Enhanced low-temperature nitrogen removal by anammox with iron‑sulfur modified biochar: Performance and mechanisms

The inferior nitrogen removal capacity of an anaerobic ammonium oxidation (anammox) system in low-temperature environments restricts its application in cold regions. In this work, iron–sulfur biochar (FeS–BC) was synthesized to improve the performance of anammox treatment at 15 °C. Compared with the control group, dosing with 2.5 g/L FeS–BC led to a 26.97% increase in nitrogen removal efficiency (NRE) and a 23.62 mg N/(g MLSS·d) increase in specific anammox activity (SAA). In addition, FeS–BC promoted the secretion of extracellular polymeric substances (EPS), including proteins (PN), polysaccharides (PS), and heme c. Enzymatic hydrolysis experiments revealed that both PN and PS in EPS contributed to increased SAA. Moreover, FeS–BC-derived dissolved organic matter (BDOM) positively contributed to increased SAA. FeS–BC addition improved microbial community richness and diversity, increased the relative enrichment level of Candidatus_Kuenenia , and resulted in 8.98-fold and 14.13-fold increases in the relative abundances of the genes encoding hydrazine dehydrogenase (HDH) and hydrazide synthase (HZS), respectively. The addition of FeS–BC increased the relative abundance of cold shock proteins and the proportions of functional genes linked to iron and sulfur transformations to induce complex and variable morphological transformations to mediate anammox and nitrate nitrogen reduction. Accordingly, the experimental findings of this study offer a viable technical strategy for increasing the NRE of anammox systems operating in low-temperature (15 °C) environments.