In 2006, engineers at HDR began planning efforts to increase capacity at the city of St. Charles, Mo.’s Missouri River Wastewater Treatment Plant. The landlocked plant had limited opportunities for physical expansion. HDR needed a solution that fit within the existing infrastructure, which included three paralleloperated oxidation ditches, each 12 ft deep with 1.06-million-gal capacities.
HDR specified the need to replacethe plant’s oxygenation system of brush aerators and internal “boat clarifiers.” The brush aerators were experiencing problems with icing and shaft breakage, and many of the aeration “stars” were corroded and had broken off. The boat clarifiers—originally designed as a pre-clarification step to lower the solids loading rate on the existing final clarifiers— were inefficient and interfered with channel velocity, causing mixed liquor suspended solids to settle in the basins.