The city of Longmont Wastewater Treatment Plant generates 140,000 cubic feet per day of biogas. Putting more of it to beneficial use became a priority five years ago.
Today, the city uses 70-80% of the biogas, thanks to a project that converts that fuel to renewable natural gas for the Waste Services truck fleet. Eleven new trucks in a fleet of 21 now run on the biogas-based fuel. The city expects to complete the replacement of the diesel vehicles by 2025, using nearly all the available biogas.
“We are significantly reducing greenhouse gas emissions with the transition of our trash, recycle and compost collection trucks from diesel fuel to renewable natural gas,” says John Gage, P.E., senior civil engineer and project manager.
There’s a benefit on the financial side, as well: After the expense of producing the fuel, the city reaps a net of about $600,000 per year from the sale of renewable fuel credits and the avoidance of diesel fuel costs. The $8.3 million project was partially funded by a $1 million grant from the Colorado Department of Local Affairs and a $385,000 grant from the Colorado Regional Air Quality Council.
Longmont operates an activated sludge treatment plant (13 mgd design). Primary sludge is gravity thickened and waste activated sludge is thickened in a dissolved air flotation process before the materials are sent to the anaerobic digesters. The digested solids are dewatered in centrifuges (Centrisys/CNP) to 16-19% solids. Denali Water Solutions hauls the cake to a blending facility that serves multiple area treatment plants; the finished product is land-applied on farms.
In 2017, the city hired Carollo Engineers to study alternatives for making optimal use of the plant’s biogas. At that point the plant used about one-third of the gas for process heating and flared the rest. The engineers looked at installing a combined heat and power facility, and supplying biogas to local industry, before settling on the biogas-to-RNG project.
The city undertook a design-build project with Carollo and CGRS, which handled all construction and designed the truck fueling station and a new building to fuel and store the Waste Services fleet. Unison Solutions supplied a biogas treatment system that removes hydrogen sulfide, moisture, carbon dioxide, siloxanes and volatile organic compounds.
Biogas treatment process is highly automated. It can be monitored remotely, and operating data is fed to the city’s reporting system. “Every day we get an automated report showing a list of 15 to 20 parameters, their expected ranges, and whether any are falling outside those ranges,” says Gage. “So we know quickly whether it was operating the way it should have on the previous day.”
A key technical challenge to the project was matching fuel supply to demand so as to make the best use of the biogas generated. The treatment plant digesters produce gas continuously, while the trucks operate four days a week and are fueled overnight. This made it necessary to provide gas storage upstream and downstream of the biogas treatment system.
On the upstream side, the floating cover on one digester was fitted with a level indicator that communicates the cover’s elevation to the biogas treatment system. The treatment system’s production then increases or decreases to maintain a digester cover setpoint. If the digester with the floating cover is out of service, the gas treatment system is controlled based on digester gas pressure.
On the downstream side, the truck fueling system has two compressors and two gas-storage systems. A low-pressure storage tank is located upstream from the first compressor, which sends RNG to a high-pressure storage bank with six aboveground tanks holding a combined 104,000 standard cubic feet at 4,500 psi. The second compressor distributes RNG to the trucks as needed; it can fill up to 30 trucks over seven to eight hours.
Ultimately, all of the Waste Services trucks will be converted to operate on RNG. “We have designated a life cycle for each truck, and once the remaining diesel trucks have reached the end of that period, they will be replaced and upgraded as compressed natural gas vehicles,” Kamenides says.
Gage believes the project’s success could encourage other utilities to embrace the biogas-to-RNG technology. “When it comes to renewables projects like this one, mid-range utilities might feel like it’s too much for them to handle,” he says. “One thing we’ve learned is that we have a lot of power in our database management. Also, finding really good partners, like our contracted maintenance teams, can ease the burden on operations staff.”
For now, the clean-fueled collection trucks moving through the streets of Longmont are helping to show the way to more sustainable communities.