Zinc removal in anaerobic sulphate-reducing liquid substrate process

Zinc and sulphate removal from synthetic wastewater was investigated by using four laboratory parallel upflow-mode reactors, referred to as R1 to R4, where R1 contained carriers to retain biomass, and R2–R4 were operated as suspended reactors. All reactors were inoculated with anaerobically digested cow manure. Initially, R1 and R2 were fed glucose- and sulphate-containing feed for 48 days. Subsequently, all reactors received wastewater with zinc concentrations of 50 mg/L in R1–R3 and 200 mg/L in R4, and were operated for an additional 96 days under controlled hydraulic retention times, organic loading rates, and sulphate loads. Results indicated that during the runs, 30–40% of sulphate and over 98% of zinc were effectively removed, while up to 150–200 mg H2S was produced in all reactors, with effluent pH dropping to between 3 and 5. Notably, significant fluctuations in zinc removal were observed only in the latter part of the runs, with R1 and R2 achieving zinc levels below detection limits. Qualitative X-ray diffraction analysis suggested the formation of ZnS as the primary precipitate, supporting simultaneous zinc and sulphate removal through sulphide precipitation.