The Successes and Challenges of the First North American Full Scale Post Aerobic Digester

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 and ammonia effluent concentrations of less than 50 mg/L without alkalinity or supplemental carbon addition. When operational controls were optimized for total inorganic nitrogen (TIN) removal, the SCRWRF PAD demonstrated effluent TIN concentrations of less than 40 mg/L and 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, even considering the high metal salt load in the influent.Additional evidence of additional VSR destruction in the PAD is that final biosolids hauled per pound of raw influent biochemical oxygen demand (BOD) decreased during periods when PAD was functional and that the average rate of cumulative solids production decreased by 12-43% when PAD was online.The main operational challenge experienced with this first of its kind installation at SCRWRF was regarding temperature due to significant biological heat generated from the resultant process. Proper automation and controls are critical to maintain proper operation for the key parameters within the PAD of ammonia, nitrate, dissolved oxygen, temperature, and pH. Foam is another challenge experienced during startup for which both operational and engineering solutions have been developed.Capital cost for the construction and design of the PAD system at SCRWRF was approximately $2,040,000. Capital cost savings for a PAD system include elimination of carbon and alkalinity chemical storage and feed systems.The average annual cost to operate PAD blowers at SCRWRF is $53,200/year and averages $0.80/kg ($0.36/pound) of ammonia removed. The annual aeration cost is overcome by the estimated $65,000/year to $200,000/year savings in final biosolids disposal costs resulting from the additional total solids destruction.