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Fast Track to New Digester Gas-To-Energy Facilities at the F. Wayne Hill Water Resources Center, Gwinnett County, Georgia

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The F. Wayne Hill Water Resources Center (FWHWRC), owned and operated by the Gwinnett County, GA, Department of Water Resources (DWR), is an advanced wastewater treatment plant which currently discharges into the Chattahoochee River. A new outfall to Lake Lanier is currently under construction and scheduled to be complete in late 2010. The FWHWRC maximum month design flow is 60 million gallons per day (mgd) and currently about 30 mgd of wastewater is received.

In light of rising energy costs and declining revenues reflective of the continuing, severe economic downturn that began in 2007, the Gwinnett County DWR began an initiative to make the best possible use of resources under DWR control, including renewable energy resources. DWR retained CH2M HILL to identify and evaluate opportunities to improve resource utilization and reduce energy costs at the FWHWRC. The results of the evaluations, procedures for capturing stimulus funding, and technologies employed are discussed in this paper.

The energy types considered for the FWHWRC were biogas derived from anaerobic solids digestion, solar, wind, and low-head hydropower. A screening analysis concluded that biogas combustion to produce power and heat was the optimum alternative.

Next, a Business Case Evaluation (BCE) was conducted to determine if the construction and operation of a gas-to-energy facility would be economically feasible. The BCE considered several different scenarios for generating power from biogas, including biogas production with and without addition of fats, oil & grease (FOG) and high strength waste (HSW) to the existing, anaerobic sludge digesters.

The BCE concluded that a gas to energy facility based on an internal combustion engine (ICE) was feasible. The proposed system, in addition to continuously generating electrical energy for use at the FWHWRC, would be capable of producing sufficient heat to keep the anaerobic digesters operating in the mesophilic temperature range of 95-100 degrees Fahrenheit (F). By capturing the heat produced by the ICE, in addition to generating power, the system would have a total energy-recovery efficiency approaching 80%.

The BCE recommended a gas to energy facility of approximately 2 megawatts (MW) in capacity at the FWHWRC. The biogas requirement at a nominal 600 British Thermal Units (BTU) per cubic foot (ft3) for an ICE of this capacity is approximately 520 standard cubic feet per minute

(scfm). However, as the FWHWRC is at only about 50% of its total design capacity, the currently available biogas is considerably less than 520 scfm, and a purchased natural gas fuel blend would be required to obtain full power generation and heat recovery benefits.

To minimize purchase of natural gas, maximize biogas, and as a result improve the return on investment in the cogeneration system, DWR next investigated addition of FOG and high strength waste (HSW) to the anaerobic digesters to supplement the solids feed. The project was made even more attractive by DWR's successful pursuit of funding under the American Recovery and Reinvestment Act (ARRA), as administered by the Georgia Environmental Facility Administration (GEFA), and from the U.S. Department of Energy (DOE).

A schematic design of the system with specifications was prepared for competitive selection of a design-build contractor. The design-build contract was awarded in September 2009 and construction began in early 2010. The gas-to-energy facility is expected to reach substantial completion by the end of 2010 with contractual completion in May 2011. A Request for Proposals for the FOG and HSW Receiving and Handling Facilities has been advertised with proposals to be submitted in early April 2010. The project is expected to be awarded for design and construction in June 2010. Its completion and startup will closely follow the completion and startup of gas cogeneration facilities.

Once operational, the FOG/HSW handling and cogeneration facilities will have the potential to save over one million dollars annually in power costs and generate more revenue in FOG and HSW disposal fees. When operating at its rated capacity, the resulting power production will offset the amount of fossil fuel used to generate over 17,000 MW-hours of electrical power annually.
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Keywords: Digester gas; biosolids; cogeneration; energy; fats; oil & grease (FOG)

Document Type: Research Article

Publication date: 01 January 2010

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