Abstract:
To investigate the effects of COD and sulfate concentrations on sulfate reduction level and to estimate the concentration of effluent sulfide from anaerobic process. Three separately indentical laboratory scale UASB reactors were fed with a synthetic wastewater which was made from sucrose and sodium sulfate as organic carbon and sulfate source respectively. In all experiments, sodium bicarbonate was used as a buffering chemical. All reactors were seeded with 1-3 mm. diameter granulated anaerobic sludge. There were two sets of experiments in this study at ratio of COD/sulfate 4 and 2. In each ratio, five levels of 400, 600, 800, 1000 and 1200 mg COD/l were applied. At the ratio COD/sulfate of 4 experiment, the sulfate reduction efficiencies were 86.8, 89.3, 92.0, 91.9 and 92.7% and effluent sulfide/influent sulfate ratios were 0.264, 0.258, 0.268, 0.266 and 0.259 for COD concentrations of 400, 600, 800, 1000 and 1200 mg./l respectively. At the ratio COD/sulfate of 2 experiment, the sulfate reduction efficiencies were 91.1, 93.5, 94.3, 95.1 and 95.1 and effluent sulfide/influent sulfate ratios were 0.264, 0.273, 0.275, 0.262 and 0.270 for COD concentrations of 400, 600, 800, 1000 and 1200 mg/l respectively. The soluble organic COD removal efficiencies were over 95% in all experiments and % electron flow between methane producing bacteria and sulfate reducing bacteria were 82.0%MPB/18.0%SRB and 62.9%MPB/37.1%SRB for COD/sulfate ratio of 4 and 2 respectively. It showed that if there was no toxic effect from sulfide, the increased role of SRB would not affect soluble organic removal efficiencies of the system. In conclusion, the variation of COD and sulfate concentration in this study did not significantly affect the fairly high sulfate reduction efficiencies in the range of 86.6-95.1%. Moreover, the empirical equation for estimation of sulfide concentration from anaerobic process at the COD/sulfate ratio over 2 was obtained as follows: Effluent sulfide (mg/l) = 0.265 x Influent sulfate (mg/l).