Activated sludge treatment of paper machine effluent for mill re-use

Abstract

The SAPPI Ngodwana mill is the largest integrated pulp and paper mill in Africa and is situated in the Elands River catchment in the Mpumalanga province. A need has arisen at the mill to reduce effluent volumes requiring discharge. The combined mill effluent is currently irrigated onto pastures and over the years percolation of attenuated effluent has resulted in elevated chloride levels in the Elands River. Elevated chloride levels in the river have in turn impacted on the tobacco farmers downstream of the mill. Reclamation of Kraft Liner Board effluent was identified as a means of reducing the hydraulic load onto the irrigation pastures and at the same time reduce the intake of fresh water into the mill. Two possible uses for the reclaimed effluent, namely washing of bleached or unbleached pulp, were identified. The major reclaimed water quality variables of concern for re-use are; biodegradable organic material, colour and suspended solids. Approximately 9 months of pilot-scale activated sludge treatment of the effluent demonstrates that sufficient organic material can be removed for re-use, as wash water. Removal of the organic material was not influenced by sludge age in the range 2 to 8 days. A high degree of colour removal was achieved, which is contrary to reports in the literature. Even with the high colour removal achieved, however, insufficient colour was removed to allow washing of bleached pulp. Pulp and paper effluents are often reported to result in filamentous bulking activated sludges. This was confirmed in the testwork. Severe sludge settleability problems were experienced throughout the pilot-scale testwork. Filamentous bulking was identified as the cause of poor sludge settleability and preventative or control strategies would have to be implemented on full-scale.