Challenges Faced and Solutions Found During the Salt Lake City Water Reclamation Facility Anaerobic Digester Upgrade

Salt Lake City Water Reclamation Facility (SLCWRF) retained CH2M HILL to develop a Biosolids Study in 2008. SLCWRF had numerous concerns regarding the solids handling system and biosolids reuse outlets, as well as the future impacts of potential effluent nutrient limits on the solids handling system. SLCWRF also recently converted the liquid stream process from trickling filter/solids contact to trickling filter/activated solids (currently treating an average dry weather flow of 35 mgd), and wanted to model future waste activated solids production – with and without nutrient limits – along with primary solids production to determine the impact on the solids handling system.One of the recommendations of the 2008 Biosolids Study was replacement of the buoyant digester covers with new water seal gasholder covers. This project was designed by CH2M HILL with Gerber Construction being the successful construction contract bidder. Shortly into the construction in January 2010, it was determined the anaerobic digester tanks were structurally compromised. The team developed an innovative approach, which included building a new tank within the old tank, using the old tank walls as the outer forms for construction of the new tank. The tank size was increased significantly by increasing the height so that SLCWRF would have enough capacity for facility build-out.The new digesters included the use of water-seal gasholder covers to increase the system operation pressure, provide gas storage to optimize use of the cogeneration system, and eliminate the release of biogas. Because the tank size was increased a new mixing system was required. In cooperation with CH2M HILL, SLCWRF chose to specify the new linear motion mixers by Ovivo. These mixers use significantly less energy than other mixers on the market.Through the cooperative approach and leadership primarily from SLCWRF and Salt Lake City Department of Public Utilities (SLCDPU), but also CH2M HILL and Gerber Construction, the team leveraged the problem into a prudent, economical, and sustainable approach that will serve Salt Lake City citizens well into the future.