Anammox in nature-based systems for urban water management: A review and Bayesian assessment of environmental drivers

Nitrogen pollution from stormwater runoff and domestic/industrial wastewater poses major challenges for sustainable urban water management. Decentralized nature-based solutions (NBS), including bioretention systems (BRS), constructed wetlands (CWs), and roadside swales (RS), provide sustainable treatment options but often show inconsistent nitrogen removal due to environmental variability. The anaerobic ammonia oxidation (anammox) process, a critical component of the global nitrogen cycle, has emerged as a promising pathway to enhance nitrogen removal in NBS. This review synthesizes current knowledge on the occurrence, mechanisms, and drivers of anammox in urban NBS. A Bayesian modelling approach was applied to quantify the combined effects of key environmental factors -dissolved oxygen (DO), system depth, hydraulic retention time (HRT), pH, temperature, and influent nitrogen composition-on anammox activity. Results indicate that low DO generally favours anammox, while greater depth and longer retention times help maintain stable anoxic conditions. Activity is promoted under mesophilic temperatures but declines sharply at colder ranges. Nitrite availability is a consistent limiting factor, whereas elevated ammonium concentrations and high C/N ratios can suppress activity through acidification and microbial competition. Plant presence indirectly benefits anammox by moderating DO and stabilizing pH. Additionally, this review highlights the potential of advanced pathways-simultaneous nitrification-anammox-denitrification (SNAD) and partial denitrification-anammox (PD-A)-to further improve nitrogen removal efficiency in urban NBS. By consolidating mechanistic insights, quantifying environmental drivers, and identifying key research gaps, this work provides a framework to guide the optimization of NBS design and advance sustainable nitrogen management in urban water systems.