Simultaneous removal of arsenic and nitrate in absence of iron in an attached growth bioreactor to meet drinking water standards: Importance of sulphate and empty bed contact time

Abstract

Abstract There are several reports on simultaneous occurrence of arsenic and nitrate in drinking water sources especially in groundwater at wide range of concentrations. However, there is no report available, so far, on simultaneous successful removal of arsenic and nitrate from contaminated groundwater in absence of iron, and effects of one contaminant on the overall performance of a biological reactor. The present study investigates the roles of sulphate and empty bed contact time (EBCT) on simultaneous removal of nitrate and arsenic to meet the drinking water standards in an attached growth bioreactor in absence of iron. An attached growth reactor (AGR) was fabricated using Perspex cylinder, inoculated with mixed bacterial culture and operated in downflow mode in absence of oxygen at 30 °C for more than 400 days under varying influent arsenate (200–750 μg/L) and nitrate concentrations (50–200 mg/L), and EBCT of 45–60 min. Acetate was used as external carbon source and electron donor in this study. Complete nitrate removal was observed at all tested concentrations. Arsenic removal was high (up to 99.8%) and was well below drinking water standards from initial concentrations of up to 750 μg/L. The arsenic removal efficiency was found to depend on sulphate reduction and EBCT of the reactor. Results of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analyses suggested that arsenic precipitation in the form of arsenosulphides (orpiment and realgar) was the removal mechanism.