Microaerophilic activated sludge system for ammonia retention from high-strength nitrogenous wastewater: Biokinetics and mathematical modeling

It is crucial to shift from conventional ammonia removal to recovery to solve environmental problems and resource scarcity. Recovering ammonia from high-strength nitrogenous wastewater can simultaneously save the energy and cost involved in ammonia oxidation, produce ammonia without CO2 emissions, and prevent N2O emissions. A technology for concentrating and recovering ammonia by retrofitting a conventional activated sludge (CAS) system is promising but faces significant challenges in oxidizing organic carbon but not ammonia in wastewater. This study employed biokinetic analysis and mathematical modeling to comprehensively assess the operation and effectiveness of a microaerophilic activated sludge (MAS) system for simultaneously removing chemical oxygen demand (COD) and retaining ammonia. We show that heterotrophic bacteria (HB) in the MAS system had higher maximum specific growth rate (mu HB), biomass yield coefficient (YHB), and decay coefficient (bHB) but lower affinities for oxygen and COD than HB in CAS. Simulations using the developed model demonstrated that COD removal efficiencies of > 90%, ammonia retention efficiencies of > 90%, and low N2O emission factors (N2OEF) of < 0.01% can be achieved at solids retention times (SRTs) of 3-50 d, hydraulic retention times (HRTs) of 0.1-1 d, and a volumetric oxygen transfer coefficient (KLa) of (24.58 +/- 0.30)HRT-1. The combination of the biokinetics and mathematical modeling revealed the favorable MAS operating conditions allowing efficient ammonia retention and COD removal, which paves the way for turning a CAS system into a MAS system by retrofitting the configuration without high capital expenditures, requiring future intensive study.