Feammox for nitrogen removal: replication-aware meta-analysis and evidence authorization from pathway certainty to climate and deployment readiness

ABSTRACT

Anaerobic ammonium oxidation coupled to ferric iron reduction (Feammox) is a potentially low-aeration route for nitrogen removal, rapid publication growth does not necessarily translate into transferable engineering performance. This critical review combines bibliometric mapping, claim-level evidence auditing, and replication-aware meta-analysis to examine pathway causality, nitrogen–iron conservation, robustness, climate implications and deployment readiness. The frozen Dimensions corpus contained 228 unique records published from 2020 to 13 August 2026. Annual output increased from 9 records in 2020 to 64 in 2025, with a further 36 records indexed during partial 2026. Meanwhile, the broader evidence review retained 168 studies for qualitative synthesis and 72 experimental records for claim-level audit. Formal quantitative inference was restricted to 10 independently replicated study-level contrasts. The pooled synthesis yielded RR = 1.437 (95% CI 1.197–1.726), although the exploratory 95% prediction interval (0.790–2.615) showed substantial between-system uncertainty. Importantly, the overall direction remained stable across alternative heterogeneity estimators, variance assumptions, leave-one-out analysis and a 13-specification multiverse. Endpoint diagnostics were also positive for total nitrogen removal (k = 5; RR = 1.241) and NH4-N removal (k = 4; RR = 1.779), although only three selected endpoints reported numerical variance for the exact contrast. Across the 72-record audit, evidence remained concentrated in intermediate certainty tiers, while only 2 of 15 source-audited engineering phases supported a defensible fresh-iron-intensity calculation. Overall, Feammox shows credible engineering potential, but the present evidence does not support a universal design coefficient, a pathway-pure causal multiplier or a universal low-greenhouse-gas claim. Progress toward deployment will therefore require stronger replication, direct pathway attribution, complete iron accounting, long-term real-wastewater validation and climate-resolved measurement.