Enhanced denitrification performance of constructed wetlands for wastewater treatment plant effluent using zero-valent iron-based mixing solid carbon sources.

Here, a novel zero-valent iron-based mixing solid carbon sources (ZVI-MSCSs) embedding cellulose, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and ZVI was developed to advance nitrogen removal from municipal wastewater treatment plant effluents. Leaching experiments showed that ZVI-MSCSs combine the bioavailability of carbon release components and moderate carbon release rate. When applied in constructed wetlands, ZVI-MSCSs achieved nitrate removal rates up to 97% with less than 10% fluctuation under decreasing hydraulic retention time. Mechanistic studies revealed that ZVI promoted cellulose degradation into labile organic compounds to supply more NADH and upregulating electron carriers (complex I/III and Cyt c), thereby enhancing electron transfer and denitrifying enzyme activities (NAR, NIR, NOR, and NOS). High-throughput sequencing displayed that the biological basis for enhanced nitrogen and carbon metabolism lies in the shift from Proteobacteria to Clostridioides, driving a transition to iron-carbon mixotrophic denitrification. This study provides key technical support for efficient bio-denitrification using solid carbon sources in constructed wetlands.