The feasibility of sequential sodium hypochlorite decolorisation and anaerobic biological treatment, as a low-cost process for decolorisation of extraction-stage bleach plant effluent, was studied. Treatment of E-stage effluent with chlorine can reduce colour by 50-90%, depending on the reaction pH. At pH 10, where the hypochlorite ion is the reactive species, the highest levels of colour removal were observed and, compared with pHs 4 and 7, the effluent produced was less inhibitory to anaerobic bacteria. A chlorine dosage of 0.1 kg active Cl/kg colour was sufficient to achieve about 90% colour removal, although the AOX concentration of the wastewater increased by about 50%. Subsequent anaerobic treatment of the decolorised wastewater was able to reduce the AOX concentration to close to its original level. Only a small amount of colour reversion was observed and an overall colour removal of about 80% was achieved. The hypochlorite treatment caused a substantial increase in the concentration of low molecular mass AOX, which was amenable to anaerobic dechlorination. Anaerobic treatment of untreated E-stage effluent had no effect on the concentrations of AOX or colour. These results suggest that this treatment method may have potential as a low cost technology for providing a short-term solution to bleach plant colour problems.
