The 2007 Sludge Thickening Adventure in Boulder, Colorado

The City of Boulder, Colorado 75th St. WWTP (Boulder WWTP) was recently converted from a trickling filter/solids contact (TF/SC) process to a biological nutrient removal (BNR) activated sludge (A/S) process with dissolved air flotation thickeners (DAFTs) for secondary sludge thickening prior to digestion. One challenge during construction was switching from an existing operation of sludge co-thickening in gravity thickeners (GTs) to primary sludge thickening only in the GTs and waste activated sludge thickening in the DAFTs. This paper focuses on how the sludge thickening process evolved during construction and final configuration of the BNR A/S and DAFT processes by evaluating three distinct operational scenarios: before, during, and after construction. Overall, the sludge thickening processes operated successfully during conversion to A/S. An additional goal was to evaluate impacts on the digester operation. Boulder WWTP staff as well as staff from other WWTPs will benefit from this study by learning how to optimize thickener operation so they are best prepared to handle future thickening challenges, planned and unplanned.