Suspended carrier blended with a high loading of zero-valent iron demonstrates augmentation of anammox bacterial enrichment and bioactivity

The application of anammox process is often constrained by the slow growth rate and low bioactivity of anammox bacteria (AnAOB). While zero-valent iron (ZVI) supplementation is a recognized strategy to enhance anammox, its direct addition poses operational challenges. This study proposed a novel approach by developing suspended carriers with melt-blended ZVI to provide sustained iron release and physical support for biofilm growth. Three types of ZVI5@HDPE, ZVI10@HDPE and ZVI15@HDPE carriers were prepared with varying ZVI loadings (5, 10 and 15 wt% respectively) using high-density polyethylene (HDPE), and evaluated in the anammox systems over 87 days. Results demonstrated that ZVI modification significantly enhanced carrier surface hydrophilicity (static water contact angle was 92.55 ± 2.33° for HDPE carriers and 77.76 ± 2.74° for ZVI15@HDPE carriers) and roughness (arithmetical mean deviation of microscopic profile was 37.0 ± 2.4 nm for HDPE carriers and 70.3 ± 4.2 nm for ZVI15@HDPE carriers), which facilitated microbial attachment and biofilm development. Moreover, these ZVI-modified carriers promoted the specific anammox activity (0.13 ± 0.06 g N/(gVSS·d) for HDPE carriers and 0.20 ± 0.01 g N/(gVSS·d) for ZVI15@HDPE carriers), increased heme c content, and enhanced key anammox enzymes activity. Nitrogen removal performance was superior in reactors with ZVI-modified carriers, with ZVI15@HDPE system achieving over 80 % total nitrogen removal efficiency. Microbial community analysis revealed a selective enrichment of Candidatus_Kuenenia (70 % relative abundance) on ZVI15@HDPE carriers, alongside an upregulation of functional genes related to energy metabolism and biofilm formation. This study confirms that ZVI-blended suspended carriers are an effective strategy to augment AnAOB enrichment and bioactivity, overcoming the limitations of direct ZVI supplementation.