Biological attenuation of arsenic and nitrate in a suspended growth denitrifying-sulfidogenic bioreactor and stability check of arsenic laden biosolids

Co-occurrence of arsenic and nitrate in groundwater sources at wide range of concentrations are reported. In this work, performance of suspended growth semi-batch reactor was assessed for co-removal of arsenic and nitrate from simulated groundwater to meet the drinking water standards in absence of iron. The bioreactor was inoculated with mixed bacterial culture and operated in absence of oxygen for more than 450 days under varying influent arsenate (200-800 µg/L), nitrate concentrations (50-250 mg/L), and hydraulic retention time of 3-6 days. Complete nitrate removal was observed at all tested concentrations. Arsenic removal was found to meet drinking water standards from initial concentrations of up to 600 µg/L. The extended toxicity characteristic leaching procedure leaching experiments indicated that arsenic laden biosolids would not constitute a hazardous waste. The arsenic leaching was found to be increase with increase in dissolved oxygen and the final leachate concentrations of arsenic were below 150 µg/L. The leaching experiments suggested maintaining non-alkaline conditions for minimum arsenic release from arsenic biosolids formed under sulfidogenic conditions. This study is first to report that nitrate and arsenic can be simultaneously removed to meet drinking standards in a suspended growth bioreactor.