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By Heidi Bauer, Bruce Johnson, and Tom Johnson A sidestream is any flow stream resulting from the treatment of biosolids that is returned to the liquid treatment train. Sidestream flow streams at facilities with anaerobic digestion are targeted for nutrient removal because they exhibit relatively small flow with concentrated nutrient loading back to the liquid treatment train. Post-aerobic digestion (PAD) and anaerobic ammonium oxidation (anammox) are two sidestream treatment technologies that are beneficial for the reduction of nitrogen recycled back to the liquid stream without the need for supplemental carbon or alkalinity. However, because the two treatment options differ greatly, careful evaluation will help determine which is the most beneficial and cost-effective option for wastewater treatment facilities. Post-Aerobic Digestion PAD is a recently developed, advanced digestion process, where aerobic digestion is designed to follow anaerobic digestion. The most significant reason for implementing PAD is the reduction of nitrogen recycled back to the liquid stream without a need to supplement with carbon or alkalinity. Other benefits include volatile solids reduction, odor reduction, and struvite stabilization. Challenges, which have been overcome by operational controls and engineered solutions, include significant biological heat generated by the process and foam. Spokane County Regional Water Reclamation Facility (SCRWRF) implemented a full-scale PAD facility in late 2011 to help achieve strict nutrient removal, including a maximum monthly effluent limitation of 10 mg/L total nitrogen. (Figure 1) Denver Metro Wastewater Reclamation District's (MWRD) Northern Treatment Plant is also implementing PAD technology, which will be placed into service in the fall of 2016. The MWRD selected PAD as a cost-effective way to help achieve strict nutrient removal criteria, including a maximum daily nitrate value of approximately 10 mg/L total nitrogen, while also reducing biosolids hauling costs. Typically, MWRD hauls biosolids for 30 to 60 miles to both private and District- owned land application sites. Anammox Anammox harnesses a specific species of autotrophic bacteria that can achieve partial nitritation-deammonification (or, the conversion of ammonia and nitrite to nitrogen gas) under anoxic conditions. Anammox bacteria work alongside ammonia-oxidizing bacteria under partial aerobic/partial anoxic conditions to ultimately convert ammonia to nitrogen gas without fully nitrifying and denitrifying. Like PAD, one of the key benefits of anammox is the reduction of nitrogen recycled back to the liquid stream without supplemental carbon or alkalinity. Other benefits include lower energy consumption and less oxygen compared to conventional nitrification. Challenges to overcome include the slow growth of the anammox bacteria, as well as its competition with nitrite-oxidizing bacteria. Alexandria Renew Enterprises (AlexRenew) Water Resources Recovery 12 wateronline.com n Water Innovations Two Powerful Sidestream Treatment Technologies Battle To Be The Best A comparison of post-aerobic digestion (PAD) and anaerobic ammonium oxidation (anammox) for sidestream nutrient removal at wastewater facilities Centrifuge Nitrogen Removal & Solids Reduction Centrate Anaerobic Digesters Aerobic Digester Waste Solids Raw Chemically Enhanced Primary Treatment Step Feed Bioreactor Membranes Figure 1. SCRWRF process flow diagram with post-aerobic digestion