The Downers Grove (Illinois) Sanitary District is on the verge of a sustainability milestone: The wastewater treatment plant is producing more electricity than it uses, nearly enough to offset natural gas consumption to become a net-zero energy facility.

Nick Menninga general manager of the district in a southwestern suburb of Chicago says the plant approached net-zero energy consumption by the end of 2017. “We certainly have days when we make more energy than we use, but our objective is to get 12 months in a row where we’re at a negative number,” he says.

Achieving that will bring in a $500,000 grant from the Illinois Clean Energy Community Foundation, which has helped the district with several previous grants totaling $589,400 as it cut electricity consumption and boosted its generating capacity.

“Downers Grove is almost like the poster child of what we would want some of the large plants to do,” says Gabriela Martin, program director for energy with the foundation. “They have been working with us for many years and have gradually been improving the plant’s energy efficiency. They got to the point where there wasn’t a lot more they could do to reduce energy consumption, so they began to expand biogas capability.”

A long process

The process has taken about 10 years. In the beginning, Downers Grove was just trying to save money: Electricity made up about 15 percent of operating costs. The activated sludge plant (11 mgd design) has tertiary filters and hypochlorite disinfection and discharges to the East Branch of the DuPage River. 

District leaders considered replacing electric motors in the plant with steam or biogas but decided to stick with the basic electric infrastructure and look for efficiency opportunities. “We first looked at modifying the aeration system,” Menninga says. “We put in fine-bubble diffusers (Sanitaire - a Xylem Brand) and high-speed turbo blowers (Sulzer Pumps Solutions) and installed better process controls (Allen-Bradley) so we could modulate the airflow to the process more closely.”

The old blowers were either on or off. The new instrumentation monitors the air demand from the aeration tanks and automatically modulates blower speed. The district also changed the blowers in the aerated grit chamber, installed more efficient lighting, and improved the buildings’ HVAC system efficiency.

“The biggest thing was the aeration project,” Menninga says. “That made a very noticeable difference in our electricity usage. Once we had a handle on how efficiently we could run, we started looking at making electricity on site using digester gas.”

Biogas to electricity

The district had some experience as early as the 1980s using digester gas to operate a blower, but the equipment was difficult to maintain and was inefficient. “We started looking at other uses of biogas,” Menninga says. “We decided to install a combined heat and power, or CHP, facility. It’s basically a couple of V8 engines driving synchronous generators. The hot water off the engines is used to heat the anaerobic digesters.”

The biogas is fed through a cleaning system (Unison Solutions) before being fed to the engine gensets, a 280 kW Tech 3 Solutions unit and a 375 kW Nissen model. To boost gas production, the district installed a receiving station to accept fats, oils and grease from restaurant grease traps for feeding to the digesters.

“Restaurant grease traps get pumped out pretty routinely,” Menninga says. “We permit haulers to deliver it here, and they pay us to take it off their hands. We put it into a tank and bleed it into the digesters along with our sludge. It dramatically increases gas production.”

The grease receiving equipment required a significant investment: “We bought special pumps and macerating equipment and a mixer for the tank. Grease receiving has become a pretty substantial piece of infrastructure.” The CHP system brought the treatment plant close to energy neutrality. In December 2017, net energy usage was 13 MWh (40 MWh in natural gas used for heat and 27 MWh of electricity sold back to the grid), while plant operations totaled 632 MWh of energy.

The final hurdle

The next piece of the puzzle, which Menninga thinks will bring the plant to net-zero, is FOG storage. After the second engine-generator came online in 2017, the plant was using up all its FOG over weekends.

“We found our gas production would fall off on Sunday afternoons,” Menninga says. “We took our final delivery on Friday. By Sunday, our gas production would drop. There would be a day of the week when we weren’t making that much electricity.”

The solution was to put in a second grease receiving station with enough storage to keep the digesters producing gas throughout the weekends. “We’re expecting to be able to run seven days a week pretty much at capacity instead of six days a week,” Menninga says. “With that, we’re pretty confident we’ll be able to run at net-zero from now on.”

Helping pay the way

The Illinois Clean Energy Community Foundation offers grants to wastewater treatment plants in the state that are serious about becoming energy neutral or better.

The independent foundation was established in 1999 with a $225 million endowment from Commonwealth Edison. Part of its mission is to improve energy efficiency and advance the development and use of renewable energy resources. It has awarded more than 5,000 grants totaling more than $250 million.

However, the end of the money is in sight. “We’re spending ourselves out of existence,” says Gabriela Martin, program director for energy. “We estimate that we have another five to seven years of funds left. The board decided that we wanted in the last few years to make an impact in a few sectors and really push them.” The members chose to focus on net-zero energy buildings and wastewater treatment plants.

Illinois has more than 1,000 wastewater treatment plants, large and small, and many may be capable of producing more energy than they use. “Some just need a large solar array to offset their electric consumption, or they may want to consider a wind turbine,” Martin says. “We have a number of smaller plants where that is definitely feasible.”

The Downers Grove Sanitary District is the only grant recipient so far under the net-zero program, but other projects are in the design phase. The foundation is looking for more qualified applicants. The grants require matching funds and are performance-based: The foundation releases the grants only when the project produces results.

“Anyone who comes in looking for funds has to have an energy plan in place that commits the facility to becoming net-zero,” Martin says. “It has to be a part of their way of doing business. Sometimes it’s a long haul to get there. We’re not looking to make many grants. We want to make grants that catch people’s attention, that make a difference, and hopefully get others to think about doing the same thing.”