Impact of calcium-to-magnesium ratio on the performance of submerged membrane bioreactors

Abstract

The performances of membrane bioreactors (MBRs) were compared in bench-scale submerged MBRs treating synthetic wastewater at three Ca:Mg ratios of 3:1, 1:1, and 1:3 with constant divalent cations of 200 mg/L. The dosages of Ca2+ were 150, 100, and 50 mg/L, corresponding to Mg2+ of 50, 100, and 150 mg/L for Ca:Mg ratios of 3:1, 1:1, and 1:3, respectively. The results showed that Ca:Mg ratio had a slight impact on effluent quality, while membrane fouling was effectively mitigated by enlarging Ca:Mg ratio due to the decline of fouling layer resistance. When Ca:Mg ratio declined from 3:1 to 1:3, the concentration of soluble microbial products (SMP) significantly increased, while the particle size of flocs decreased. SMP and particle size correlated well with the variation of membrane fouling. The carbohydrate in SMP and bound extracellular polymeric substances (BEPS) changed significantly with the decline of Ca:Mg ratio, while the protein in SMP and BEPS varied slightly, suggesting that the divalent cations were more prone to bridging carbohydrate compared with protein. In comparison with Mg2+, Ca2+ had a more significant impact on mitigating membrane fouling, because Ca2+ was more beneficial to binding and bridging negatively charged functional groups in extracellular polymeric substances such as carbohydrate.