2010 Issue

24 T HE UTAH WATER QUALITY BOARD selected this project as one of six projects that will use the $20 million available through the American Recovery and Reinvest- ment Act of 2009 (ARRA). Constructing this project will support major ARRAgoals including supporting over two hundred jobs and using American products and materials. Figure 1 shows a water-sealed gasholder cover CH2M HILL designed for Chino, California. Salt Lake City’s design will be similar. The SLCWRF stabilizes wastewater residuals with 2-stage mesophilic anaerobic digestion to produce approximately 10 dry tons of biosolids per day. Biosolids are air-dried and reused for landfill cover and land reclamation. Stimulus Funds Go Green!!! GILES DEMKE, DRURY WHITLOCK, and DANNIE POLLOCK Salt Lake City is going “Green” utilizing Federal stimulus funding for a $6 million project to replace the existing floating covers on the anaerobic digesters at the Salt Lake City Water Reclamation Facility (SLCWRF). In addition to providing a stable biosolids product, the anaerobic digestion system produces about 300,000 cubic feet per day of methane-rich biogas that is piped into the cogeneration system. The cogeneration system was upgraded and began operation in 2004 with two new 700-kilowatt lean-burn engine generators. The cogeneration system produces about one half of the facility power demand and almost all of the heat demand, which significantly reduces facility-operating costs and improves the environment by re- covering and utilizing a renewable energy source. The combination of anaerobic diges- tion and cogeneration effectively convert a waste product into energy, which is a huge benefit to the citizens of Salt Lake City. The existing anaerobic digestion system in- cludes three 95-foot-diameter digesters with floating steel covers (constructed in 1964) for primary digestion and one 100-foot-diameter digester with a fixed concrete cover for sec- ondary digestion (constructed in 1984). The existing steel covers on the primary digest- ers are 45 years old. Their age is beyond the original design life and deterioration is threatening to compromise the structural integrity. The covers are in need of replace- ment not only due to deterioration, but are also in need of replacement to better support the new cogeneration system. The new covers will provide increased gas storage, which is critical to optimize the use and performance of the cogeneration sys- tem. The gas system currently operates at a pressure of about 6 inches of Water Column (W.C.). There is about 1-inch W.C. headloss between the digesters and the cogeneration system, reflecting a minimal pressure dead- band in the system, making it a challenge to operate. The new system will increase the operating pressure to 12 inches of W.C. and significantly increase the available deadband, making the system more forgiving and more operator friendly. Figure 1: Example Water-Sealed Anaerobic Digester Cover Anaerobic Digestion and Energy Recovery System