Utilization of fly ash for treatment of pulp mill effluent

The discharge of highly colored effluents by kraft pulp and paper mills is a major environmental concern. A process utilizing fly ash to treat kraft effluents was developed in this study. A caustic extraction stage effluent obtained from the pulp mill at Prince Albert had a color of 12000 platinum-cobalt or American Public Health Association (APHA) units, a chemical oxygen demand (C01)) of 2480 ppm, and a total organic carbon content (TOC) of 832 ppm. The effluent was treated with lignite­derived fly ash produced at the Saskatchewan Power Corporation's thermal generating station at Boundary Dam. The untreated raw fly ash was ineffective for treatment of the effluent. A raw fly ash dosage of 1000 g/l removed only 51% of the color and 49% of the COD and TOC. The ineffectiveness of the fly ash was a result of its low carbon content. The Boundary Dam fly ash contained only 1.1% carbon by weight. Fly ash acidified by treatment with hydrochloric acid was effec­tive for color removal. Treatment with a 3.0 g/l dosage of acidified fly ash resulted in 94% color removal and 86% COD removal. After treat­ment with a 3.0 g/l dosage of acidified fly ash, the pH of the effluent was 4.2. The highest color removal was obtained when the pH of the effluent was adjusted to 5.0 by the addition of sodium hydroxide during treatment with acidified fly ash. At a pH of 5.0 by 1.9 g/l dosage of acidified fly ash removed 98% of the color from the effluent. The hydrochloric acid requirement for complete acidification of fly ash at room temperature (22 C) was 20 mg equiv/g of fly ash. It was determined that the mechanism for color removal using acidified fly ash was chemical precipitation of dissolved colored bodies in the effluent by aluminum and ferric salts released from the fly ash during acidification. It was also determined that color removal by raw fly ash was not solely an adsorption phenomenon. The color removal ob­tained using raw fly ash was partly due to chemical precipitation by lime released tram the fly