More than 30 municipalities around New England can be sure their wastewater biosolids are put to good use and not just filling up expensive landfills.

They send their biosolids to the Hawk Ridge Compost Facility in Unity, Maine. There the biosolids are mixed with wood chips and other bulking agents, composted, and turned into useful products for landscapers, contractors, gardeners and others.

Owned by Casella Organics, the 15-acre facility uses an in-vessel tunnel composting system developed by Gicom Composting Systems of the Netherlands. The technology was developed to provide a climate-controlled environment for growing mushrooms. Since then it has been adapted to composting and is used in locations all over the world.

Hawk Ridge is licensed to process 4,800 cubic yards of biosolids a month (about 3,500 tons). It yields about 80,000 cubic yards per year of finished product, distributed under the earthlife brand.

Winner of a 2020 Biosolids Management Award from the New England Water Environment Association, Hawk Ridge started operation in the early 1990s with open-windrow composting. The Gicom tunnel technology came on board 1994; facility manager George Belmont says it improved operations, odor control and product quality. “With Gicom, we have more control over our composting system,” he says.

In January 2009, Hawk Ridge became the nation’s first privately operated biosolids management facility to become certified and admitted into the National Biosolids Partnership’s Environmental Management Program.

Taking care of customers

Thirty-five municipalities in Maine, New Hampshire and Massachusetts use the Hawk Ridge facility.  

“Some of our customers operate small municipal treatment plants and cannot afford the investment of money and staff time to achieve necessary biosolids certification on their own,” Belmont says. “It made sense for us, as their biosolids management partner, to attain certification and offer them the security and confidence they deserve.”

Municipalities pay Hawk Ridge a tipping fee based on wet tons. Contractors hired by Casella Organics haul the dewatered material to the composting site. Some clients produce as little as five tons a week; others generate several hundred tons. The percent solids range from the low teens to the high twenties. “We don’t handle liquid biosolids,” Belmont says.

Success depends heavily on good relationships with customers. Belmont says the keys are “being responsive to our customers’ needs, providing reliable and timely service and offering support to answer any questions customers may have a bout operations or regulatory compliance.”

The wastewater treatment plant in Camden, Maine, has sent its belt-pressed biosolids to Hawk Ridge for several years. David Bolstridge, plant superintendent, says the facility is great to work. “We’re a tourist community and can have increased biosolids in the summer months,” he says. “Hawk Ridge always finds a way to help us out if we need an extra roll-off to keep us running.”

Sophisticated process

Biosolids arriving at Hawk Ridge are tipped across a certified scale and then moved to an enclosed receiving area where, in Belmont’s words, the “initial recipe” starts, A bucket loader turns over the biosolids, which are then blended with bulking agents including sawdust, wood chips and kiln-dried hardwood shavings (called turnings) from a nearby golf tee manufacturing plant.

The turnings work great, Belmont says, because of their porosity and ability to absorb moisture: “We’re lucky to have them so close to us.” At this point an inoculant — screenings from the cured compost — is added to help the composting process get started.

A bucket loader tosses and turns the material once more, and operators make sure the proper porosity, moisture and carbon-to-nitrogen ratios are maintained. Next, the material goes into a mixing system where it is blended further before it drops off a discharge belt into the in-vessel composting system.

There are six concrete tunnels, each 15 feet wide, 17 feet high and 110 feet long. Blowers (Air Industries of New England) force air through plenums in the floor up through the contents, which are piled 8 feet high. The computer-controlled aeration system first operates at near capacity for six hours to fluff the material.

Then the blowers return to a lower setpoint and the composting process begins, as the microbes feeding on the organic matter start to generate heat. All the while, the oxygen content of the mix is closely monitored; the flow of air can be adjusted as needed during this “warming up” phase.

Heating and cooling

Pasteurization of the material at 55 degrees C for three days is a requirement. Hawk Ridge brings the mixture to 65 degrees C to ensure that the required temperature is maintained. Then things are cooled down. “Pasteurization is great for killing pathogens, but it’s not great for composting,” Belmont explains. “During the cooldown phase, outside air is introduced into the tunnel to bring it back to a better composting temperature, around 50 degrees C, to get the microbes going again.”

After seven days in the tunnels, the material is moved to an aerated curing area, where it is held for about 21 days. There the microbes continue the composting process. Then the material is stacked in 14-foot- high block piles. Finally it is broken into batches that are windrowed for another 45 to 90 days before being pushed up into a final pile for screening and blending.

A Compost Manager probe system (Freeland Scientific) monitors CO2, oxygen, moisture and temperature, indicating when the time is right to turn a windrow to add more oxygen. Properly aged, the final compost product is transported to finished cure piles, passed through a Komptech screener, and prepared for shipment to customers throughout the year; sales peak in the spring.

The final earthlife brand products include a contractor-grade compost that can be added to the loam pile, and a premium grade sold to nurseries and landscapers. Hawk Ridge also sells compost blended products such as Gro-Max, Super-Mulch and Super-Peat. All are sold in bulk.

Belmont says Hawk Ridge has provided more than 2 million cubic yards of compost products to garden centers, nurseries, golf courses, athletic fields, landscapers and contractors over the years. The facility operates from 6 a.m. to 3:30 p.m. Monday through Friday.

The staff includes Keith Hunter, yard manager; Jim Stevens, facility operator; Richard Howell and Richard Kaufmann, equipment operators; and Robert Bowman, operations manager. They are supported by sales, compliance and other staff members who are part of Casella Organics.

Controlling odors

The enclosed tunnels of the Gicom system and the use of recirculated air are keys to eliminating odors. “Even though we’re in Unity Plantation, an unorganized township, we have about 13 or 14 homes nearby, the nearest within 1,500 feet,” says Belmont.

The composting process can operate with 100% outside air, or it can use recirculated air, cutting down on the amount of air that needs to be treated before it is exhausted to the atmosphere. Controlled by a computerized air handling system, the recycled air is piped through a recirculation loop into the aeration plenum.  

“During startup the ratio is about 50% recirculated air and 50% outside or fresh air,” Belmont says. Exhausted air passes through a Ceilcote (Verantis) odor-control scrubber. The pH of the wash water strips ammonia, and a biofilter filled with wood chips and compost further treats the air before it is discharged through roof-mounted dispersion fans.

Keeping up with rules

While every waste processing facility needs to address odors and respect its neighbors, many other environmental regulations require constant attention. Belmont says the changing regulatory environment is one of his facility’s biggest challenges, especially the nationwide focus on PFAS.

Most composting facilities and municipal wastewater treatment operations are finding small amounts of these “forever chemicals” in their biosolids. “In 2019, we began testing for these compounds. We currently test twice a year for PFAS and report our results to the state,” says Belmont. “We have had to evaluate the risk assessment based in what we are finding in the products. We have reviewed loading rates, adopted best management practices, and have changed our labeling.”

While emphasizing safety, he points out that Hawk Ridge needs to keep products going out the door. And PFAS or not, municipalities still need to find a home for their biosolids. That’s why Camden’s David Bolstridge appreciates working with Hawk Ridge.

“Their customer service is second to none,” he says. The previous plant he worked for also used the composting facility, so he has several years of experience with it. “They go beyond normal to keep us happy,” he says. “We don’t have a lot of storage here, and it’s a big deal if we have to stop biomass going out. If we need something in a hurry, they respond.”

Camden uses some earthlife composted products and offers them free to residents. “I’m pleased that the Maine Department of Environmental Protection is allowing us to continue to land spread it,” says Bolstridge. “As far as costs go, it’s about the same as landfilling, but it would be a travesty to fill our landfills with biosolids. It just makes sense to continue to land spread it.”