Sidestream Nitrogen Removal from Reject Water Using Partial Nitritation and Anammox Process Integration

Abstract

Reject water generated from sludge dewatering processes in wastewater treatment plants contains high concentrations of ammoniacal nitrogen, creating significant challenges for conventional biological treatment systems. This study investigates sidestream nitrogen removal from reject water through the integration of partial nitritation and anaerobic ammonium oxidation (anammox) processes under controlled operating conditions. The research focuses on evaluating nitrogen removal efficiency, microbial activity, and process stability for high-strength ammonium wastewater treatment. Experimental analyses were conducted to examine the influence of dissolved oxygen concentration, temperature, hydraulic retention time, pH, free ammonia concentration, and influent nitrogen loading on process performance and nitrogen conversion pathways. Results demonstrated that partial nitritation successfully converted approximately half of the ammonium into nitrite, creating favorable conditions for the subsequent anammox process, which efficiently converted ammonium and nitrite directly into nitrogen gas with minimal nitrate formation. The integrated system achieved high nitrogen removal efficiency with reduced oxygen demand and lower external carbon source requirements compared with conventional nitrification–denitrification systems. Microbial community analysis confirmed stable coexistence of ammonia-oxidizing bacteria and anammox bacteria under optimized operating conditions. Kinetic evaluation revealed that temperature and dissolved oxygen control play critical roles in maintaining selective nitrite production and sustaining anammox activity. The process integration also resulted in lower sludge production and improved energy efficiency in sidestream treatment operations. Comparative assessment with traditional nitrogen removal methods indicated that the proposed approach offers substantial operational and environmental advantages for high-ammonia wastewater management. The findings highlight the potential of partial nitritation–anammox integration as a sustainable and cost-effective technology for advanced nitrogen removal in modern wastewater treatment facilities.