Application of Secondary Flotation-Filtration and Coagulant Recycle for Improvement of a Pulp Mill Primary Waste Treatment Facility

An Asian pulp mill uses the bleached Kraft pulping process and has three major wastewater streams: (a) alkaline wastewater, FBE; (b) acidic wastewater, FAE; and (c) neutral wastewater, RWE. Its existing primary wastewater treatment (WWT) system cannot meet the government’s effluent discharge limitations on color (100 units), COD (100 mg/L), BOD (100 mg/L), TSS (100 mg/L), and pH (neutral) for ocean disposal. This research has proven that addition of secondary dissolved air flotation-filtration (DAFF) to the existing primary WWT system will allow the pulp mill to meet the current ocean discharge requirements. This publication also introduces a waste sludge recycle system for coagulant recovery at the pulp mill. The recycle system involves the following steps: (a) collecting and purifying the waste sludge from primary sedimentation and/or secondary flotation-filtration, (b) dividing the dried combined sludge into two fractions for aluminum solubilization: one fraction in an acid treatment unit and another fraction in alkaline treatment unit, (c) filtering the inert silt for ultimate disposal, and (d) returning the solubilized chemicals from the acid and alkaline treatment units in proper proportions for reuse as primary coagulants. The newly developed technologies are designed to provide a cost-effective simultaneous WWT and coagulant recycle. Experimental results tend to suggest that practical designs based on the technologies of secondary DAFF-WWT and coagulant recycle are both technically and economically feasible for improvement of the pulp mill’s existing primary wastewater treatment facility.