Enhancing the total nitrogen removal efficiency of anaerobic ammonium oxidation (anammox) with two types of biochars

Although anaerobic ammonium oxidation (anammox) process has the advantage of energy neutrality and resource positivity, the accumulation of nitrate in the effluent decrease the total nitrogen (TN) removal effi-ciency. Biochar has been reported to improve nitrogen removal efficiency in anammox process. Biochars derived from different raw materials are different in electron transfer capacity, soluble composition, and functional groups. However, the specific effect of biochar characteristics on anammox bacteria and the mechanism are still elusive. The present study evaluated the effect and potential mechanism of coconut shell and coal biochars on facilitating TN removal of anammox process. Results showed that quartz sand (42.29% +/- 12.31%) and PE carrier (42.47% +/- 11.85%) had no evident effect on the TN removal compared with control reactor (41.46% +/- 11.33%). Both types of biochar could promote TN removal efficiency (58.90% +/- 13.81% and 55.80% +/- 12.55% in coconut and coal biochars) despite the difference in surface groups and electron transfer capacity. Biochar could provide electron to initiate nitrite reduction and then receive electron from hydrazine oxidation, leading to a lower Delta NO3--N/Delta NH4+-N in coconut (0.14) and coal biochar (0.16) than control reactor (0.26). Genus Candi-datus Brocadia (2.18% and 2.68% in coconut and coal, respectively) was outcompeted by Candidatus Jettenia (11.10% and 20.19%) in the presence of biochar. The abundance of Limnobacter was also increased in the presence of biochar, which was likely because they could improve the electron transfer between biochar and anammox bacteria. The findings of the present study provide insights into the role of biochar in mediating nitrite reduction in the anammox process.