Thermophilic sulfate reduction process with molasses as an electron donor was investigated for the removal of zinc from rayon industry wastewater. Sulfide rich effluent from the process was used to remove zinc as zinc sulfide precipitate. The investigations with sulfate reduction process were conducted with synthetic (Stage I) as well as real wastewater from Rayon industry (Stage II) as feed. The effect of feed COD: sulfate ratio, which is the key factor for operating the sulfate reduction process, was focused on in this study. The experimental results showed that the process could achieve a high sulfate conversion rate of about 7.22 +/- 1.91 g SO4 l(-1).d(-1) at COD: sulfate ratio of 1.5:1 and 7.20 +/- 2.27 g SO4 l(-1).d(-1) at COD: sulfate ratio of 3:1 during stage I and stage II, respectively. At the end of the operation of stage II, a maximum sulfide production of 496.2 mg S l(-1) was achieved at COD: sulfate ratio of 3:1. Furthermore, sulfide rich effluent from sulfate reduction process was used for zinc sulfide precipitation. The results showed that more than 95% of zinc was removed from a high sulfate-containing wastewater.
A biological sulphate reduction process, with molasses as an electron donor, was used for the removal of zinc and sulphate from Rayon industrial wastewater. The process involved reduction of sulphate to sulphide under anaerobic conditions. The sulphide-rich effluent was used to remove zinc as zinc sulphide precipitate. The investigation was conducted at pilot scale with real wastewater from the Rayon industry as feed. The effects of sulphate loading rate and temperature of feeding wastewater were evaluated. The experimental results showed that there was no significant difference in sulphide production when the reactor was operated at 50 +/- 2 degrees C and 65 +/- 2 degrees C. Sulphide production was in the range of 500-515 mg L(-1). In addition, an increase in sulphate loading rate from 6.3 +/- 0.7 kg SO4 m(-3) d(-1) to 14.9 +/- 2.4 kg SO4 m(-3) d(-1) resulted in a dramatic decrease in sulphate removal efficiency. Furthermore, zinc sulphide precipitation at pH 7 removed more than 96% of zinc.
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