When the state of Massachusetts prohibited the landfilling of organic material, the Greater Lawrence Sanitary District made plans to use food waste to boost biogas production from its anaerobic digesters.

The plan has worked so well that the wastewater treatment plant is a net producer of electricity. The district, in North Andover, added a fourth digester, installed a combined heat and power system, and built capacity to accept processed food waste for co-digestion. The project won an Energy Management Achievement award in 2020 from the New England Water Environment Association.

The project cost about $30 million, but about a third of it was paid through grants and incentives from the state Department of Energy Resources and Department of Environmental Protection, the Massachusetts Clean Energy Center, gas and electricity provider National Grid, and the Clean Water Trust.

The district had been using three anaerobic digesters to produce biogas to heat the digesters and the rotary drum dryer (Baker-Rullman) that turns biosolids into a Class A farm fertilizer. “In 2019 we added a fourth digester,” says Cheri Cousens, P.E., executive director. “We mix biosolids with the food waste and send a completely mixed feedstock to each digester.”

High-power fuel

The food waste is collected and processed at a Waste Management CORe facility in nearby Charlestown and delivered to the district as a slurry in a tanker truck. The mix of food waste and biosolids produces much more biogas than the biosolids alone. Cousens says it’s like the difference between regular gas and high-octane premium fuel.

“If you put in a higher-octane fuel, you get more power out of the same machine,” she says. “That’s what’s happening with these digesters. If you put in a different feedstock that has more volatile solids in it, you can get more biogas. We have more than tripled our gas production.”

The biogas goes through a cleaning process to remove hydrogen sulfide, siloxanes and moisture and then is used in two Caterpillar 3520C generators to produce electricity, plus heat for the digesters and other purposes.

“We produce more power than we need at this time,” Cousens says. “We are putting power back on the grid. We are able to net-meter that power, and it has administratively paid for the energy used at our offsite pumping station.”

The district, known as GLSD, already had some solar power at the treatment plant and has tried to be efficient with the use of electric power, but adding food waste to the digesters enabled the plant to go beyond net zero in terms of electricity use. “This has been years in the making, and to actually say this now, it’s amazing,” says Cousens.

The district has also significantly reduced natural gas usage by harvesting heat from the CHP engine cooling systems and exhaust. The treatment plant, which opened in 1977, has a design capacity of 50 mgd and an average flow of about 30 mgd. The effluent, after chlorine disinfection, is discharged into the Merrimac River.

Other measures

The biosolids are dewatered in a centrifuge (WGEA) and then in the rotary drum dryer to make fertilizer pellets. Although the digester volume has increased significantly with the addition of food waste, production of fertilizer pellets has not changed much. “What’s really interesting about adding food waste is that it almost entirely converts to biogas in the anaerobic digestion process,” Cousens says.

In addition to the Organics to Energy Project, the district embarked on major upgrades to its main pumping station, Riverside, about a half-mile from the main treatment plant. The district has significantly reduced the electrical demand there by optimizing pump operation.

Eddy current clutches (Dynamatic) were added to two 800 hp pumps (Grundfos) that work in tandem with two 1,200 hp pumps (also Grundfos) that already had eddy current clutches. The 800 hp pumps were also rebuilt, and the pumps were staged differently so that the smaller pumps work more often than the larger ones. The result was a 21% reduction in electrical demand.

Resilient and sustainable

The CHP system shuts down automatically if the electrical grid goes down, but it can be restarted off-grid, if necessary, to keep the plant operating even during an electrical outage.

“We can start the engines in what we call island mode,” says Cousens. “We start the engines using natural gas, and we can power the whole plant without being connected to the grid.

“We have tested it several times. There was one instance where National Grid had planned maintenance on our line, so we knew they were going to be down for about five days. We proactively went into island mode and ran the engines for that period. It was actually a really good test of the system.”

Keeping the system running during an outage means the food waste digestion project has made the plant more resilient as well as more sustainable. “The state organics ban was the impetus for this project,” Cousens says. “Without feedstock we wouldn’t have been able to produce the biogas we are using to make the power. In the end, we take the wastewater and bring in food waste. We remove screenings and grit, and that does get landfilled; we have no other option for that. But the biosolids are converted to a fertilizer product, and then we make heat and power — and clean water. The environmental benefits are tremendous.”