Microbial toxicity effects of reverse transported draw solute in the forward osmosis membrane bioreactor (FO-MBR)

Reverse transport of draw solute in an FO-MBR system can affect the microbial community and treatment efficiency. Eight commonly used inorganic draw solutes and four novel surfactant draw solutes were investigated for toxicity to Escherichia coli. A bench scale FO set-up with a flat sheet membrane was installed to measure the specific reverse solute transport (Js/Jw). Five different concentrations (0.005, 0.01, 0.025, 0.5 and 0.1 M) of all 12 draw solutions were prepared. E. coli micro-organisms were allowed to react with these solutions for 16 h at 35 °C and agar plates were developed, incubated and bacterial colonies were counted. Chlorides of calcium and magnesium depicted more support to growth as compared to sodium and potassium. However, all four chlorides may be used without inhibitory effects in the FO-MBR. Ammonium bicarbonate solution could only be used with a pledge of oxygen. Sulfate salts of ammonium, sodium and potassium can potentially be used in the FO-MBR. Surfactants are suitable alternatives; due to the high molecular weight of micellar aggregates, they exhibit a value for Js/Jw which is 100–150 times lower than that for inorganic salts, but they should be used at concentrations above their critical micellar concentration (CMC).

► Draw solutes were investigated for toxicity to biomass in FO-MBR.
► NaCl, MgCl2, KCl, CaCl2, (NH4)2SO4, Na2SO4, K2SO4 are suitable for use in FO-MBR.
► NH4HCO3 can be used with high dissolved oxygen and adequate mixing in FO-MBR.
► Surfactants have 100–150 times low Js/Jw then inorganic draw solutes.
► Surfactants can only be used at concentrations above their CMC.