Soybean-processing wastewater as an electron donor for denitrification

Soybean-processing wastewater (SPW) has a high COD concentration that has the potential to be an electron-donor substrate to drive tertiary denitrification. However, SPW also has a high concentration of organic nitrogen that will be released as ammonium-level N. In this work, SPW was utilized as the electron donor to drive denitrification in the anoxic stage of a two-stage denitrification (AN) + nitrification (OX) process, which contained two alternations between denitrification and nitrification. The total N concentration in the effluent was below the discharge standard after two-stage denitrification + nitrification. An important finding is that amino acids in SPW could supply sufficient electron donor to reduce exogenous nitrate and nitrogen released from the amino acids, based on our analysis of electron flow during the alternations of denitrification and nitrification. SPW contained soluble and solid COD that could be electron donors for denitrification. Their simultaneous oxidation gave faster kinetics than either by itself, and soluble COD gave faster denitrification kinetics than solid COD. The two fractions combined gave the fastest kinetics because different bacteria utilized them. In AN, Dechloromonas and Brooklawnia likely utilized only soluble COD, while Paludibacter and Christehsenellaceaue could utilize solid COD for denitrification. Although present in small abundances, Nitrosospira, Nitrosomonas, and Nitrospira were able to oxidize ammonium to nitrate in OX, while Nakamurella, Micropruina, and Ottowia probably were responsible for the biodegradation amino acids.