellular polymeric substances(EPS) distribution characteristics and membrane fouling control mechanism.Comparing with the CMBR which has only suspended biomass in the reactor,the HMBR has additional attached biomass and thus represented a good performance of membrane fouling control.Results are summarized as follows:1.Through the comparative experimental study between the CMBR and HMBR, the contaminants removal enhancements by the biocarriers was studied.Results showed that,the HMBR can remove the organic substances effectively,COD and BOD5 removal was 94.2%and 98.5%,respectively,which was both enhanced by 3.8% comparing with the CMBR;the HMBR can remove the nutrients effectively,NH₄⁺-N, TN and TP removal was 97.2%,51%and 81.5%,respectively,which was enhanced by 4.2%,13.7%and 1.7%,respectively,comparing with the CMBR;the turbidity removal by the HMBR was as same as the CMBR while the color removal by the HMBR was enhanced by 8.2%comparing with the CMBR;the additional biofilm and the increased total biomass in the reactor was the main reason that the HMBR can enhance the contaminants removals.2.The membrane resistances in the CMBR and HMBR was compared and results showed that,with the increase of attached biomass in the HMBR,the transmembrane pressure(TMP) increased much slower than that of the CMBR.When the TMP reached to 20 kPa,the CMBR lasted only for 57 days,while the HMBR lasted for 92 days in the first period after chemical cleaning.In the second period,the TMP in the HMBR was only 16 kPa when the reactor was operated for 140 days.These evidence indicated that the HMBR can retard membrane fouling effectively.3.The EPS distribution characteristics in the CMBR and HMBR was studied and results showed that,among three EPS namely the soluble one(S-EPS),the loosely bond one(LB-EPS) and the tightly bond one(TB-EPS),TB-EPS was the main component without reference to the extraction method and accounted for more than 60%of the total.The S-EPS and LB-EPS in the HMBR were both decreased by about 42%and the TB-EPS in the HMBR was decreased by about 2%comparing with the CMBR,indicating that the HMBR can remove EPS effectively,especially for S-EPS and LB-EPS.4.The effect of bound EPS(B-EPS) and S-EPS on the membrane resistance was analyzed and results showed that,B-EPS had a tightly correlation with sludge physical properties.With the decrease of B-EPS,the sludge bioflocculation and settleability was enhanced,the floc diameter was increased and the floc was more compact.B-EPS was subdivided into LB-EPS and TB-EPS.LB-EPS had a more tightly correlation with the sludge properties than TB-EPS.After the same operational period,the cake layer resistance R_c in the HMBR accounted for 43%of that in the CMBR.It can be concluded that,the decrease of B-EPS enhanced the sludge properties and finally slowered the increase of the cake layer resistance,this was the main reason that the HMBR can retard membrane fouling effectively.For S-EPS,its decrease was supposed to affect the pore resistance Rp significantly.After the same operational period,R_p in the HMBR accounted for 56%of that in the CMBR.This was another important reason that the HMBR can retard membrane fouling effectively.5.The constituents of total membrane resistance in the CMBR and HMBR was studied and results showed that,among three membrane resistances namely intrinsic membrane resistance R_m,pore resistance R_p and cake layer resistance R_c,the last one was the main fraction and accounted for more than 70%of the total.Therefore,a cake layer resistance model in the HMBR and CMBR was founded respectivly and was exploited to simulate the variation of TMP in the reactor.Results showed that,when R_m and R_p was ignored,the model calculation was more lower than the experimental results in the first period of filtration,indicating that R_m and R_p affected membrane fouling significantly in this period;the model calculation was nearly equal to the experimental results in the second period of filtration,indicating that R_c was the main fraction of the total in this period;in the end of filtration,the model calculation was more lower than the experimental results and Rp probably turned into the main fraction that affected TMP variation in this period.However,the model calculation reproduced that the HMBR can retard membrane fouling effectively and can prolong the filtration period.At the same time,through the contrast between the model calculation and experimental results,it can be concluded that if the membrane was operated in the period when Rc was the main fraction of the total,the TMP increase can be controlled effectively.