Impact of influent feed and mixed liquor characteristics on fouling in submerged membrane bioreactors for wastewater treatment

Membrane fouling was investigated in membrane bioreactors (MBRs) with respect to characterization of membrane foulants, the relation of sludge properties, nature and extent of fouling and fouling mechanisms. The physical and biochemical properties of sludge, including floc size distribution, relative hydrophobicity, extracellular polymeric substances (EPS), soluble microbial products (SMP) and modified fouling index (MFI) were examined to assess their impact on membrane fouling in the present research.;Influent feed characteristics studied include calcium concentration, Mg/Ca, monovalent to divalent (M/D) cation and protein/carbohydrate (P/C) ratios. Improvement in membrane filtration performance was achieved by introduction of 280 mg/L of calcium in the feed due to formation of larger flocs and reduction in SMP concentrations. However, higher influent calcium concentrations (at around 800 mg/L) resulted in substantial inorganic fouling of the membrane. It was revealed that the membrane fouling was lower for the MBR at Mg/Ca ratio of 5:1 compared to Mg/Ca ratios of 1:5 and 1:1. The increase in the EPS concentration, the larger floc size, and lower SMP concentration and hydrophobicity by increasing magnesium concentration was very likely responsible for lower fouling rate at higher Mg/Ca ratio. The fouling rate for the MBR at M/D ratio of 3:1 was found to be higher compared to 1:1 due to decrease in EPS concentration and size of the activated sludge flocs as a result of replacement of divalent cations with sodium ions. Higher SMP concentration and RH was also found responsible for the increase the fouling resistance at M/D of 3:1 relative to M/D of 1:1.;Membrane fouling was found to increase by increasing the P/C ratios from 2:1 to 8:1. P/C ratios correlated strongly with EPS composition. Higher hydrophobic protein EPS concentrations at the higher P/C ratios caused higher attachment of EPS on membrane surface and reduced membrane permeability. Furthermore, increased concentrations and RH of SMP at higher P/C ratios of 8:1 accelerated the fouling process. Increasing the P/C ratio of the feed slightly increased the P/C ratio of EPS, but the protein and carbohydrates decreased. It can be concluded that there are several factors that play a role in the changes of EPS concentration and composition and consequently impact MBR fouling.;Compared to a conventional MBR, higher fouling rates were observed in a simultaneous nitrification and denitrification (SND) MBR despite the presence of larger floc size in the SND reactor. Higher EPS concentration and RH was found to be responsible for higher attachment of EPS on the membrane surface for the SND MBR and increased fouling rates. Furthermore, higher retention of SMPs by the membrane pores proved that the pore blocking was higher in the case of SND MBR. For MBRs, rejection and pore retention of carbohydrate SMP was found to be the highest, indicating that carbohydrates were the major contributors to pore blocking resistance.