Simultaneous precipitation in an ECF kraft pulp mill : Long-term experiences

Long-term experiences of simultaneous precipitation (SP) of phosphorus using ferric sulfate in the secondary treatment of softwood E C F kraft pulp mill wastewater were examined from a time period of 7 years. Phosphorus reduction in an activated sludge process (ASP) was improved from monthly average level 43%-67% to 72-84% by adding ferric iron in the aeration basin. Due to periodic changes in wastewater qualities, the demand for iron addition to wastewater varied between 0 and 10 mgFe/L, depending mainly on influent P/BOD7 ratio. Introduction of SP has not caused any notable disadvantages to daily wastewater treatment plant operation, excluding minor increase of total sludge production. Experiences and conclusions from long-term operation were similar to our preliminary findings from pilot-scale and short-term full-scale studies. Thus, it can be verified, that SP offers an alternative to decrease phosphorus discharges from pulp mill ASP instead of more expensive tertiary treatments.