Effects of nitrogen load on nitrogen removal efficiency of biochar-enhanced anammox systems
To investigate the effects of biochar on the nitrogen removal efficiency of Anammox,batch experiments were conducted.The total nitrogen removal efficiency of the anammox system under three different nitrogen loads(100,300,and 500 mg/L total nitrogen)was examined.The underlying mechanisms were analyzed based on changes in biochar performance before and after reaction,bacterial extracellular polymeric substances secretion levels,and microbial community succession characteristics.Results showed that biochar addition increased the total nitrogen removal efficiency in all groups(TN100,TN300,and TN500)by 13.6%,4.5%and 1.8%,respectively compared with the control group.Additionally,the final nitrate concentration was significantly lower in biochar-amended groups than in the control group.This improvement could be attributed to biochar's electron conductivity and redox properties,which interacted with EPS,promoting extracellular electron transfer and enhancing nitrate reduction,with the most significant effect observed under low nitrogen loads.Analysis of extracellular polymeric substances showed that the TN 100 group with biochar had the highest average extracellular polymeric substances secretion concentration and better sludge structure stability.However,extracellular polymeric substances(EPS)concentration decreased with increasing nitrogen load.This was explained by the higher nitrate accumulation in high nitrogen-loaded anammox systems,leading to increased EPS consumption as an endogenous electron donor during nitrate reduction.High-throughput sequencing and KEGG prediction analysis showed that biochar significantly increased the relative abundance of anammox bacteria Candidatus Kuenenia,as well as the denitrifying bacteria Acinetobacter and Afipia under low nitrogen load conditions.Furthermore,the functional genes for nitrate reductase EC:1.7.5.1 and EC:1.9.6.1 exhibited higher expression levels in the TN 100 group with biochar,revealing the reason for the more significant nitrogen removal efficiency enhanced by biochar under low nitrogen loading conditions.