Colour formation in pulp and paper wastewater treatment systems

Pulp and paper wastewaters are coloured, and further colour can be formed during biological treatment. Colour discharge increases the attenuation of sunlight, thus affecting the ecology and aesthetics of the receiving water. The higher reported colour formation in activated sludge systems (AS) compared to aerated lagoons (AL) was studied. Colour behaviour in four laboratory-scale reactors, treating wastewater from a bleached kraft mill, together with a control treating a non-coloured synthetic wastewater, was monitored. Two reactors simulated well mixed AL operation at different hydraulic retention times (HRT), one the contribution of the sediment layer to ALs, and one an AS system. Results indicated that: (i) increasing aeration time increased colour formation, (ii) at short HRTs, increasing the sludge age did not increase colour formation, (iii) at longer HRTs, increasing the sludge age did increase colour formation, and (iv) activated sludge operation did not form colour, whereas colour approximately doubled in the aerated lagoon simulations, consistent with full-scale observations. One mechanism for colour formation in aerated lagoons may occur through the adsorption of colour precursors from the wastewater with subsequent intracellular microbial transformations. On settling, the formed colour may be released back into the wastewater where it may undergo further transformation.