Digester Gas to Biomethane: Design, Construction, and Operation Lessons Learned

The Lulu Island WWTP Sludge Gas Treatment System upgrades surplus biogas produced from the digesters and delivers pipeline quality biomethane to the natural gas utility as a preference to flaring, to support the Board-established regional greenhouse gas emission reduction goal. The business case for the project is presented, which estimated a 16-year payback. The focus of this discussion is on the challenges faced during the design, construction, and operation of the treatment system, including technology selection, fluctuating surplus biogas flow, and process design and control considerations. Construction of the system was completed in September 2021, and it is currently in operation. This innovative project will contribute to regional environmental objectives, all without subjecting the region’s sewer ratepayers to unreasonable financial risk. The system has been consistently producing greater than 98% methane and is projected to generate 60,000 gigajoules (5.69 x 1010 BTU) of RNG for the natural gas utility each year, enough to supply 600 typical homes.