Simultaneous removal of arsenic, iron and nitrate 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, iron 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, iron and nitrate from contaminated groundwater, 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 on simultaneous removal of arsenic, iron and nitrate to meet the drinking water standards in an attached growth bioreactor. An attached growth reactor was fabricated using Perspex cylinder, inoculated with mixed microbial culture and operated in downflow mode in absence of oxygen at 30 °C for more than 500 days under varying influent arsenate (500–1500 μg/L as arsenic), iron (2.0–10 mg/L) and nitrate concentrations (50–200 mg/L) in simulated groundwater, and empty bed contact time of 45–120 min. Acetate was used as external carbon source and electron donor in this project. Iron and arsenic removal met the drinking water standards, while complete nitrate removal was observed at all tested concentrations. Roles of sulphate concentration and empty bed contact time were found to be very important to improve efficiency of the reactor. The reactor was also fed with real groundwater containing arsenic and iron of up to 295 μg/L and 13.2 mg/L respectively. The reactor could remove both the contaminants to meet drinking water standards after supply of sufficient amount of sulphate and adjustment of empty bed contact time. Results of field emission scanning electron microscopy and X-ray diffraction analyses suggested that precipitation in the form of arsenosulphides (orpiment and realgar) and iron sulphides (pyrite and pyrrohite) were the removal mechanisms for arsenic and iron. Hence, this study demonstrates an excellent potential of application of biological process on simultaneous removal of arsenic, iron and nitrate to meet drinking water standards.