Improvements ranging from new light fixtures to a breakthrough solar installation help a Colorado plant cut energy usage by nearly 20 percent.
“We’re starting to reap some of the benefits,” says Virgil Turner. He’s talking about the Montrose Wastewater Treatment Plant, which has seen a 17.5 percent reduction in the cost of energy since 2008 — saving the plant some $70,000 in a $1.3 million annual budget.
“We’ve been doing quite a bit with our wastewater plant,” says Turner, director of innovation and citizen engagement for the City of Montrose, population 19,000, in the high desert of the Uncompahgre Valley of western Colorado.
The cost-saving measures include water conservation, high-efficiency lighting, upgrades to large motors and an advanced solar photovoltaic system.
Focus on utilities
A regional drinking water plant serves the area, but Montrose has its own wastewater plant. When Turner began working for the city in 2005, he noticed that nobody in city government had the job of focusing on energy use: “As I was looking at the budgets of all our departments, the utility line item really stood out to me, particularly the wastewater treatment plant, which was running around $213,000 annually. That was a pretty good chunk of our budget.”
Built in 1984, the activated sludge facility has a design capacity of 4.32 mgd and average flows of 2.1 mgd. One money-saver was reduction of the plant’s influent load through a citywide education effort that has cut water consumption by 25 percent over the last two years.
After the 2008 recession hit, the city got more serious about energy savings. “Across the city, we were able to cut back about 10 percent,” Turner says. “We didn’t do much other than making people aware that keeping lights on or running heaters when they didn’t need to was costing us money. With everyone keenly aware of our budget issues, people really started to pay attention to it and started thinking about things differently.”
At the wastewater plant itself, the simple measures reduced the utility costs by 8 percent to just under $200,000. One step was to cut back on outside lighting. “The entire complex was lit up, but we didn’t normally have people there when the lights were running,” Turner says. As a remedy, the lights were placed on timers so now they burn only as needed.
The plant team also made changes in the aerated grit chamber. “The plant superintendent had never been happy with the amount of grit trapped in the chamber,” says Turner. “He started experimenting with aeration rates and found that lowering the aeration actually improved grit separation. He kept pushing it and finally turned off the aeration. That saved us quite a bit.”
In 2009, Montrose signed a joint city and countywide energy efficiency performance contract with Ameresco. One task was an evaluation of energy use at the treatment plant. Again, the efforts started small. Several small maintenance and equipment buildings that staff members used infrequently have wall-mounted electric space heaters. These were fitted with twist timers like those on hotel hot tubs to help limit how long they would operate.
“If the thermostat is calling for heat but nobody is there to turn the twist timer, no heat comes on,” says Turner. “We used to set those thermostats at 70 degrees or so, and the buildings were heated even if nobody was in them for weeks at a time.”
All the fluorescent lights were switched from T12 to the higher-efficiency T8 lamps. “We typically saw an 18-month payback from those changes,” Turner says. “We also installed occupancy sensors on the lighting controls. Those weren’t huge savings, but we’re seeing the accumulated benefits.”
Looking to equipment
After taking care of the easy sources of savings, the team looked at the large motors throughout the plant. “We’d been talking for years about adding variable-frequency drives,” says Turner. “We had them on our sludge pumps, but we have three 40 hp screw pumps that move the effluent into oxidation ditches, and one or two are running most of the time.”
VFDs from ABB Industrial Drives were added to the screw pumps, while others from Siemens were installed on nine oxidation ditch rotors. The VFD project cost about $80,000 but yielded 12 percent savings on electricity and a six-year payback. Up next is an evaluation of VFDs for the aeration pumps on the four digester cells.
Facing the sun
Solar energy is another big contributor to efficiency. On the roof of the digester building sit 28 small photovoltaic solar arrays that manufacturer BrightLeaf Power calls the most efficient solar energy units on the market. The specially designed concentrated solar units have a dual tracking system to follow the sun. They also capture waste heat for cogeneration, making them 70 percent efficient, according to BrightLeaf, versus about 18 percent for a typical flat-panel system.
Besides turning the sun’s energy into electricity, each cell warms cooling fluid, which is pumped to the plant’s truck bay for space heating. The plant team is looking at other solar cogeneration options in the future, such as hot water heating or supplementing a geothermal system.
“I’m a real advocate of renewable energy,” says Turner, an alumnus of the Energy Execs program of the National Renewable Energy Laboratory. “One of the important principles is that the cost of renewables is typically higher than conventional electricity. In order to make sense, you really have to work on your energy efficiency first.
“The system concentrates the sun’s rays onto very small gallium arsenide chips, about the size of your pinkie fingernail, that were developed for use in space. It gives us the same amount of electrical conversion as traditional PV in a quarter of the space.” The 23 kW system gets about 300 days of sunshine a year and is expected to save about $8,000 a year on electricity.
It’s part of a sound energy strategy that has reduced the plant’s environmental footprint while helping keep costs down for residents and businesses.