Various electron donors for biological nitrate removal: A review.

Nitrate (NO) pollution in water and wastewater has become a serious global issue. Biological denitrification, which reduces NO to N (nitrogen gas) by denitrifying microorganisms, is an efficient and economical process for the removal of NO from water and wastewater. During the denitrification process, electron donor is required to provide electrons for reduction of NO. A variety of electron donors, including organic and inorganic compounds, can be used for denitrification. This paper reviews the state of the art of various electron donors used for biological denitrification. Depending on the types of electron donors, denitrification can be classified into heterotrophic and autotrophic denitrification. Heterotrophic denitrification utilizes organic compounds as electron donors, including low-molecular-weight organics (e.g. acetate, methanol, glucose, benzene, methane, etc.) and high-molecular-weight organics (e.g. cellulose, polylactic acid, polycaprolactone, etc.); while autotrophic denitrification utilizes inorganic compounds as electron donors, including hydrogen (H), reduced sulfur compounds (e.g. sulfide, element sulfur and thiosulfate), ferrous iron (Fe), iron sulfides (e.g. FeS, FeS and FeS), arsenite (As(Ш)) and manganese (Mn(II)). The biological denitrification processes and the representative denitrifying microorganisms are summarized based on different electron donors, and their denitrification performance, operating costs and environmental impacts are compared and discussed. The pilot- or full-scale applications were summarized. The concluding remarks and future prospects were provided. The biodegradable polymers mediated heterotrophic denitrification, as well as H and sulfur mediated autotrophic denitrification are promising denitrification processes for NO removal from various types of water and wastewater.