Long-Term Effect of Seawater on Sulfate Reduction in Wastewater Treatment

In sulfate-rich saline wastewater, biological sulfate reduction can occur spontaneously or applied beneficially for its treatment. Although application of sulfate-reducing bacteria (SRB) on saline domestic wastewater, obtained by seawater-based toilet flushing for instance has been suggested repeatedly, no study on the effect of applicability in the complete range of seawater concentration has been performed (salinity range of 0-3.5% and sulfate concentration of 0-2500mg/L). The present article examines the long-term effect of seawater on biological sulfate reduction using three sequencing batch reactors fed with different volumetric fractions of seawater and freshwater. To evaluate this effect, the effluent quality, sulfate reduction rate, and microbial population in the reactors were investigated. The biomass-specific biological sulfate reduction rate decreased significantly (approximate to 45%) when salinity increased from 0.7% to 3.5%, while total organics removal remained unaffected. Differences in microbial population were observed for sludge adapted to 0.7% or 3.5% salinity. Even at high salinity (3.5%) and moderate temperature (20 degrees C), biological sulfate reduction occurs and organics are removed sufficiently (>97%) by SRB. Therefore, biological sulfate reduction can be considered as a feasible process in treatment of saline- (3.5%) and sulfate (2500mg/L)-rich wastewater.