Monday, July 11, 2011

Prince William County Landfill and Renewable Energy

On Friday, July 8th at the Potomac Watershed Roundtable meeting Thomas Smith, Solid Waste Division Chief of the Prince William County Public Works Department gave a presentation on Utilizing the Landfill Gas for Renewable Energy Production followed by a tour of the Prince William Landfill.

Prince William County residents pay a flat fee ($70) every year on their property taxes so there is no tippage fee at the landfill. This fixed fee that grows with population allows the landfill to have a steady cash flow to maintain their monitoring and remediation programs and plan smooth expansion and improvement programs. Trash generation in general grows with population, but it is surprisingly subject to the economy. The population of Prince William County was 281,287 in 2000 and grew more than 43% to 402,002 in 2010. In 2000 238 thousand tons of trash was disposed at the PW Landfill. The peak tons of trash disposed the PW Landfill was 367 thousand tons in 2007 in 2010 285 thousand tons of trash were disposed at the landfill. To put that in perspective, in 2007 it was estimated that Prince William Residents generated about 6 lbs of trash per day per person; today that number is less than 4 lbs of trash per day per person. Approximately, 32% of the trash is recycled, 10% is recovered as energy, and 58% is buried in the landfill. According to Mr. Smith much of the trash going to the landfill could have been recycled, but recycling and the recycling center at the landfill is a topic for another day.

Prince William Landfill is right off of Dumfries Road in Manassas, VA and has operated at this location since 1972. The oldest section of the landfill contained 57 acres that were closed in 1991 when the state law known as HB 1205 went into effect. That area is currently used for little league fields and have been undergoing retrofit with liners and leachate and landfill gas collection systems to protect the environment. Today Prince William County Landfill is engineered and built as a series of cells. The cells include liners of plastic membranes and watertight geosynthetic clay liner fabric on the bottom along with a leachate collection system. At the end of each day, earth covers the trash deposited in the cell, to keep animals away and improve aesthetics. When a cell if full it is capped to prevent (or at least limit) the rain that percolates through the landfill and covered in soil. Currently, the landfill is capping the Phase I section and opening up a newly lined cell in the Phase II area.

The PW Landfill has 48 groundwater monitoring wells that are observed and/or sampled quarterly to ensure that groundwater is not impacted or any impact is contained and 78 landfill gas extraction wells. Landfill gas is generated during the natural process of bacterial decomposition of organic material contained in the trash buried in the landfill. Landfill gas is approximately forty to sixty percent methane, with the remainder being mostly carbon dioxide. Landfill gas also contains varying amounts of nitrogen, oxygen, water vapor, sulfur, and other contaminants. The gases produced within the landfill are either collected and flared off or used to produce heat and electricity. The landfill gas cannot be allowed to build up in the landfill because of the explosive potential. PW Landfill has operated for almost 40 years and has more than 7 million tons of trash buried at the landfill. That trash currently generates 2,700 standard cubic feet per minute of landfill gas up from 1,600 scf/m in 1999.

Landfill gas can be used as a source of energy to create electricity or heat. It is classified as a medium-Btu gas with a heating value of 350 to 600 Btu per cubic foot, approximately half that of natural gas, and can be used in place of propane and natural gas in some application. It is a reliable source of energy because it is generated 24 hours a day, 7 days a week. Landfill gas is a renewable energy source. Landfill gas that is used to produce energy does not have to be flared and wasted to prevent explosive gas build up. Flaring of landfill gas at PW landfill is done in a candle flare an open air flame that you have probably seen at night. In 1998 the County formed a partnership with NEO Prince William to install a landfill gas collection system and a 1.9 MW energy recover facility which is a two engine turbine that burns the gas to make electricity that is sold to NOVEC, the local electric cooperative. The 1.9 MW energy recovery system was utilizing less than 25% of the currently available landfill gas for energy recovery.

NEO was acquired by Fortistar Methane Group. Fortistar has since obtained the VDEQ permits for four additional engine turbines. Negotiations with NOVEC were more difficult because Virginia has no renewable energy requirements, but 3 MW of additional engines will go on line this year for a total constant energy production of 5 MW. This will produce $40,000 annual revenue to the Solid Waste Fund and $20,000 in personal property tax revenue in the first year. Even after the new engines go online this year there will still be 200 scf/m excess landfill gas available. A new gas pipeline has been installed to provide landfill gas to heat the landfill’s Fleet Maintenance Building and extended to provide fuel to the County animal shelter cremation incinerator to replace propane. The payback for the pipeline installation is estimated to be 8-10 years. Propane use will be reduced 56% and the savings to the general fund will be $25,000-$35,000 annually.

As the landfill continues to operate, adding closed cells and opening new cells, more landfill gas will be produced for generations. Prince William County views the landfill as an under utilized resource and is studying options for creating a Prince William Renewable Energy Park. The PW Landfill has applied to EPA for technical assistance in performing feasibility studies for the potential options.

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