Promoting performance of Anammox by iron loaded sludge biochar with hydrothermal carbonization (HTC-Fe-BC) addition

Anaerobic ammonia oxidation (Anammox) treatment cannot always achieve optimal efficiency due to its sensitivity to environmental conditions. The nitrogen removal efficiency of Anammox reactors with different iron loaded sludge biochar modified by hydrothermal carbonization (HTC-Fe-BC) addition dose of 0, 2.5 and 5.0 g/L (named BC0, BC1 and BC2) were analyzed. The effects of adding HTC-Fe-BC on the performance of Anammox reactor, extracellular polymeric substances (EPS), microbial community and functional genes were investigated. The results showed that the addition of HTC-Fe-BC could reduce the environmental sensitivity of Anammox reactor, the removal efficiency of total nitrogen increased from 51.20 (BC0) to 70.83 % (BC2), and the activity of Anammox sludge improved by 49.58 %. HTC-Fe-BC increased the activity of Anammox sludge by promoting the secretion of protein, humic acids and cytochrome c in EPS and alleviated the inhibitory effect by mediating the Anammox electron transfer process. After EPS removal and enzymatic hydrolysis, the activity of Anammox sludge in BC2 decreased by 61.38% and 79.80 %, respectively, significantly higher than the reduced values of BC0. Further, the relative abundance of Candidatus_Brocadia in Anammox reactors supplemented with HTC-Fe-BC was increased by 35.56–55.56 % compared to the control group, as were the relative abundances of the key functional genes connected to N transformation. These results deepened the understanding of the potential mechanism by which HTC-Fe-BC improved the efficiency of Anammox system.