The effect of microbial entrapment method on sulfate reduction and sludge production rates of sulfate-reducing bacteria at different COD/SO42- ratios.

This study investigates the effect of chemical oxygen demand/sulfate (COD/SO42-) ratios on sulfate reduction performance in both entrapped and non-entrapped sulfate-reducing bacteria (SRB) systems for treatment of mining-influenced waters. Two types of entrapped SRB systems were evaluated: chemically crosslinked (C-ESRB) and physically crosslinked via freeze-thaw (FT-ESRB), along with a non-entrapped SRB system as the control. At a COD/SO42- ratio of 2.0, C-ESRB and FT-ESRB achieved significantly higher sulfate reduction rates of 5.82 ± 0.73 g SO42-/L/day and 5.73 ± 0.18 g SO42-/L/day, respectively, exceeding the non-entrapped SRB system (1.56 ± 0.23 g SO42-/L/day) by more than threefold. When the COD/SO42- decreased from 2.0 to 1.5, C-ESRB exhibited the highest sulfate reduction rate (4.70 ± 0.54 g SO42-/L/day) due to lower acetate accumulation. Both entrapped systems maintained structural integrity over 169 days of operation, with FT-ESRB demonstrating greater mechanical strength and stiffness. Also, entrapped systems supported higher SRB community abundance and significantly reduced sludge production, which were 88 ± 4% lower in FT-ESRB and 82 ± 4% lower in C-ESRB, as compared to the non-entrapped SRB system. These findings demonstrate the benefits of microbial entrapment for enhancing sulfate reduction performance, process stability, and biomass retention, and suggest that further evaluation of SRB-based technologies towards mining-influenced water treatment applications is warranted.