High Solids Loading for a DAFT Retrofit for Co-Thickening Operation

This paper presents results from upgrades to the City of Tacoma's dissolved air flotation thickeners (DAFTs) to allow co-thickening of primary and secondary sludges. This allowed the City to resolve ongoing operational issues within the plant related to the hydraulic loading of the anaerobic digester. The design of the DAFT upgrades was completed in 2010, and construction started shortly after. The DAFTs upgrades were installed by May 2011 and the units have been in operation for about two years. DAFT performance has shown that for solids loading rate up to 90 pounds per day per square foot (lb/d/sf) the solids capture rate exceeds 94 percent and the float thickness averages 4.5 percent. Subsequent upgrades to the saturation tanks and other related systems made by the Operations and Maintenance Staff for the City have further improved operability and performance.