Influence of Mg2+ catalyzed granular sludge on flux sustainability in a sequencing batch membrane bioreactor system

The present study was designed to achieve steady state high membrane flux values by controlling the organic and inorganic foulants during MBR operation. Three granular sludge sequencing batch membrane bioreactors, namely, R-1, R-2 and R-3, were run at a constant flux of 40 LMH for 90 days. R-1 was operated without the addition of Mg2+, R-2 with a continuous dosage of 50 mg/L Mg2+, whereas R-3 was run with stoichiometric amounts of Mg2+ (18-22 mg/L) in correlation with SMP contents. The particle sizes of aerobic granules were approximately 725 and 600 μm in R-2 and R-3, respectively, compared with 250 μm in R-1. The ratios of EPS proteins/polysaccharides in R-1, R-2 and R-3 were 1.63, 3.90 and 3.76, respectively, whereas the SMP concentrations in R-1, R-2 and R-3 were 40, 5 and 5 mg/L, respectively. The results highlighted that in R-3, the controlled addition of Mg2+ along with the emergence of aerobic granules tremendously increased the membrane permeability, which was approximately 6 and 3 times higher than R-1 and R-2, respectively. In R-3, a molar ratio of 1:2 between Mg2+ and SMP was found to be optimal for the successful sustainability of high flux values during the long term membrane treatment. FTIR, SEM and ICP spectroscopic investigations revealed that the deposition of fine sludge flocs and high amounts of proteins and polysaccharides on the membrane surface in R-1 and Mg2+ in R-2 were mainly responsible for their low permeability.