Enhanced performance is achieved as arsenate (AsO43-, As5+) rather than arsenite (AsO2-.As3+). Arsenite is therefore usually oxidized to arsenate prior to precipitation. Effluent arsenic levels of 0.05 mg/l are obtainable by precipitation of the arsenic as the sulfide by the addition of sodium or hydrogen sulfide at pH of 6 to 7. In order to meet reported effluent levels, polishing of the effluent by filtration would usually be required.