Post-aerobic digestion is proving to be a viable alternative in the advanced digestion arena, as seen at the Spokane County Regional Water Reclamation Facility

By Bruce Johnson and Adam McClymont, CH2M HILL, Dand avid Moss, Spokane County Utilities

Bruce Johnson and Adam McClymont along with David Moss of Spokane County Utilities will present their paper “Startup and Operation of North America’s First Full Scale Post Aerobic Digester” on Wednesday, July 31st during the 2013 WEF/IWA Nutrient Removal and Recovery Conference in Vancouver. Learn more about CH2M HILL’s entire conference presence.

Advanced digestion processes have gained recent interest because they provide improved volatile solids reduction and reduced biosolids for disposal.  Furthermore, increasingly stringent effluent nutrient limits have focused industry attention on the management of phosphorus and ammonia-rich recycle streams typically associated with anaerobic digesters.  Post-aerobic digestion (PAD) is a recently developed advanced digestion process where aerobic digestion is done after an anaerobic digestion step, thus improving overall nutrient and VSS removal. Post aerobic digestion uses the endogenous decay of biomass to provide a carbon source for the denitrification of the nitrified anaerobically digested sludge and gives clients “free” nitrogen removal.

SCRWRF Process flow diagramOur paper focuses on the start-up of the first full scale post aerobic digester at the Spokane County Regional Water Reclamation Facility.  The PAD has proven that it can exceed expectations, when temperature issues are not impacting performance, in its ability to reduce soluble nitrogen recycle to the head of the facility, and data indicates VS destructions of approximately 65% in the combined anaerobic/aerobic digestion system, which is at the higher end of anaerobic digestion performance, especially considering the high level of metal salts in the feed.

Operation of the first PAD system has uncovered a number of design aspects that should be considered in future facilities.  Even after a 15 day anaerobic detention time upstream, significant biological heat is generated in the PAD, to the extent that it is possible to kill off nitrifiers (at approximately 44 degrees C) during summer operating conditions if not cooled.  It was also determined that maintaining good nitrification is critical to stable operation, since nitrification failure can easily lead to ammonia toxicity and full PAD failure.

This first-of-its-kind digestion process is proving out as a viable alternative in the advanced digestion arena.  The post aerobic digester has shown dual benefits of reduced nitrogen recycle with no supplemental carbon needed, as well as improved VSS destruction.

Bruce Johnson, CH2M HILL Senior Principal Technologist

Bruce Johnson

Bruce Johnson is a Senior Principal Technologist and process engineer specializing in the process modeling, design, and sizing of biological treatment systems, solid-liquid separation equipment, and waste sludge reduction. He has over 20 years of global experience operating, troubleshooting, and designing water and wastewater treatment plants and equipment.

Adam McClymont is a wastewater operations supervisor and has supported the operation of facilities in Texas, Oklahoma, Virginia, and Alberta, Canada.  He is currently assistant project manager at the Spokane County RWRF which includes the new PAD facility in Spokane County, Washington.