Effect of tertiary coagulation and flocculation treatment on effluent quality from a bleached kraft mill

Since January 1995, the tertiary color-removal process at Crestbrook Forest Industries mill (Skookumchuk, BC) has removed an average of 60 +/- 7% of effluent color. The tertiary treatment system is a dual-polymer process that uses coagulation and flocculation technologies, which are relatively nonspecific. Consequently, besides removing the chromophores of concern, tertiary treatment of kraft effluent can also significantly reduce the levels of other major parameters such as chemical oxygen demand, chlorinated organic compounds, and target contaminants such as TCDF. A study of the system's performance showed that it functioned effectively over a broad range of temperature, pH, polymer dose, and influent BOD. The tertiary treatment system reduced COD, AOX, 2,3,7,8-TCDF, and mixed-function oxygenase induction by 24%, 23%, 80%, and 18%, respectively. Treatment of segregated D- and E-stage effluents did not reduce polymer consumption, but this approach shows promise as a means of reducing equipment size and thus capital cost.