Decolourization and removal of chemical oxygen demand (COD) with energy recovery: Treatment of biodigester effluent of a molasses-based alcohol distillery using inorganic coagulants

The removal of molasses-derived colour and chemical oxygen demand from the biodigester effluent of a molasses-based alcohol distillery effluent treatment plant was studied using inorganic coagulants—FeCl3, AlCl3 and polyaluminium chloride (PAC). The coagulation/flocculation yield about 55, 60 and 72% COD reductions and about 83, 86 and 92% colour reductions, with the use of 60 mM/l AlCl3, 60 mM/l FeCl3 and 30 ml/l of polyaluminium chloride, respectively, at their optimum initial pH. The critical pH of the effluent–coagulant mixture plays a very significant role in the coagulation/flocculation process, with pH0 5.5 being the optimum for PAC. The solid residue, obtained by filtration and drying from the use of PAC has specific energy of 13.4 MJ/kg and can be used as a medium energy fuel material. The filtration characteristics of the flocculated effluent are poor. High COD reduction of the waste water by flocculation with PAC may be a better alternative to the conventional aerobic treatment process of the biodigester effluent.