Review of biochar as a novel carrier for anammox process: Material, performance and mechanisms

Anaerobic ammonium oxidation (anammox) is globally considered as the most promising nitrogen removal technology for its high efficiency and low energy consumption. Nonetheless, the mainstream application of anammox process still faces obstacles, such as long cell doubling time, poor retention of biomass, environmental sensitivity and contaminant inhibition. Recently, biochar as a novel carrier with high specific surface area, abundant functional groups and environmental friendliness, has received cumulatively attention for promoting anammox process. The current review compiles the preparation methods of biochar with pros and cons for biochar-anammox system. Previous studies are systematically reviewed to clarify the role of biochar in facilitating anammox process in terms of shorting start-up time, improving nitrogen removal performance and alleviating contaminants inhibition. The possible mechanisms are also comprehensively summarized and discussed, including providing a habitat for microorganisms, stimulating extracellular polymeric substances secretion, facilitating extracellular electron transfer and promoting functional genes expression. Additionally, this review proposed several promising strategies for standardization, harmlessness and stabilization of biochar to help overcome the engineering challenges in application. Future research effort should include biochar modification and the development of composite carriers.