Effect of Chemical Oxygen Demand (COD) to Sulfate (SO4-2) ratio on sulfate reduction in anaerobic digestion of sulfate rich wastewater

Abstract

Effluents discharged from some industries are both high in chemical oxygen demand. If sulfate is not treated up to the standards before discharging, it will contaminate with the surface or ground water bodies. Although sulfate does not cause any direct impact on the environment, biological transformation of sulfate under anaerobic conditions produces toxic hydrogen sulfide. There exists a competition between Sulfur Reducing Bacteria (SRB) and Methanogenic Bacteria (MB) for same available substrate. COD/SO4-2 ratio represents the fraction of electron flow for SRBs and MBs, which determines the sulfate reduction. This research study, investigates the influence of COD/SO4-2ratio on the performance of the sulfate reduction in anaerobic digestion. Laboratory experiments were conducted for high and low concentrations of substrate loading at COD/SO4-2ratioof 7 and 3. Total percentage sulfate reduction for high and low concentrated synthetic wastewater at same COD/SO4-2 of 7 and 3are 50.5%,48.9%, 24.8% and 4.8%respectively. The average sulfate reduction rates for high and low concentrations at COD/SO4-2 of 7 and 3 are 111 ± 11.1 mg/l/d and 55 ± 5.8 mg/l/d, 62 ± 5.4 mg/l/d and 10 ± 1.6 mg/l/d respectively. Experimental results indicated that percentage sulfate reduction is efficient at COD/SO4-2 of 7 than at 3 and rate of sulfate reduction is high in high concentrated influent than low concentrated influent, while the COD/SO4-2 is kept at a constant. Therefore when these results are commercialized, for sulfate rich wastewater treatment, precautions have to be taken to add sufficient amount of electron donors to maintain the COD/SO4-2ratio for complete sulfate reduction.