Role of Magnesium in Mitigating Membrane Fouling in Membrane Bioreactors

In this study, membrane fouling in membrane bioreactors (MBRs) was investigated with focus on the impact of three Mg/Ca molar ratios (1/5, 1/1, and 5/1). Calcium concentration was maintained constant at around 1.7 mM. The MBRs were operated at a hydraulic retention time (HRT) of 4.5 hours, a solids retention time (SRT) of 15 days and a constant flux of 14 LMH. Membrane fouling rate was almost the same for the Mg/Ca ratio of 1/1 and 1/5. The increase in the EPS and the concomitant decrease in SMP concentrations both as protein and carbohydrates due to divalent cation binding and enhanced bioflocculation were not significant enough to change the membrane fouling rate. More hydrophobic EPS was formed due to lower divalent cations at the low Mg/Ca ratio, resulting in higher EPS deposition on the membrane surface and membrane fouling. Comparison of the reactors with Mg/Ca ratios of 5/1 and 1/5 showed that the fouling rate is lower in the reactor with the higher Mg/Ca ratio.