BACKGROUNDCoal gasification wastewater (CGW) is the most important wastewater of the coal chemical industry and if disposed of without adequate treatment can cause serious environmental pollution. Membrane bioreactors (MBRs) have been widely used for high strength wastewater treatment; however, conventional MBRs potentially fail when the BOD/COD of the biologically pretreated CGW (BPCGW) is low. In the present study, strategies were employed to enhance MBR performance. RESULTSA long-term bio-augmented submerged membrane bioreactor (BSMBR) has been developed for treatment of BPCGW. With the optimal methanol dosage of 30%, hydraulic retention time (HRT) of 24 h, granular activated carbon (GAC) dosage of 5 g L-1 and NaHCO3 dosage ratio of 3:1 (amount NaHCO3 to NH4+-N ratio, mol:mol), the concentrations of COD, total phenols and NH4+-N reached 39, 23 and 11 mg L-1 with removal efficiencies of 74%, 40% and 46%, respectively. HRT of 24 h and GAC dosage of 5 g L-1 facilitated membrane fouling mitigation while the co-metabolism process with methanol speeded up membrane fouling. High NaHCO3 dosage ratio (2:1) promoted nitritation-type nitrification. CONCLUSIONThis study suggests that it is technically feasible to treat BPCGW by BSMBR, which can provide an environmentally acceptable way to further engineering application in coal chemical industries. (c) 2015 Society of Chemical Industry