Anaerobic biodegradability and toxicity of wastewaters from chlorine and total chlorine-free bleaching of eucalyptus kraft pulps

Abstract

Chlorine bleaching effluents are problematic for anaerobic wastewater treatment due to their high methanogenic toxicity and low biodegradability. Presently, alternative bleaching processes are being introduced, such as elemental chlorine-free (ECF) and total chlorine-free (TCF) bleaching. The methanogenic toxicity and anaerobic biodegradability of ECF and TCF bleaching effluents from oxygen-delignified eucalyptus Kraft pulp were compared with those of different chlorine bleaching sequences. The effluents from chlorine and ECF bleaching sequences had similar methanogenic toxicities, with 50% inhibiting concentrations (50%IC) of 0.65–1.48 g-COD/litre. Only the TCF bleaching effluent was distinctly less toxic, with a 50%IC of 2.3 g-COD/litre. The fact that the ECF bleaching effluent was not less toxic than that of chlorine bleaching, as well as the residual toxicity of TCF, indicate that other substances aside from organohalogens contribute to the high methanogenic toxicity in bleaching effluents. Literature data is presented suggesting that wood resin compounds released by alkaline extraction stages might be important inhibitory substances in all bleaching processes. Oxygen-delignified bleaching effluents of eucalyptus were found to be highly biodegradable. When tested at subtoxic concentrations, the chlorine and TCF bleaching effluent COD were anaerobically degraded by 67 and 75%, respectively. In both cases, methane production accounted for most of the COD removed. Therefore, the TCF process also enhances the anaerobic biodegradability of bleaching effluents to a small extent.