How'd They Do It? Montpelier Team Slashes Plant Energy Use

By refusing to be satisfied with established practices, the plant team in Vermont’s capital has slashed energy use and helped the city toward its net zero goal.
How'd They Do It? Montpelier Team Slashes Plant Energy Use
The primary settling tanks at the Montpelier plant with a movable bridge for primary sludge collection.

In his five years at the Montpelier Water Resource Recovery Facility, Chris Cox has learned the value of asking one simple question: “Is this how it has to be?”

Since 2010, that question has helped the plant staff cut total energy use almost in half.

That helped the plant win the 2015 Governor’s Award for Environmental Excellence and the 2014 Energy Management Award from Efficiency Vermont, which has helped in the energy reductions over the last six years.

The assistance has included funding for projects and an energy audit in December 2013. “That helped get the ball rolling, and it got all the operators thinking about being more energy conscious,” says Cox. “We took what they had and went running with it.”

Most of the success has come from about 16 small projects over the last three years at the city’s conventional activated sludge plant (3.97 mgd design/1.8 mgd average flow). Cox became chief operator last summer and gives much of the credit for the energy savings to his predecessor, Bob Fischer.

Average daily electrical use has been reduced by 34 percent in six years, from 5,040 kWh in 2008 to 3,350 kWh in 2014. In the last five years, the plant cut its average gasoline use by 50 percent, from 30,000 gallons in 2010 to 15,000 gallons a year today.

Questioning attitude

Thinking outside the box has helped the plant staff come up with ideas that go against long-standing past practice. For instance, the dissolved oxygen level in the aeration basins had been maintained at 4 mg/L because of problems with filamentous bacteria. Since blowers make up about 38 percent of the plant’s electrical demand, reducing DO to 2 mg/L meant big energy savings and did not affect the treatment process.

For many years, the plant had run two of its three Archimedes screw pumps and their 40 hp motors all the time in winter to keep them from freezing. “Two years ago, we decided to try to run just one,” says Cox. “We bump-start the other two periodically just so they won’t freeze. That works fine, and there haven’t been any issues.”

Another energy waster was a 5 hp blower that ran about 40 hours a week to ensure that the thickened waste activated sludge would stay in suspension. “In the last month, we haven’t run it once and we’ve been able to pump sludge,” Cox says.

Septage and leachate handling has also been a target. The plant takes in 30,000 gallons of leachate per day from the state’s only active landfill and 30,000 gallons of septage. Keeping it mixed used a lot of energy that turned out to be unnecessary.

“We have two 50,000-gallon tanks, and they each have a 5 hp mixer and a 40 hp blower to keep the solids suspended,” says Cox. “We just stopped mixing and aerating it. We were worried about how often we would have to clean the tank due to plugging, but we found that it’s actually cleaner not mixing or aerating it.”

The plant has even increased the amounts of septage and leachate it accepts, which means keeping a close eye on E. coli every week.  But the tipping fees provide income that helps fund energy-saving equipment. “Last year we made more than $800,000,” says Cox. “It gives us the ability to act on energy savings ideas that we have.”

Recent projects

Improvements also extend to the headworks. The addition of influent screening many years ago made it unnecessary to grind primary sludge before it was pumped to the digesters, but the facility’s grinder pump still had to be run because it was inline. In June 2015, the team removed the grinder and replaced it with a 4-foot section of pipe, eliminating another 3.5 hp motor that ran about three hours per day.

In September of that year, the plant began two projects to improve power use and monitoring. The local utility, Green Mountain Power, penalizes industrial users that don’t have a 95 percent power factor; the Montpelier plant’s rating was 86.3. Independent Capacitor Corporation installed a 600-volt capacitor to achieve the 95 percent power factor for $5,600. It will save nearly $4,000 per year on electric bills.

Efficiency Vermont provided $4,000 toward $12,000 in power monitoring equipment to measure demand from all main breakers in the facility. The plant’s average monthly peak load is 246 kW. Cox expects the monitoring equipment to enable a reduction to 225 kW, for a potential savings of $3,800 per year.

“I can set a limit of, say, 200 kW, and if power demand exceeds that an alarm will sound,” says Cox. “I’ve set up a protocol of turning off different pieces of equipment that won’t affect effluent quality yet will keep us from hitting a certain peak demand.”

Utility companies have different peak-shaving programs, and Cox advises operators to become familiar with their peak demand charges and how they affect the monthly bill.

Net zero goal

In 2014, city leaders launched Net Zero Montpelier, a quest to become the first capital city in the nation to produce or offset all of its electric, thermal and transportation energy needs with renewable sources. “The city’s Energy Advisory Committee has some expertise and is helping us to get the digesters to be more efficient,” says Cox. “We’re trying to maximize the potential for the methane we produce because we take in so much septage and leachate.”

Currently, the plant does not produce electricity from its biogas; the fuel is burned in a boiler to heat the three anaerobic digesters and one building. “We’re trying to figure out if we can take in other organic waste to increase the methane production to maybe talk about cogeneration. That would be a great help,” Cox says.

Also on the horizon is an energy efficient hybrid screw blower. Efficiency Vermont will help fund that once cost estimates and energy savings are established. Hybrid blowers are generally about 30 percent more efficient than the existing positive displacement blowers.

Efficiency Vermont will also fund $4,500 of the $15,000 cost of installing variable-frequency drives on the plant’s three lift pumps that move sewage from primary to secondary treatment. Each has a 40 hp motor and can pump 4 mgd, much more than the plant’s flow. The VFDs are expected to save more than $5,000 in the first year alone, providing a two-year payback.

Two belt filter presses are also being retired and will be replaced with new automated technology that is more energy efficient, uses less water and produces a higher percent solids material.

Cox is happy with the results but knows the work never ends. “As the state comes out with tighter regulations, we have to use more energy,” he says. “Every addition to get higher quality water is going to mean more electricity, so it’s a never-ending battle.”


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