Biogas Treatment and Renewable Natural Gas Fueling Station

The City of Longmont operates a 13-million-gallon-per-day (mgd) wastewater treatment plant (WWTP) that, on average, produces approximately 140,000 cubic feet per day (cfd) of biogas, an energy-rich by-product of the solids treatment process. In 2017, the City implemented the Digester Gas Utilization Project to make better use of this renewal source of energy, of which two-thirds was being flared to the atmosphere.

As part of this project, Carollo Engineers evaluated various alternatives to beneficially reuse the biogas, including combined heat and power systems, digester gas supply to local industries, and biosolids drying. Ultimately, the City decided to treat and convert 100 percent of their WWTP’s biogas into renewable natural gas (RNG) that fuels the City’s brand new natural-gas-fueled trash and recycling trucks. To this end, the City installed a new biogas-upgrading system adjacent to the WWTP’s anaerobic digesters and constructed a new compressed natural gas vehicle-fueling station and maintenance building to serve their relocated waste services fleet.

The City decided to use a progressive design-build delivery model and selected CGRS and Carollo Engineers to comprise their contractor and designer team. Carollo Engineers provided design services for the single-pass membrane biogas-upgrading system and RNG pipeline to the vehicle-fueling station, making sure to incorporate ample amounts of gas storage upstream and downstream of the biogas-upgrading system.

Since the biogas-upgrading system and vehicle-fueling station came online in March 2020, the City offsets nearly 100,000 gallons of diesel fuel per year, not only saving $200,000–$300,000 annually in fuel costs but also reducing greenhouse gas emissions by roughly 1,000 metric tons of carbon dioxide equivalents per year—the equivalent of removing 200 cars from the road. And, because RNG can also be sold as valuable renewable identification numbers or “credits” through the U.S. Environmental Protection Agency’s Renewable Fuel Standard program, the City receives an additional $150,000–$250,000 per year.

Today, the biogas-upgrading system is operated almost continuously, making sustainable use of the WWTP’s biogas for the City’s Waste Services trucks. The City plans to complete the transition of the remaining 10 waste services trucks to RNG in 2024.