Solar energy provides most of the energy for the Rifle water reclaim plant, while operators’ innovations cut down electricity consumption.
Since the sun provides much of its energy, the Rifle Regional Wastewater Reclamation Plant finds it doesn’t need much electricity from the utility grid. But that hasn’t kept the staff from looking for ways to save energy: By tweaking plant operations, they have cut energy use by 20 percent.
Put online in November 2009 to meet new ammonia nitrogen discharge requirements for the Colorado River, the $23.3 million facility in West Rifle, Colo., uses three oxidation ditches to clean the wastewater from 10,000 residents about three hours west of Denver. With a design flow of 2 mgd, the plant averages 1 mgd.
Plenty Of Power
The construction project included a contract with SunEdison to build a solar photovoltaic system. In exchange for land and a 20-year purchased power agreement, SunEdison designed and built single axis, ground-mounted solar arrays totaling 2.3 MW on two tracts of land on the plant grounds. A 1.7 MW array powers the wastewater treatment plant, and a nearby 0.6 MW system runs a pumping station. The equipment can produce about 4,300 MWh of electricity per year; any excess is sold to the local municipally owned Glenwood Springs Electric System.
“On a clear, sunny day in summer, we get about 300 percent of our plant’s electrical demand generated by solar,” says Devin Jameson, lead operator. Even in winter solar fulfills nearly all the plant’s needs. That is better than the projections of 50 to 70 percent of the plant’s demand.
But solar energy was only the beginning. Garfield Clean Energy, a countywide clean-energy partnership of 10 local governments, offered to help the Rifle team better understand the plant’s energy use. “Garfield Clean Energy offered to put in data loggers across the plant,” says Jameson. “They provide a website, Garfield Energy Navigator, that gives us daily, weekly, monthly and annual data on our solar and grid power use.”
The data loggers record energy use around the plant every 15 minutes, showing energy use off the grid, solar generation, the amount of solar-generated power being sold back to the utility, natural gas usage and more. The website is being used to track energy use in 70 buildings across the county.
“We also have trending software in our SCADA system,” adds Jameson. “If we see a spike in our solar or grid usage, we can go back to our SCADA to see what equipment ramped up and figure out what happened.”
David Gallegos, plant supervisor, says the team has used the information to help detect equipment problems: “It can help find malfunctions by just tracking the energy use. Maybe a blower doesn’t turn off or a VFD has failed. We keep a real close eye on it.”
The data helps, but it still takes legwork to find ways to save energy. “After tracking energy use and talking with Garfield energy coaches, we cut back a little bit here and there,” adds Jameson. “We’re using 50 to 70 kWh less every day.” The annual 20 percent energy reduction saves about $20,000 a year and earned the plant an Active Energy Management Innovation Award from Garfield Clean Energy.
“We found we could operate without using some equipment,” says Gallegos. “We have three huge clarifiers. We tried taking two of them offline. It has worked out fine, and we’re not running two motors 24/7.” It has also reduced workload on the staff because the clarifiers don’t have to be cleaned as often.
About 30 Cutler-Hammer variable-frequency drives (Eaton) have been added throughout the plant and have been tied into the SCADA system. “They’re able to ramp down during the cold hours and months and work a little harder when it’s warmer and the oxygen dissipates,” according to Jameson. “There are 12 motors on the oxidation ditches alone. There are now VFDs on blowers, nonpotable water pumps for reuse inside the plant, and the scum and sludge pumps.”
Learning The Rates
The staff also learned that the local utility, like many others, offered lower rates outside times of peak demand. “We were running a lot of our blowers and other equipment during peak times, and that was really costing us,” says Gallegos. “Just changing the times that equipment came online resulted in a big savings.”
The peak demand charges also gave Jameson an idea to adjust use of the centrifugal blowers for dissolved oxygen control in the digesters. “We used to run them for hours at a time, then shut them off,” he says. “We had problems keeping oxygen in the digesters. After doing some math, I realized that we could do on/off aeration. We turn them on for 12 minutes, then turn them off for 12 minutes.”
That’s important because the peak demand use is measured every 15 minutes. “Keeping it from running for 15 minutes reduces the peak demand, and we found we’re able to hold more oxygen in the digesters.” This measure has also reduced overall use of the blowers: They now run six to eight hours less every day.
The staff even began questioning the plant’s lighting, even though the facility was relatively new. “We had metal halide lights that took five minutes to warm up and sucked up a lot of energy,” says Jameson. “We replaced those with instant-on fluorescent lighting and placed them in better locations. It’s more efficient, we’re using less energy, and we’re getting better lighting.”
The outdoor lighting was put on timers with local on/off switches. “We were lighting up the entire place like a prison,” Jameson says. “Now they turn off at night when we’re not there, but we can turn on the lights we need if we have a call-in.”
Rifle’s accomplishments have drawn interest from others in the area, such as when other members of Garfield Clean Energy meet at the plant. “People from the other towns picked our brains for ideas,” says Gallegos. They also come to see the solar installation, which is one of the most extensive among state clean-water facilities.
Good data, questioning attitudes and the willingness to experiment have paid off for the Rifle plant, according to Gallegos: “I think we even surprised ourselves a little bit.”