Coupling of solar-assisted advanced oxidative and biological treatment for degradation of agro-residue-based soda bleaching effluent.

This study evaluates the effect of integrated solar-assisted advanced oxidation process (AOP) and biological treatment on the extent of degradation of effluents from chlorination (C) and first alkaline extraction (E(1)) stages of soda pulp bleaching in agro-residue-based pulp and paper mill. Biodegradation of the effluents was attempted in suspended mode using activated sludge from the functional pulp and paper industry effluent treatment plant acclimatized to effluents in question. The photocatalytic treatment was employed using zinc oxide (ZnO) in slurry mode for decontamination of effluents in a batch manner and the degradation was evaluated in terms of reduction in chemical oxygen demand. The biological treatment (24 h) of C and E(1) effluent resulted in 30 and 57 % of degradation, respectively. Solar-induced AOP of C and E(1) effluents resulted in 53 and 43 % degradation under optimized conditions (2.5 g L(-1) ZnO at pH 8.0) after 6 h of exposure. For C effluent, a short duration of solar/ZnO (1 h) prior to biological treatment reduced the time required at biological step from 24 to 12 h for almost same extent (92 %) of degradation. However, sequential biological treatment (24 h) followed by solar/ZnO (2 h) resulted in 85.5 % degradation. In contrast, in the case of E(1) effluent, sequential biological (24 h)-solar/ZnO (2 h) system effectively degrades effluent to 95.4 % as compared to 84.8 % degradation achieved in solar/ZnO (2 h)-biological treatment (24 h) system. In the present study, the sequencing of photocatalysis with the biological treatment is observably efficient and technically viable process for the complete mineralization of the effluents.