Influences of sludge retention time on the performance of submerged membrane bioreactors with the addition of iron ion Original Research Article

A bench-scale submerged membrane bioreactor (MBR) treating synthetic wastewater with FeCl3 dose of 15 mg/L was operated under different sludge retention times (SRTs), i.e. 10, 20 and 30 d, in order to evaluate the impacts of SRT on the performance of MBRs with the addition of FeCl3. The results showed that the effluent quality was slightly influenced by SRT, while membrane fouling was effectively mitigated by lengthening SRT under the condition of FeCl3 adding. The concentration of soluble microbial products (SMP) significantly decreased as SRT extended from 10 to 30 d, which correlated well with the variation of membrane fouling. SMP played a more important role in membrane fouling in this study compared to other sludge properties. Fe accumulated in the MBR as SRT prolonged, and the accumulation of Fe enhanced flocculation and resulted in the decrease of MLVSS/MLSS ratio and the increase of particle size. The accumulated Fe in mixed liquor was mainly located in bound extracellular polymeric substances (BEPS). It is suggested that the negatively functional groups such as polysaccharide in SMP were adsorbed by the bridging effects of iron ion, thus leading to the reduction of SMP and increase of BEPS at longer SRT.