Every minute or so, Chattanooga, Tenn.'s Summitt Landfill belches about 1,000 cubic feet of methane - equal to approximately 1.5 million cubic feet per day.
The 25-year-old, approximately 100-acre municipal landfill used to dispose of its methane in the traditional way, by piping it from landfill wells to a flare house for incineration.
However, five years ago, Summitt sold its methane well, pipeline and flare system to Soil Restoration and Recycling (SR2), Chattanooga, in a 15-year deal that is expected to produce $10,000 per month in cash flow for the landfill and helps SR2 with its sludge recycling operations.
SR2 set up shop in an abandoned dye factory adjacent to the landfill and installed a 1,000-foot pipeline to transport gas from the landfill to its plant. SR2 uses the methane to help fire a 1.1 megawatt generator that produces electricity to power the company's soil and sludge recycling processes.
"We need about 600 kilowatts per hour of electricity," explains Glenn Palmer, SR2's business development director. "Our generator usually runs on a 50/50 mix of natural gas and methane."
The methane-natural gas mix consumes about 400 cubic feet of methane per minute. According to Palmer, SR2 could generate all of its electricity with Summitt's supply of methane, but it prefers the mixture because the methane feeds can be inconsistent.
Nevertheless, SR2 generates ample electricity and even manages to defray its costs by selling excess kilowatts to area utility companies.
"The difference between what we produce and what we use goes onto the power grid and is sold," Palmer says. "We flare any unused methane."
SR2's operations produce additional benefits for the landfill. "The landfill receives about 50,000 tons of biosludge per year," Palmer says. "We don't process all of that, but what we do process saves landfill space for other wastes."
The SR2 recycling process, called "low-temperature thermal desorption," literally cooks contaminants out of petroleum-contaminated soil and biosludge.
An 8-foot diameter rotary drum serves as the primary treatment unit. As material flows into one end of the drum, a burner at the other end raises the temperature inside the drum and burns off the contaminants.
The temperature and cooking time inside the primary unit depend upon desired throughput and economics.
"When you elevate the temperature of the contaminants, they will change states," Palmer says. "With more heat, this happens more quickly but costs more. With less heat, it happens more slowly and costs less. We aim for the right balance of heat and mass for the material we're processing.
"Processing time also depends upon the kind of material," he continues. "Sandy material moves faster, while clay doesn't move as well. These considerations affect the temperature requirements to volatize the hydrocarbons or other contaminants."
The primary treatment releases volatile organic compounds (VOCs) from material contaminated with gasoline, fuel oil, lubricating oil and other petroleum products.
In the secondary treatment, a multi-cone vapor recovery collector captures the VOCs and then incinerates them at temperatures from 1,500 degrees to as high as 2,000 degrees Fahrenheit. Processing is completed in a secondary treatment combustion unit.
Whether the plant processes petroleum contaminated soil or biosludge, the result is a Class A material.
"It's not real rich in terms of nutrients," Palmer says, "but it is good as a soil amendment." And, the material doesn't pose any of the potential liabilities associated with Class B materials.
SR2 has received permits to build a second desorption facility at Knoxville, Tenn.'s municipal landfill and hopes to operate approximately 10 facilities within the next five years.
"We're not planning a large number of plants," Palmer says. "There is a high capital investment involved with each one. Once we get the plants that are planned operating and refine the business model, it might be worth looking into licensing the technology."
Will this method of methane disposal and sludge recycling become a part of landfill operations in the future?
"The income the city receives for the well, gas transmission and flare system is a substantial benefit," says Eugene Wright, director of waste resources for the city of Chattanooga. "For a large landfill that produces substantial quantities of gas, this might be a good idea."