Loofah sponge carriers: Uncovering the mechanisms of enhanced anammox performance in low-nitrogen wastewater treatment

This study investigated eco-friendly immobilization carriers for AnAOB to enhance nitrogen removal from low-nitrogen domestic wastewater. Natural loofah sponge was evaluated as a novel biofilm carrier, with polyurethane sponge and polyethylene carrier as references. Microbial morphology, community structure, and nitrogen metabolism-related functional genes were systematically analyzed. Although the loofah sponge biofilm possessed the lowest abundances of Planctomycetota (39.38%) and Candidatus Brocadia (38.35%), it achieved over 90% TNRE and a maximum TNRR of 0.067 kgN/(m 3 ·d). The loofah sponge biofilm carrier also exhibited a denser, more uniform biofilm structure by SEM and higher relative abundances of key functional enzyme genes ( hdh , hzs , nirS , and nirK ) and ammonium transporter genes ( amt and FNT ) via metagenomic analysis. Long-term operation and typical cycle experiments validated its superior and stable anammox performance, providing a promising, sustainable, and easily applicable carrier strategy for practical anammox wastewater treatment systems.