Coagulation and precipitation of a mechanical pulping effluent—I. Removal of carbon, colour and turbidity

This study examined the effects of metal coagulant concentration, effluent dilution and pH on the removal of total carbon, colour and turbidity from a combined bleached chemithermomechanical/thermomechanical (BCTMP/TMP) effluent. Both chloride and sulphate salts of iron and aluminum were effective in treating the mechanical pulping effluent in batch jar screening tests. pH had a pronounced affect on precipitation. In terms of minimizing the total carbon (TC), colour and turbidity levels, the optimum adjusted pH ranges were: 4.0–6.5 for ferric chloride, above 7.4 for ferrous sulphate, 5.0–6.0 for aluminum chloride and 5.8 to 6.8 for aluminum sulphate. Removal of TC, colour and turbidity of up to 88, 90 and 98%, respectively, were observed. The precipitation yielded a supernatant which was virtually clear and colourless.