Functional carriers in simultaneous partial nitrification, anaerobic ammonium oxidation and denitrification: Nitrogen...

Simultaneous partial nitrification, anammox and denitrification (SNAD) couples multiple pathways for nitrogen removal from wastewater with limited organic carbon. However, competition among functional guilds for O₂, NO₂^-, organic carbon and space makes it difficult to infer the actual role of carriers from total nitrogen removal or microbial abundance. This review follows a conceptual chain from carrier properties and the carrier-microbe interface to local microenvironments, niche partitioning and nitrogen flux allocation. It examines how carrier architecture and surface chemistry, extracellular polymeric substances (EPS), substrate transport, signal bioavailability and redox activity influence biomass retention and pathway competition. Particular attention is given to NO₂^- supply and consumption, the competing fates of NO3- through complete denitrification, partial denitrification-anammox (PdNA) or dissimilatory nitrate reduction to ammonium (DNRA), and changes in the anammox contribution under different operating conditions. A claim-specific E1-E3 analytical evidence framework distinguishes performance associations, mechanistic support, and pathway-resolved or controlled-intervention evidence, while defining the interpretive scales of nitrogen balance estimates, ex situ 15N potential activity and in situ reactor fluxes. Current evidence consistently supports carrier-mediated effects on biomass retention and local mass transfer, whereas signalling regulation, direct extracellular electron transfer and carrier-specific causality require targeted validation. Biofilm ageing, interfacial drift and reactivation after maintenance affect the long-term preservation of target functions, while material loss and life-cycle burdens constrain engineering value. We therefore propose an evidence chain linking material properties, local processes, target nitrogen fluxes and long-term operating performance as the basis for evaluating functional carriers in SNAD.