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Self-sustaining biosolids drying

Self-sustaining biosolids drying

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    The Class AA product is a source of organic material and nitrogen that is sold in bulk to central Florida's fertilizer industry. The pellets can also be sold as fuel for biomass incinerators. Photos: West Palm Beach Solid Waste Authority

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    Capable of turning 600 wet tons of sludge into 60 tons of marketable fertilizer every day, Palm Beach County's Resource Recovery Facility is processing 400 wet tons daily. Solid waste and wastewater treatment managers plan to add another 900 scfm by capturing gas from a second landfill.

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    Palm Beach County's North County Resource Recovery Facility is centrally located, lowering the hauling costs and vehicle emissions between five wastewater treatment plants and the landfill, which owned enough land to house a biosolids drying and pelletizing facility.

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    The compression system runs 24 hours a day, six days per week, recirculating gas to a continuous pilot that automatically switches back to flaring when the pelletization facility isn't operating.


$500,000 saved annually

Municipal landfills are the second-largest source of human-related methane emissions in the United States, and methane is about 21 times more potent than CO2 by weight.

Almost a decade ago, up the coast from Florida, managers in Virginia looked for ways to lower operating costs and methane emissions. They launched a program similar to Palm Beach County's that is saving $500,000 annually.

In 1994, the Department of Public Works of Fairfax County, Va., landfill gas/electricity project was generating 6.4 megawatts. At the time, the 260-acre I-95 Landfill accommodated 18 million tons of waste and was flaring an average of 1,400 scfm of gas daily.

On March 12, 1996, EPA promulgated New Source Performance Standards for landfill emissions. While they could have satisfied the regulations by flaring, managers in the Fairfax County Division of Solid Waste Disposal and Resource Recovery wanted to go above and beyond the law's minimum requirement. So they began to explore beneficial uses for their gas. They found it in the county's 54-mgd Noman M. Cole Jr. Pollution Control wastewater treatment plant in Lorton, Va., which was spending an average of $5 per million Btu (mmBTU) on natural gas to power incinerator after-burners. At $2.60/mmBtu, landfill gas was about half the price of natural gas. It wasn't hard to decide to switch fuel sources.

It took two years to plan, permit, and install the biosolids drying project. The biggest challenge was acquiring right of way for the 3-mile pipeline from the landfill to the pelletization operation. County and contractor employees met with residents several times to talk about their concerns, most of which related to safety.

The county's operating budget covered the $1.2 million pipeline construction; the plant's developer paid $500,000 for the compressor/dryer station to transmit the gas to the control plant. The Fairfax County Wastewater Treatment Division spent $100,000 on burner modifications to create a dual-fuel system that enables operators to use natural gas as a backup when the LFG supply fluctuates.

In 2005, solid waste managers installed a $50,000 infrared system that burns landfill gas to heat the landfill's maintenance shop and truck washing facility and are saving $20,000 annually on propane. After investing $150,000 on the infrastructure necessary to switch from natural to landfill gas at their transfer station's auto maintenance shop, they're saving $50,000 annually on fuel.

Keeping odor to a minimum

In Florida, wastewater treatment plants deliver dewatered sludge in 40-cubic-yard trailers to a fully enclosed, odor-controlled, automated receiving area at the recovery center's pelletization operation.

Screw and belt conveyors transport the material into two 460-cubic-yard bins and then into a pug mill, which mixes oversized and fine dried pellets with the incoming sludge. The mixture then enters one of the two rotary drum dryers, which evaporate the moisture and condense the remaining solid material into 2-centimeter pellets. A separator cyclone then screens dried solids. Pellets meeting the size criteria are cooled and transported to storage silos, ready to be sold.

Because the operation resides next to residential neighborhoods, pack tower scrubbers are used to remove odor-causing water vapor, particulates, and ammonia. The scrubbers use alkaline/hypochlorite to remove acidic and oxidizable substances, especially sulfur compounds such as hydrogen sulfide. A regenerative thermal oxidizer that's heated by the landfill gas helps further by destroying odor-causing compounds and organic vapors.

A potent 50/50 mixture of methane and carbon dioxide and traces of other organic compounds, the gas is saturated with water vapor and must be conditioned. A system comprising a compressor, a heat exchanger to reduce the gas temperature, a cooling/dehydration zone to remove moisture, and a filter to remove solid impurities removes the water and impurities and creates a vacuum that pushes the clean gas through 1,000 feet of buried 7-inch PVC pipe to the facility.

There, the gas is channeled to the burners through redundant piping systems.

At 1,200 scfm, the gas provides all the energy each dryer requires: a maximum hourly fuel supply of 34 mmBtu. But like most such projects, the facility is designed to use both landfill and natural gas so operators can use either if the supply of either fuel is interrupted.

Managers are confident the facility will provide a sustainable and profitable solution for Palm Beach County.

— Ludwig (Ludwig.victoria@epamail.epa.gov) is a program manager with the U.S. EPA Landfill Methane Outreach Program, and Usherson (judy.usherson@erg.com) is a senior analyst at Eastern Research Group.


WEB EXTRA

Join LMOP

The U.S EPA's Landfill Methane Outreach Program (LMOP) provides software tools, marketing assistance, guidance materials, and tailored technical services to facilitate development of landfill gas (LFG) energy projects. Specifically, LMOP works to:

  • Assess landfills that are viable candidates for project development.
  • Estimate energy potential from landfills and match the supply to end-user demand.
  • Conduct preliminary feasibility studies for LFG energy projects.
  • Facilitate the location of corporate/industrial facilities adjacent to candidate landfills.
  • Provide technical expertise in the use of LFG in boilers and other thermal applications.
  • Promote the environmental and economic benefits of LFG energy projects.
  • But how do you know if your landfill is a good candidate for a gas-to-energy project? The LMOP defines a candidate landfill as one that is accepting waste or has been closed for five years or less; has at least 1 million tons of waste; and does not have an operational landfill gas-to-energy project or one under construction.