2010 Issue

25 Need for New Covers Replacing the current digester cover systemwith the water-sealed type of floating covers will result in: • Preventing digester system failure. The existing buoyant digester covers are beyond their useful life. Failure of a cover (or covers) would create a protracted period with the system out of service. • Providing reliable fuel supply to the cogeneration system. Without adequate biogas storage, the digester gas system periodically loses pressure. This usually happens when solids are being transferred from the digesters to the drying beds. Without biogas, the cogeneration system must be fueled with natural gas from the commercial supply system, which significantly increases SLCWRF operating costs and increases fossil fuel combustion. • Providing additional back-up power. Without reliable biogas supply, SLCWRF must provide back-up power from the diesel-fueled 2-megawatt generator and increasing fossil fuel combustion. • Mitigating the release of fugitive emissions of methane from the digesters. Methane is a source of greenhouse gas emissions with a global warming potential of 21 times greater than carbon dioxide. Salt Lake City is taking a proactive approach to preventing fugitive emissions of methane by utilizing new state of the art water-seal digester covers. • Mitigating the release of fugitive odors from the digesters. Hydrogen sulfide - among other gases - is a major source of odor from wastewater treatment plants and in particular from un-sealed anaerobic digesters. The water-seal covers will eliminate odor emissions from the digesters at SLCWRF. With the water-seal covers, the operation of the anaerobic digestion and cogeneration systems will be streamlined. The improved design of the covers will allow the system to capture a larger volume of biogas and allow storage of biogas, resulting in greater output of the cogeneration system and more efficient operation. In addition, replacing the digester covers will reduce the carbon footprint of the SLCWRF. Construction will begin in January 2010. The first of three water-sealed gasholder covers is scheduled to be operational at SLCWRF by the sum- mer of 2010. For more information about this project, contact: Giles Demke, SLCWRF, Giles.demke@slcgov.com Dru Whitlock, CH2M HILL, Drury.Whitlock@ch2m.com Dannie Pollock, CH2M HILL, dannie.pollock@ch2m.com Giles Demke is an engineer at the Salt Lake City Water Reclamation Facility (SLCWRF), Drury Whitlock and Dannie Pollock are engineers with CH2MHILL in Salt Lake City.