Shelagh Connelly has some advice for biosolids producers: Stop hauling water.

Connelly is president of RMI, a company in Holderness, New Hampshire, with 25 years of experience helping wastewater treatment plants, food producers, paper mills and other facilities recycle their residuals that might otherwise go to landfills or incinerators. The company has 25 employees.

“We characterize materials that can be recycled and that have beneficial properties, either nutrients or physical characteristics, that make for a good soil material,” Connelly says. We get hired to do all the permitting and regulatory oversight.

“Then we have the trucks that pick it up to recycle it for land application programs. Some facilities want to do improvements at the source. Some want us to pick up and process the material. Some have a product that’s fine as it is, and we’ll pick it up and deliver it.”

Making it dry

In 2018, RMI introduced a new drying technology that helps wastewater treatment plants dramatically reduce biosolids volume and improve quality. The company has run successful pilot projects in Hooksett, New Hampshire, and Brattleboro and Bellows Falls, Vermont.

In all three cases, the dryer produced biosolids at 88-92% solids, significantly cutting the volume that needs to be transported. The effort led to RMI being selected as the 2021 recipient of the Green Steps Award from the New England Water Environment Association.

“NEWEA’s Sustainability Committee was thrilled to present RMI with the 2021 Green Steps Award, which recognizes innovation and sustainability in the clean water and stormwater industry,” the association said in a statement.

“This honor recognizes RMI’s sustainable biosolids recycling technology and their demonstrated commitment to the triple bottom line, including environmental stewardship, social well-being and economic prosperity.”

Low-temperature drying

The new technology is a belt dryer manufactured in China by Shincci. RMI imported the first two full-scale dryers installed at U.S. treatment facilities. Although the company has experience managing dried biosolids from paddle dryers and drum dryers, Connelly prefers the Shincci technology because it operates at lower temperatures, uses less electricity, has a smaller footprint and is more cost-effective.

“It’s misleading to call it a dryer. It’s more like an evaporator,” Connelly says. “It dehumidifies the biosolids. It’s not comparable to conventional dryers. My partner Charley Hanson likens it to a barbecue: It’s low and slow. Low heat and slow-moving belts allow the water to evaporate. You’re left with a 90% solids Class A product that is just beautiful.”

The process takes about four hours. Connelly says the final product resembles Cracklin’ Oat Bran cereal. “When you put the wet cake, into the unit, it goes through what’s called a slitter, which is just like a giant pasta machine, and it literally turns it into long noodles.

“The beauty of those noodles is it increases the surface area, so it’s easier to evaporate the water. At the end you have these crunchy, dry, shorter noodles. It’s very friable, very stable and doesn’t have odor. It ends up being a very nice biosolids product.” 

Truckloads eliminated

After a pilot project at Brattleboro, one of the dryers was moved to Bellows Falls, where the machine fit into the same space as the 30-cubic-yard dump container that had been receiving the biosolids from an anaerobic digester.

“We use a PWTech 302 screw press,” says Rob Wheeler, chief operator at Bellows Falls. “We’ve done some fine-tuning, and we’re getting 28-30% solids off that press. That goes into the drier, and the drier takes it to 88-92% dry product.”

The press and the dryer were linked to communicate with each other. “When the dryer can’t take any more product, it shuts the press off,” he says. “They work in unison. You don’t need somebody out there watching it all the time.”

The plant (average flow 0.5 mgd) was landfilling 30 to 40 container loads of biosolids per year. Now the plant fills only one container per month of Class A product that can be applied to farm fields. Farmers are eager to take it. Although Bellows Falls delivers the product to fields, Wheeler envisions the biosolids being conveyed directly into a storage building where farmers could come pick it up.

Back to the headworks

The water from the dewatered material is condensed, collected and sent back to the headworks. Connelly says that in different climates it could be suitable for reuse, such as for irrigation. “It’s very clean, and it would be OK to put into receiving water,” she says. “Probably it’s a little cleaner than the effluent.”

Helping biosolids producers with technology is a new direction for RMI. “Our company hasn’t dealt with technology before,” Connelly says. “We’ve done land application, but we saw the opportunity and value because this technology makes such a nice final product.”

Although the pilot projects have used electricity for a heat source, Connelly says the dryer could also be set up to use waste heat from other processes. The pilots have been at relatively small plants, but it could be scaled up with larger machines or by connecting modules.

“This is really a simple technology to evaporate water out of whatever product you have,” Connelly says. “I see it as the wave of the future. I just want to stop hauling water. It’s as simple as that. It doesn’t make sense to truck water around. We have about 19,000, wastewater treatment plants in the country. If we can shrink four truckloads down to one, that’s huge.”