Time, sunlight, and the fate of biotreated kraft mill organochlorines (AOX) in nature

A system of gas- and light-permeable sealed sample bags was employed to monitor the fate and persistence of the color and organochlorines (AOX) exiting the secondary treatment systems of three modern pulp and paper kraft mills. Placed in a remote lake at various depths, the bags of effluent showed that the “stable” biotreated AOX from elemental chlorine-free bleaching was largely but not completely mineralized in 4 months, largely by sunlight-dependent processes. Mill AOX and color was partially protected from photomineralization by natural organic matter (NOM) in the water. Effluent chromophores were more rapidly and completely degraded by light than AOX, and neither AOX nor color were very susceptible to biological degradation. The biotreated effluents, even when undiluted, were not acutely toxic and over time gave rise to diverse, flourishing microalgal, protozoan, and metazoan communities in the sealed bags. Thus these concentrated effluents apparently had no acute or chronic toxicity for a diverse assemblage of aquatic organisms over a 4-month period.