A combined anaerobic-aerobic process for the cotreatment of effluents from a piggery and a cheese factory

Food processing industries can cause serious environmental problems. In particular, piggeries and milk/cheese factories need improved treatment of their waste-water, especially if land-spreading of wastes is not possible, limited or forbidden. This paper reports on a study of a treatment plant whose originality lies in the combination of two treatment stages, the first anaerobic, the second aerobic. The technical problems encountered during the installation of the digester and the optimization of the working of the whole treatment plant during 2 years are analysed and discussed. Besides a daily monitoring of the anaerobic stage, complete measurements were carried out on the treatment process in order to control the steady state performance of the plant. The biogas produced is used in a cheese factory and allows propane consumption to be reduced by 26%. The anaerobic digestion of the slurry, followed by a settling process, leads to changes in the nature of the effluent entering the aerobic stage. In particular, the C/N ratio would be unfavourable for the denitrification process, without the cheese factory washing-water, which brings the necessary organic carbon. The reduction in organic load achieved by the digester (94% reduction of BOD and 74% of soluble COD) allows sufficient concentrations to maintain a good aerobic oxidation and a good nitrification process in the oxidation ditch. Overall the whole treatment process achieves 98% COD reduction, more than 99% BOD reduction and 93% N reduction.