| Membrane fouling has become a major obstacle to the application of membrane bioreactors (MBR). Carriers typically used in MBR have two effects in relation to membrane fouling. One, carriers’ direct physical effect, which means the cake layer can be reduced by the collision between the membrane surface and the carriers. Two, carriers’ biochemical effect, which means carriers can influence membrane fouling by changing sludge characteristics. To investigate the contribution of carriers’ two effects on membrane fouling, three sets of submerged membrane bioreactors (MBRs), including one suspended-growth MBR, called SMBR, and two attached-growth MBRs (one called AMBR, and another with baffles called AMBRb) were operated at various media volume fractions. The results are as follows:1. Three reactors showed high removal efficiencies in all operating conditions, with COD and NH3-N over 95% and 92%, respectively. This finding suggests that the three membrane bioreactors worked well.2. The operation times of AMBR and AMBRb were 4.2 and 3.5 times longer, respectively, than that of SMBR at an optimum media volume fraction of 30%.3. Resistance analysis showed that the total resistance (Rsm) reduced by carriers accounted for a large proportion (from 60% to 78%) of total resistance (Rt), indicating that the carriers positively mitigated membrane fouling. In addition, the resistance reduced by the carriers’ effect on sludge characteristics (Rcm) accounted for a significant proportion (from 84% to 91%) of Rsm,which further demonstrated that the carriers’biochemical effect on sludge characteristics played a more dominant role in reducing membrane fouling compared with the carriers’physical effect.To investigate the effects of sludge characteristics on membrane fouling, Linear Regression analysis on sludge characteristics including Particle size, soluble microbial product (SMP), sludge volume index (SVI), filament index (FI) and bound Extracellular polymeric substance(bEPS) were conducted. The results are as follows:1. Sludge Particle size decreased sharply from 64 to 40 μm with the increase in media volume fractions, and increased slightly at media volume fractions greater than 20%. Meanwhile, no significant correlation exhibits between sludge Particle size(r= 0.73, P= 0.137) and membrane fouling rate (MFR). These results suggested that the sludge Particle size has no significant effect on membrane fouling.2. The amount of SMP in both AMBR and AMBRb only slightly differed from that in SMBR. No significant correlation was observed between SMP (r= 0.09, P= 0.455) and MFR. SMP in this study was suggested to have no apparent effect on membrane fouling.3. The amounts of bEPS in AMBR and AMBRb were observed to be the lowest than that of SMBR at the media volume fraction of 30%. which accounted for 72% and 76% of bEPS in SMBR, respectively. The amount of bEPS (r= 0.94, P= 0.032) and bEPSp (r= 0.96, P= 0.021) demonstrates a significant positive correlation with MFR, confirming that bEPS in the study heavily affected membrane performance, and that protein rather than carbohydrate was a major factor in membrane fouling.4. FI (r= 0.66, P= 0.169) had no significant correlation with MFR, that is, FI had no apparent effect on the membrane fouling.5. A huge increase in SVI was observed in AMBR and AMBRb compared with SMBR, peaking (from 60 mL/g to 108 and 102 mL/g, respectively) at the media volume fraction of 30%, indicating that the settling characteristics of sludge floc in AMBR and AMBRb were poorer than that in SMBR. SVI (r=-0.78, P= 0.112) was observed to have no statistical correlation with MFR, indicating that the settling characteristics had no remarkable effect on membrane fouling.Above all, the results showed that the protein in bEPS primarily contributed to membrane fouling. |
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