Bleaching of color of kraft pulp mill effluents and natural organic matter in lakes

Abstract

In situ changes in the color of lake water and biologically treated kraft pulp mill effluents mixed in lake water were examined in three lakes. In 1.7 m3 enclosures, the color (400–700 g Pt·m–3) of the mixture of pulp mill effluent and lake water remained similar for 4 months in darkness, whereas exposure to solar radiation resulted in 17–42% bleaching of color. During the summer stratification of a humic lake, the color (160 g Pt·m–3) of hypolimnetic water remained similar to that found during spring turnover, but the color of epilimnion decreased 13%. The measured rates of photochemical bleaching of color by solar radiation could explain the decrease of epilimnetic color. Attenuation of photobleaching in the water column was similar to that of blue irradiance. Measured photobleaching was well described by a model based on quantum yields. The model revealed that visible solar radiation accounted for 29% of photobleaching at the surface and 66% in the whole water column. The results indicated that solar radiation, rather than plankton, bleached color of lake water and pulp mill effluents in surface strata.