Beyond Nitrogen Removal: Emerging Contaminants and Environmental Risk in Anammox-Based Wastewater Treatment

Anaerobic ammonium oxidation (anammox) is an energy-efficient and low-carbon process for nitrogen removal, but its wider application exposes anammox biomass to more complex contaminant mixtures. Existing reviews have examined contaminant inhibition and microbial responses, while their links to environmental risks have received less integrated attention. This review focuses on how representative chemical contaminants and particles affect anammox performance and microbial responses and how these changes may lead to environmental risks beyond nitrogen removal. Responses depend on contaminant type, concentration, exposure pattern, and biomass form (e.g., granules and biofilms). Common responses include contaminant retention in extracellular polymeric substances (EPS) and biomass, changes in granule or biofilm structure, cellular and redox stress, disruption of core anammox metabolism, and shifts in microbial interactions. These changes may lead to ammonium or nitrite leakage, risks related to antibiotic resistance genes and antibiotic-resistant bacteria, potential N2O emissions, and contaminant accumulation in solids. Recovery of nitrogen removal alone does not confirm recovery of microbial activity or a reduction in retained contaminant burdens. We therefore propose evaluating contaminant-stressed anammox systems across liquid, gas, and solid phases, using a source-exposure-response-risk-control framework for monitoring, risk control, and recovery. This framework supports the risk assessment and long-term management of anammox-based treatment.