Colour formation in pulp and paper wastewaters during biological treatment

Wastewater colour is of concern in the pulp and paper industry as regulations place strict limits on wastewater quality and aesthetic appeal. In some treatment systems increases in colour occur, meaning that maximum daily limits are exceeded and mills may fail discharge standards. This study specifically addressed the colour formation issue for the first time.;A multi-site industrial survey confirmed previous anecdotal evidence that colour formation of up to 40% occurs in whole mill effluents treated using aerated stabilisation basins. In-depth field studies of a specific treatment system showed that colour formation was initiated in low dissolved oxygen zones. The importance of biological anaerobic activity was confirmed using continuous and batch reactors, where colour formation was shown to be correlated to oxygen availability.;Batch studies showed that in-mill wastewaters high in lignin-derived material, such as bleaching or thermomechanical pulping effluents had greater colour forming potential. Wastewaters low in lignin, such as papermachine effluents, showed less colour forming potential. Combinations of effluents with high and low colour forming potential led to greater colour increases than predicted from the individual effluents alone.;Fractionation of pulp bleaching wastewaters based on molecular weight showed that colour formation occurs in high molecular mass (HMM) material. Absence of HMM material effectively removed the potential for colour increases during biological treatment.;Previously reported mechanisms ascribing colour formation to reactions between anaerobically produced monomers were not supported experimentally. Reactions between such monomers and HMM also did not produce colour increases.;Studies on fractionated bleaching wastewaters showed HMM acted as an electron acceptor for bacterial growth and could increase its colour through structural changes, such as the formation of quinonoid chromophores. Increases in colour were inhibited when electron acceptors of greater energy than HMM were present. Colour increases also correlated well with the availability and removal of suitable electron donor material.;Overall, this study of colour behaviour in these complex systems has presented complex challenges. Combining fundamental studies on colour function and behaviour with laboratory assay-directed studies and in-mill assessments will assist the industry in identifying realistic colour management strategies that provide overall environmental benefits to the sector.