Anaerobic Digesters in Longmont, Colorado, to Provide Fuel for City Waste Service Trucks  

Biogas to RNG project furthers sustainability and resource recovery aims while generating significant revenue for Longmont, Colorado.

Anaerobic Digesters in Longmont, Colorado, to Provide Fuel for City Waste Service Trucks  

Charles Kamenides, left, and John Gage in the final fueling area in the service vehicle garage.

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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; by the end of 2022, three more RNG-fueled trucks will replace diesel units that have reached the end of their life cycle. The city expects to complete the replacement of the diesel vehicles by 2025, using nearly all the available biogas.

“This project is directly in line with the city’s Sustainability Plan,” says John Gage, P.E., senior civil engineer and project manager. “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.”

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.

Operations teams at the wastewater treatment plant and the city’s Waste Services and Fleet Management departments had input from the early stages, ensuring that their concerns were addressed and contributing to the effort’s success. For its accomplishment, the city earned a 2021 Project Excellence Award from the Water Environment Federation.

Tapping a resource

Longmont, a city of about 100,000 in north-central Colorado about 30 miles north of Denver, operates an activated sludge treatment plant (13 mgd design). Effluent discharges to the St. Vrain Creek.

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.

The cleaned gas, at 95% methane, is sent through a newly built pipeline to the fueling station on city land next to the wastewater treatment plant. Compressors pressurize the gas into storage vessels at 4,500 psi; the fuel then discharges to the trucks while they are parked overnight.

Close collaboration

Operations team members were intimately involved in the project planning, notes Charles Kamenides, waste services manager.

“We developed strong relationships early on,” Kamenides says. “We were very collaborative, and we had good conversations through the planning process about what we needed from our operations teams. We knew there would be some challenges because a facility like this was new to the city, but we had a lot of input and meetings where we talked about how to make the project work.”

Gage adds, “When exploring the direct fueling of vehicles, we knew it was going to be a partnership. It was important to get the whole team together and ask: What is this going to take?” Gage, Kamenides, treatment operations assistant manager Karl Heil, fleet manager Cash Johns visited a similar process in Grand Junction, Colorado. “It really built confidence for all of us to see things on the ground and how all the pieces fit together,” Gage recalls.

A key objective was to minimize impact on the operations teams. The new Waste Services building was constructed near the treatment plant to avoid installing a four-mile pipeline through a congested area to the existing fleet facility. The Waste Services trucks were stored indoors before the project, and that is still the case in the new building. “The operations staff has a work environment similar to what they previously did,” Gage says. “We built the new building and pretty much matched what they had been doing.”

The 2,680-square-foot building has five indoor fueling bays with 16 fueling positions. A natural gas supply is available as backup to the RNG system.

Highly automated

On the treatment plant side, Gage observes, “Most people in the industry say that staffs are lean. More and more utilities are relying on automation. Feedback from our treatment operations staff was to keep the focus on what the plant was built for, which is to meet the discharge permit. Their main goal is to treat wastewater effectively.

“The utility is starting to lean more toward being holistic on the energy side, but their concern was that they didn’t want the project to be a distraction, where they would have to learn this new process and touch it and inspect it every day.”

Accordingly, the 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.”  

CGRS handles the maintenance under a contract. “We don’t necessarily want to be experts on gas systems; that’s not what we’ve been trained to do,” says Gage. “CGRS specializes in working on high-pressure gas systems. They take care of routine maintenance.

“We worked with CGRS and the manufacturer to make sure we understood what all the planned maintenance tasks were. We occasionally go with them just to observe what they’re doing so we have some capacity within our organization, but we don’t spend staff time maintaining the system.”

Optimum usage

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.

Looking forward

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.

He credits a sound design and collaboration across departments for the project’s smooth performance: “It has really been hands-off; it was not so difficult a project to incorporate into our operations. They built a great facility for our operation. A collaborative approach in the design-build process was really important.”

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.   


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