Anaerobic treatment of kraft bleaching plant effluent

Anaerobic treatment of a kraft bleaching plant effluent was studied with focus on the removal of chlorinated organic compounds and biochemical O2 demand (BOD). Experiments were carried out using a laboratory-scale anaerobic fixed-film process operated at different hydraulic retention times (HRTs). The process efficiently removed a variety of chloro-organic compounds when the HRT was 10 h or longer. However, a very long treatment time was required for significant removal of chlorofor; in fact, at the shortest HRT tested (6 h), there was a net production of chloroform. The overall removal of organically bound chlorine, measured as adsorbable organic halogens, was about 50%, and 60% removal of the extractable organic chlorine was achieved at an HRT of 15 h. About 70% of the BOD7 and 20% of the chemical O2 demand (COD) were removed. Up to 0.19 Nl of methane was produced per gram of COD removed by the treatment. When th HRT of the anaerobic process was shortened from 10 to 6 h, the methane production ceased.