while some might not be thrilled to discover their neighbor is home to sewage, engineers at the University of Nebraska–Lincoln saw it differently—as an opportunity to repurpose the city’s wastewater into a source of renewable energy.
With plans under way to turn Nebraska’s state fairgrounds into the university’s new Nebraska Innovation Campus, engineers realized the construction site was sitting next to prime real estate: the city’s wastewater plant.
The campus’ Centralized Renewable Energy System, which launched on Aug. 24, uses “borrowed” treated wastewater from the Theresa Street Wastewater Facility in Lincoln, Neb., to heat or cool the campus’ buildings. The system is one of only a few in the world, modeled after an office building in Chicago that uses the Chicago River for its air-conditioning cooling towers.
Olsson Associates—the engineering team behind the $12 million renewable energy system that can provide heating or cooling for up to 1.8 million square feet of space—had experience working with geothermal systems, heat exchangers, and wastewater grids, though typically as separate projects.
“We had all of the pieces, but this was the first time we pulled all of them together,” says Dave Roberts, Olsson’s head mechanical engineer on the project.
The wastewater facility, which serves 70 percent of Lincoln—including the University of Nebraska–Lincoln campus—discharges up to 15,000 gallons of treated wastewater (also referred to as clean, gray water) per minute during daytime peak hours, which amounts to 28 million gallons of water per day. The wastewater is treated using bacterial microorganisms that break down and feed on the waste. Once the microorganisms are removed, the water is disinfected using ultraviolet light.
But instead of being immediately released to the nearby Salt Creek as it had been in the past, the water is diverted to a building just off the Lincoln wastewater facility property.
The CRES functions like a geothermal ground loop—a central heating or cooling system that uses the earth as a heat source by transferring heat to or from the ground. In the case of CRES, wastewater becomes the heat source. When the water leaves the plant, it ranges between 57 and 75 degrees Fahrenheit, says Roberts. Once diverted, it goes through a series of heat exchangers that either add or subtract heat from the water based on the campus’ heating or cooling needs.
The water then circulates through the campus in a 6,000-foot closed loop of pipes and is later discharged back into Salt Creek. According to Daniel Duncan, executive director of the Nebraska Innovation Campus, the Nebraska Department of Environmental Quality has approved the discharge temperatures of the water returning to the creek.
As for the energy-saving features of the CRES, the impacts seem to be especially noticeable this winter.
“Today, in Lincoln, it’s 15 degrees, so rather than having to heat the building an extra 55 degrees to keep the inside temperature at 70, you only have to heat the water about 10 degrees,” says Duncan. During summer months, extracting heat from the wastewater and pumping the cooler water through the building will be the alternative to air-conditioning.
In the long term, the system is expected to reduce cooling costs by 25 percent and heating costs by 30 percent when compared with standard energy costs. It also will free up additional office space by eliminating the need for air-conditioning units or heaters and could be a boon for private developers and future tenants.
An additional highlight of the project for Roberts was discovering how the system naturally operates in tandem with residents’ daily schedules.
“When designing it, we noticed that as people wake up and take showers, that’s also the same time we start waking up our buildings,” he says. “Flows are highest when people wake up, and then, when people go to bed, the buildings are slowing down as well. The system follows our habits and the way we socially work.”