Coagulation and anaerobic digestion of tannery wastewater

Coagulation and digestion of tannery wastewater were investigated. Coagulation at pH 7.5 removed at least 32% of chemical oxygen demand (COD), 64% of suspended solids (SS), 77% of chromium, 80% of sulphide and 85% of colour. The results indicated that coagulation considerably reduced the concentration of sulphide and improved the anaerobic treatability. Both aluminium sulphate and ferric chloride coagulants provided excellent sulphide removal (>71%), even at a low dose of 50 mg l(-1). Subsequent anaerobic digestion was carried out on initial samples and supernatants from the coagulation at an hydraulic retention time (HRT) of 10 days with a loading rate of 0.33 kg COD/m(3)/day. Incorporation of coagulation prior to digestion resulted in an increased capacity of the digesters and improved digestion performance. A methane yield of 0.21 CH4/(g COD removed) was achieved, while COD removal was 77% and COD removal rate was 0.24 kg COD/m3/day. The combined system provided a residual COD of less than 760 mg l(-1) and a residual sulphide of less than 200 mg l(-1). The results also demonstrated that a sulphide concentration in excess of 250 mg l(-1) completely inhibited methane production. These results may lead to a reduction in waste disposal costs for tannery companies.