Operating Experience of North America’s First Post Aerobic Digestion for Nitrogen Removal and Biosolids Reduction

The first North American full scale post aerobic digester (PAD) is successfully operating at the Spokane County Regional Water Reclamation Facility (SCRWRF). The SCRWRF PAD has demonstrated ammonia removals of 90% or above without alkalinity or supplemental carbon addition. When operational controls were optimized for total inorganic nitrogen (TIN) removal, the SCRWRF PAD demonstrated effluent TIN removals of greater than 95%. Data from the SCRWRF PAD also demonstrated an additional 30%-40% volatile solids reduction (VSR) beyond typical anaerobic digester VSR of over 60%, resulting in a total VSR of at least 80% in the SCRWRF digestion system. The main operational challenge experienced was temperature buildup due to significant biological heat. Proper automation and controls are critical to maintain proper operation.