A Maine composting facility creates beneficial recycled products, and does it with less energy, thanks to innovations like geothermal heating and cooling.


As the largest municipal biosolids composting operation in New England, the Hawk Ridge Compost Facility’s main purpose is the beneficial reuse of the solids produced in treating wastewater.

Operated by Casella Organics, the site in Unity serves more than 30 treatment plants in Maine and surrounding states, turning biosolids into Class A compost. “Many of our customers have been with us since we opened in 1990,” says Jen McDonnell, director of sales and marketing. “Many large wastewater treatment plants around the area are anchor customers for us.”

As the first private company certified under the Biosolids Management Program through the National Biosolids Partnership, Hawk Ridge achieved Platinum Certification — the highest — in 2011. The voluntary program promotes resource recovery from biosolids and provides training and support to reduce operating costs and achieve efficiencies.

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“One of the goals we identified was reducing the energy used,” says McDonnell. “Our efforts have improved energy usage dramatically — we’ve seen more than 27 percent reduction in kilowatt-hours used per cubic yard of processed material.”

From the ground

The biggest energy innovation is geothermal heating and cooling, installed when a new office building was added to the Hawk Ridge facility in 2010. The system is a bit more complicated than normal. “We sometimes call it a geo/biothermal system,” says McDonnell. “We’re using a byproduct of our process.”

Like a typical geothermal system, the Hawk Ridge installation uses the natural temperature of the underground soil to cool the office building in summer, by way of underground piping and a heat exchanger. In winter, heat from the 110-degree process water warms the soil around the system’s underground lines so that the heat pump doesn’t have to work as hard. The hot water comes from the odor-control scrubber.

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The system was expanded in 2011 to include winter heating for the maintenance shop. The total investment of $40,000 is saving Hawk Ridge another $10,000 a year above other savings produced by operational and lighting improvements.

Going on the grid

Another big change is in the electric power supply. The facility originally was powered by on-site diesel generators, but fast-rising fuel prices prompted Hawk Ridge to hook up to the utility grid in February 2009.

“We paid to bring in three-phase power from 5 miles away, and that really helped us from an environmental impact standpoint,” says McDonnell. “We have a lot of hydro and renewable power in Maine, so transitioning to grid-based energy through Central Maine Power was a big environmental improvement for us.”

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With diesel prices above $4 a gallon, the $290,000 investment, aided by a $50,000 state incentive, provided a quick payback, saving the plant about $200,000 a year on fuel.

The next step was to seek operational efficiencies to save even more energy and money. The 4-acre composting operation includes covered storage, mixing facilities, composting buildings and six composting tunnels with computer-controlled air recirculation, an ammonia scrubber and a biofilter for odor control. The energy efficiency changes include:

  • Monitoring blower operating flow rates
  • Raising the temperature setpoint of the odor-control scrubber
  • Using premium efficiency models as motors are replaced
  • Placing timers on all equipment block heaters

Through Efficiency Maine, interior and exterior lighting was upgraded to premium efficiency lighting (Wiswell Electric). The program provided $2,300 cash incentives toward the total cost of $12,000, along with advice to help save energy and money.

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Many of the T-12 fluorescent lighting fixtures across the facility were upgraded to the more efficient T-8 lights. Others were replaced with newer T-5 high-output fixtures that produce more light with even less energy than the T-8s. “From 2010 to 2012, we reduced our electricity consumption by 23 percent, for an annual savings of about $20,000,” says McDonnell.

Proof in the product

Hawk Ridge takes in about 45,000 cubic yards of biosolids and other organic feedstocks per year. The facility is permitted to accept municipal biosolids, food waste, wood ash, leaf and yard waste, shredded paper, short paper fiber, fish waste and wood waste. It distributes 80,000 cubic yards of product annually, enough to cover 42 football fields with about a foot of compost.

“A lot of what we do is providing materials to improve soil quality, reduce runoff and improve crop yields,” explains McDonnell. Marketed under the Earthlife brand, the products from Hawk Ridge include:

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  • Premium compost — Class A compost to benefit plant growth and soil health
  • NutriMulch — Compost-enriched mulch
  • Super-Peat — A custom blend of sphagnum peat and compost
  • GrowMax — A multipurpose product made from horticultural-grade ingredients including Maine blond sphagnum peat, compost and
  • Superhumus (Casella’s blend of forest soil, fine bark, organic matter, sand and fine stone)

“When you recycle biosolids, you keep valuable nutrients in the ecosystem,” says McDonnell. “It is also a carbon-sequestration strategy, because improving the soil helps support plant growth, and plants sequester carbon. Our services are environmentally friendly and help address some of the challenges of our time.”   


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