Three years ago, an energy audit of the Freeport (Maine) Wastewater Treatment Plant found many ways to save energy by upgrading old equipment, changing processes and insulating the building. Now the Freeport Sewer District is saving more than $90,000 in electricity and $25,000 in heating oil annually.

To serve a population of 8,300, the district operates a secondary treatment facility and maintains 18 miles of gravity pipes, 11 miles of force main and 11 lift stations. The treatment plant has three independent trains, and operators can divert water to as many trains as necessary. The 0.75 mgd plant treats 0.350 mgd on average. Effluent discharges to the Harraseeket River, home to one of the richest clam beds in New England. Biosolids are co tourists,” says Leland Arris, general manager. “No one wants to fish, swim or boat in a bad river. There are 24 lobster families and 60 clamming families who work on the river. We affect a lot of people if we have a negative effect on the river.”

Over the last few years, the district has renewed its emphasis on out-performing state requirements. The outfall is in the river estuary, and with clam digging in that area, the district maintains a close relationship with the state Department of Marine Resources, which regulates the distance from the outfall at which clammers are allowed to dig.

Assessing energy

In 2010, the Woodard & Curran engineering firm conducted a plant energy audit and power assessment. Brian Cataldo, project engineer, proposed several efficiency upgrades to Arris, who was chairman of the wastewater board at the time. They included an aeration upgrade that would cut electrical costs in half.

“When I saw that the project would save us more than $80,000 annually, it caught my eye,” Arris says. “That’s like two years of savings in user rate increases of 4 percent. We didn’t have to raise rates for two years because of that savings, even with significant improvements in staffing and operations.”

The plant was built in 1974 with coarse-bubble diffusers. The upgrade separated sludge pumping from aeration and retrofitted fine-bubble diffusers to improve efficiency and save energy. The project also added hybrid positive displacement blowers (Aerzen) with premium efficiency motors (WEG) and variable-frequency drives (Allen-Bradley/Rockwell Automation). The updates, completed in January 2012, saved an average of 70,000 kWh per month, or about $100,000 per year. Full payback is expected in eight years.

About one third of the $950,000 project cost was funded by a grant from Efficiency Maine, an independent trust that promotes energy efficiency. The district borrowed the remainder from bonds through the Clean Water State Revolving Fund (SRF).

The new aeration system required a learning curve for plant superintendent Dan Bicknell and the operating team. “With the old system, there was no technology — we just turned the blowers on full speed,” Bicknell says. “It was easy to operate, but we didn’t get great results. Now we can measure and balance our units for optimum operating conditions. We have data trending and more testing available to us.”

Heating with effluent

Another upgrade was an effluent thermal system. Effluent moves through two heat pumps. Then heated or cooled water is diverted through piping to the district office buildings. Consoles in the offices allow employees to adjust temperature and humidity. “Originally we didn’t agree to the project because it was a high capital investment,” Arris says. “But then we saw it would save an incredible amount of money on heating oil. We have eliminated a $25,000 heating oil bill every year. Plus, we are reducing our carbon footprint significantly.”

The new heating system, which went online December 2011, cost $170,000 and was funded by SRF bonds. Full payback is expected in about seven years.

Woodard & Curran’s analysis also looked at insulation and other upgrades to the process building envelope. Spray foam insulation was added to the roof and other areas. A new room for the biosolids dewatering centrifuge was built with a heat recovery ventilator so that when odors and air are blown out, the heat is captured before it is exhausted.

The insulation and heat recovery projects cost $80,000, funded partially by a grant from the American Recovery and Reinvestment Act of 2009 and SRF bonds. The improvements save $6,000 per year in electricity.

The savings from the aeration upgrade gave the district board confidence to approve the additional projects in a short time frame, says Arris: “We were able to move quickly on other projects that continue to save us energy.”