Anaerobic treatment of hexavalent chromium in industrial effluent using two-phase anaerobic sequencing batch reactor

Hexavalent chromium pollution is a major environmental concern due to its toxicity and extensive use in industry. Industrial effluents containing Cr(VI) have been traditionally treated by chemical reduction, followed by precipitation, or more recently, by removal using non-viable biosorbents. This study investigates the use of a two-phase anaerobic sequencing batch reactor (ASBR) for the treatment of Cr(VI) contaminated wastewater. Simulated wastewater containing potassium chromate and sucrose was introduced into reactor one which contained phase I optimized biomass. Complete sorption and reduction of Cr(VI) to Cr(III) occurred in reactor one, which had a 1:1 volume of treated wastewater to settled cheese whey biomass granules. The chromium-free effluent from reactor one, rich in volatile fatty acids (VFA) was the feed stock or influent for reactor two, where optimum methane production was maintained. Reactor one was operational for about 40 days with a Cr(VI) loading of 25 mg/L-day. Cr(III) derived from the reduction of Cr(VI) occurred in the effluent of reactor one at levels between 1 to 2.5 mg/L only from about day 28 to the end of the experiment. Selective sequential extraction (SSE) of the spent biomass showed that 45% of the added chromium was in the form of insoluble Cr(III) hydroxides, and the remaining 55% as organo-Cr(III) complexes. VFA levels were eventually reduced in reactor one by about 50%, and soluble COD removal by about 65%. Average methane content in the reactor two biogas was greater than 80%. Based on sorption studies, and using a novel approach whereby sorbed Cr(VI) was interpreted as a "dose" in order to obtain various toxicological indices, models were developed for estimating the amount of Cr(VI)-wastewater that could be treated using plant size unit operations. Leaching tests carried out on spent biomass from reactor one over a five month period demonstrated the non-leacheability of chromium. This study indicated the potential feasibility of using a two-phase ASBR system for the co-treatment of inorganic Cr(VI) contaminated wastewater and high organic loading effluents.