Microturbines are nothing new to the Albert Lea (Minn.) Waste-water Treatment Facility on the banks of the Shell Rock River.

With more than seven years of experience using four digester-gas-fueled microturbines for cogeneration (combined heat and power), plant superintendent Rick Ashling says the equipment was a good investment and continues to provide savings now that ownership of the units has reverted to the city.

“Our utility, Alliant Energy, approached us because they were looking for renewable energy credits,” says Ashling. “They would own the turbines, do the installation, and operate and maintain them for five years.”

Funding assistance

The original plan was to install two C-30 microturbines from Capstone Turbine Corporation. After considering the idea, the city, in southeast Minnesota, decided to buy two more at a cost of $76,000. The Minnesota Conservation Improvement Program provided $85,000 toward the $250,000 project, and Alliant Energy funded the remaining $89,000. It was the first use of microturbines in the state, according to Ashling.

The first 30 kW microturbine went online in October 2003, and the other three followed in 2004. The project also included equipment for gas collection, conditioning and compression, electrical connections, and automation gear.

The turbines generate about 800,000 kWh per year toward the plant’s 8 million kWh of annual electricity usage. About 28 million Btu is captured daily from the turbines for supplemental heating of the anaerobic digesters and space heating in the plant.

“We are generating more than $100,000 in savings every year,” says Ashling. “About $40,000 of that is electricity savings. Heat recovery saves about $62,000 in natural gas. And we are reducing the carbon footprint of the plant by not burning fossil fuels to generate electricity and by not flaring off our methane.” The project achieved payback in four years, while Albert Lea recovered its investment in two years.

The cooperative effort earned the 2004 Minnesota Government Reaching Environmental Achievements Together Award, which recognizes waste and pollution prevention, conservation and recycling by government agencies.

Watching closely

After seven years, Ashling and his team are happy with the microturbines, and they’ve made it a high priority to take advantage of them. “I believe in this,” Ashling says. “We had to rethink the way we manage solids to make sure we feed the digesters with the thickest solids we can to produce the maximum amount of methane.”

Operators closely monitor dissolved oxygen in the activated sludge process to minimize the use of the two 400 hp and three 600 hp blowers. “We’ve been successful with that,” says Ashling. “We save $10,000 a month by watching the organic loading.”

Success continued even after the city took over ownership, and the $30,000 annual cost of maintenance, nearly two years ago. “We’re getting a net savings of about $77,000 a year,” Ashling says. That equals about a 25 percent reduction in the plant’s overall energy use, along with the prevention of emissions related to producing electricity at power plants or burning natural gas. “There was even a time when the utility called to check the gas meter because they thought it was broken,” Ashling recalls.

He expects the savings to increase now that the utility has received approval of a temporary rate that could hit 22 percent. Still, the plant buys energy off the grid and may have to raise customers’ rates to meet the added expense of about $9,000 a month. “Our city is faced with the same problem as other cities, keeping costs down,” he says.

Rising energy costs create a quandary for wastewater treatment plants: pay more for operations or make a large capital expenditure to reduce usage. Ashling had a plan to add three 60 kW microturbines, but funding is a problem in tight economic times.

Always exploring

The same is true of wind energy, which he has been considering for some time. He says the city was close to an agreement on two wind turbines but couldn’t reach a mutually satisfying agreement with the developer. “We also continue to look at the way we do things in order to reduce our power consumption,” Ashling says. “We work with our blowers and try to be as efficient as we can with those.”

The plant has not yet switched to variable-speed blowers but has tested them. “We keep studying ways to reduce our air demand,” says Ashling. “We constantly look at expansion of renewable energy, but we’re also looking for financial assistance to do it. Any expansion of the system will require some sort of state or federal assistance.”

And Ashling is keeping his eyes open for more opportunities to reduce costs and environmental impact while protecting ratepayers.