Insights to the roles of nitrogen-doped biochar in anaerobic digestion

Biochar has been proven as a promising additive to enhance methane production from anaerobic digestion (AD). To further improve the roles of biochar, nitrogen-doped biochar (NBC) has been widely prepared and studied. This review systematically summarized the current research progress of NBC in AD. Regarding internal nitrogen contained in bio-wastes or introducing external nitrogen sources are the two nitrogen sources for NBC preparation. The physical and chemical characteristics of NBC like adsorption, electrical conductivity, pH buffering, and redox, affect AD through improving operation conditions, promoting organic matters conversion performances, optimizing microbial community structures, and accelerating electron transfer performances. Moreover, chemical nitrogen forms like amino and oxidized nitrogen on the surface of NBC and structural nitrogen forms like pyrrolic nitrogen, pyridinic nitrogen, and graphitic nitrogen in carbon skeleton all play important roles in regulating AD process. Thus, how to quantify nitrogen forms and how the different nitrogen forms affect the enrichment performance and syntrophic relationships of electroactive microorganisms are needed to reveal the structure-efficiency relationship between NBC and AD. Overall, it is an effective alternative to evaluate the potentials of NBC in AD regarding performance evaluation, environment analysis, and economic assessment from NBC preparation to its role in AD for the industrialization of NBC.