The Same…But Completely Different: Thermophilic Aerobic Digestion Overhaul(s) in the Pacific Northwest

The McMinnville Water Reclamation Facility (WRF), located in Oregon's Willamette Valley, has long relied on thermophilic aerobic digestion to manage biosolids while supporting a community known for tourism, agriculture, and environmental stewardship. The facility previously operated a first-generation Autothermal Thermophilic Aerobic Digestion (ATAD) system consisting of three digesters with proprietary mixer-aerators and foam 'cutters'. Since startup in 1996, the legacy system (with some innovative aeration and mixer upgrades in 2006) was usually capable of achieving thermophilic conditions, but experienced persistent operational challenges, including poor oxygen transfer, incomplete digestion, odors, solids loading limitations and difficulty maintaining stable thermophilic temperatures-issues that became more pronounced during seasonal production of tertiary sludge. Despite these challenges the WRF staff was proactive, persistent, and resourceful, which enabled them to improve, tweak and operate their first generation ATAD system for nearly 30 years - much longer than many others. Ultimately, it was time to address these limitations and improve long-term reliability, so McMinnville evaluated options and ultimately selected a second-generation ThermAer ATAD system. ThermAer utilizes jet aeration (pumps, blowers and jet headers), instrumentation, and integrated process control to deliver consistent solids reduction, Class A biosolids production without offensive odors. This technology has been refined through more than three decades of full-scale operating experience and had been in use for 15 years at another facility in the Pacific Northwest - another upgrade of a first-generation ATAD system in Douglas County, Washington. The construction and phasing process was unique in that the existing steel ATAD tanks were to remain and would be operated until the new ThermAer system was completed. There were modifications to the odor control air handling systems to incorporate other nearby processes. The existing post-digested, Class A liquid biosolids storage tank remained in place with some minor (though exciting) modifications. The new tanks were all concrete with an equipment room placed between the ThermAer and SNDR. New odor control biofilters were added. At McMinnville, the ThermAer process was started up in the spring of 2025. Unlike Anaerobic Digester startups that often require extensive planning, seeding and phased feeding, the fast-growing aerobic biology established very quickly, and the first tank was at operating temperatures in just over a week. These digesters achieve sustained thermophilic temperatures of approximately 59-63 °C (140-150°F) without supplemental heat input. Like a compost pile, in this aerobic process, heat is generated internally through exothermic microbial oxidation of organic matter. Evaporative heat losses are minimized through high-efficiency jet aeration and precise oxygen delivery. Oxidation Reduction Potential (ORP), temperature, and pH are continuously monitored to dynamically match oxygen supply with biological demand, ensuring complete oxidation while conserving energy. The variable-flow jet aeration system provides high-shear mixing, excellent oxygen transfer efficiency at low air-to-liquid ratios and effective foam control via hydraulic splash cones. The ThermAer upgrade increased biosolids processing capacity at McMinnville by approximately 20% while achieving total solids reductions of 48-55% and volatile solids destruction of 60-65 %. Improved cell lysis under thermophilic conditions, reduces hauled biosolids volumes, and enables digestion of tertiary sludge streams that previously strained the original system. The combination of time, temperature, and isolation meets USEPA Part 503 requirements for Class A biosolids, expanding reuse and land application options. %. The specifications required specific performance criteria to be met and some of this data will be shared. Ammonia released during thermophilic digestion is managed through integration with a Storage Nitrification Denitrification Reactor (SNDR). Following the ATAD, biosolids are transferred to the SNDR, which operates under mesophilic conditions where nitrification and denitrification can occur. Process control based on pH, temperature, and ORP enables conversion of ammonia to inert nitrogen gas without the need for external alkalinity. Carbon dioxide generated during ThermAer digestion provides sufficient bicarbonate alkalinity to support nitrification, reducing chemical consumption. In addition, ATAD off-gas is routed through the SNDR headspace, where cooler temperatures and lower pH promote ammonia absorption back into the liquid phase, significantly reducing ammonia loading to downstream odor control systems. Final off-gas polishing is provided by a natural media-based biofilter designed for high ammonia removal efficiency, minimal headloss, and low energy demand. The integrated ATAD-SNDR-biofilter configuration delivers near-zero odor emissions, a critical performance requirement for McMinnville and any facilities located near residential, commercial areas. Operational experience at McMinnville mirrors experiences installations, including the Douglas County, Washington ThermAer system commissioned in 2009. Together, these facilities illustrate how second-generation thermophilic aerobic digestion can reliably overcome the limitations of first-generation ATADs while increasing capacity, improving odor control, and simplifying operation and maintenance. This presentation will focus on McMinnville WRF as a detailed case study allowing an opportunity for the plant to share their story while placing the project within the broader regional and national context of multiple ATAD facilities in the Pacific Northwest and nationwide. Operational data, design considerations, startup experiences, and operator perspectives will be shared to demonstrate how modern ThermAer systems have become a reliable Class A biosolids solution (where that still matters) and a viable solution to sludge mass and volume reduction challenges where land application and other beneficial use has been restricted.