Board of Adjustment approves 110-foot stack for Metro Water Recovery to meet air-permit requirements

Commerce City’s Board of Adjustment on the evening of the meeting approved a height exception allowing Metro Water Recovery to install a 110-foot exhaust stack for a regenerative thermal oxidizer at 6450 York Street.

The board voted to approve case AH24-001, finding that a 110-foot, 16-inch-diameter stack is needed so the project can meet air-permit requirements administered by the Colorado Department of Public Health and Environment (CDPHE). Metro Water Recovery said the stack height is necessary to disperse sulfur dioxide (SO2) emissions below the 1-hour national ambient air quality standard.

Metro Water Recovery and its design consultant presented the project. Becky Luna, a consultant with Corolla Engineers, described the facility and the technical reasons for the stack. She said Metro is the largest water and resource recovery facility in the Rocky Mountain West, serving some 65 local governments and about 2.2 million people and treating an average of about 138,000,000 gallons per day. Luna said the facility has produced biogas in anaerobic digesters for decades; Metro plans to upgrade biogas to pipeline-quality methane and combust the remaining non-usable gas stream in a regenerative thermal oxidizer (RTO). She said, “There will not be any visible flame or flaring off the top of that stack.”

Luna said Metro’s modeling showed a 110-foot stack (16-inch diameter) is required to meet the 1-hour SO2 standard (196 µg/m3) and to avoid degrading local air quality. Project presentation materials noted the treatment site occupies roughly 135.6 acres in an I-3 (heavy intensity industrial) zone and is centrally located on the Metro property, with industrial neighbors including a Suncor refinery and Xcel Energy’s Cherokee generating station nearby. Metro said the work is being carried out within the existing facility footprint and will not change the site’s use.

Staff planner Nick Berry explained that a height exception is an administrative approval for structures that exceed the code’s height limit when extraordinary circumstances or state/federal requirements make the code standard infeasible. He reported no public comments had been received as of Feb. 11. During questioning board members sought clarification about visible flaring, stack diameter, who pays for the facility, and whether Metro treats Commerce City wastewater; Metro representatives confirmed Commerce City sewage is processed at the plant and that Metro is a tax-exempt special district funded by connector rates.

Board members discussed the city’s review process. One board member expressed concern that the city did not perform an independent third‑party review of the air‑modeling; staff clarified that the applicant provided modeling and the city’s technical review resources were limited. After discussion the board approved the height exception based on the criteria in the staff report. Electronic voting produced four yes votes and no recorded no or abstentions.

Metro representatives said the project is being built in two phases: Phase 1 (boiler/mechanical building) is already under construction; Phase 2 (biogas upgrading and RTO) had design submitted to the city building division in December 2024. Metro expects a notice to proceed for Phase 2 in 2025 and full commissioning in 2027. Staff and Metro said a revised air‑permit application will be submitted to CDPHE as part of the equipment change.

The board’s approval allows Metro to proceed with the design and to continue work toward state permitting; final construction permits and the state air‑permit decision remain separate processes.