Organics in wheat and rye straw pulp bleaching and combined mill effluents (I) chemical characterization and biodegradation studies

Abstract

Exposure of the aquatic environment to hazardous organics originating from bleached straw pulping has been studied. Bleached neutral sulphite straw pulping results in discharge of both black cooking and spent bleaching liquors. Time proportional one‐week samples have been taken both from the combined bleaching effluent and the combined mill effluent, and subjected to chemical analyses and biodegradability studies. The bleaching effluent had a content of total organic carbon (TOC), 3400 C kg/day, and of adsorbable organic halogen (AOX), 340 Cl kg/day. Similarly, the combined mill effluent had a TOC, 53,000 C kg/day, and AOX, 260 Cl kg/day. Fourteen phenolics were identified in the combined mill effluent amounting to 430 kg/day, and twenty chlorophenolics in the bleaching effluent amounting to 4.6 kg/day. Other chlorinated organics were recognized but not identified. The biodegradability in sea water of the organics from the effluents were estimated in a BOD‐experiment from the time trends in biological oxygen demand (BOD), dissolved organic carbon (DOC), and AOX at 15°C. In the combined mill effluent 40 per cent of the initial DOC remained after 21 days of incubation, and similarly, 50 per cent remained in the bleaching effluent. In order to perform ecotoxicological studies a high‐concentrated sample of non‐biodegradable or persistent organics in the effluent was obtained from an aerobic stabilization of the wastewater in sea water from the receiving waters. It appeared that 40 per cent of DOC and 70 per cent of AOX remained in the combined mill effluent after 45 days, and similarly, that 90 per cent of the DOC and 50 per cent of the AOX remained in the bleaching effluent.