Beneficial Reuse of Fly Ash as a Replacement for Lime in the Alkaline Stabilization of Biosolids for “Class A” Exceptional Quality Sludge in Wisconsin

An existing biosolids lime stabilization process at the City of Kiel, Wisconsin has historically used lime as the alkaline material at a rate of about 318 tonnes per year (350 tons per year) and a cost of about 34,000 per year. The desire to optimize the operation of the stabilization process led Kiel to pursue the idea of substituting fly ash as the alkaline material in the process. Fly ash from a coal-burning electric generating plant about 45 kilometers (28 miles) away was found to be available free of cost to the City. By way of lab, bench and full-scale testing, fly ash was found to be effective in biosolids stabilization and in maintaining “Class A” biosolids status. The predicted annual savings to switch from use of lime to fly ash in the stabilization process was estimated to be 50,000 to 52,000 per year, while increasing the final biosolids volume for disposal by about 10%.