Fate of organic nitrogen in amino acids during alternating denitrification and nitrification

Nitrogen-containing organic compounds, such as amino acids in soybean-processing wastewater, can be used as electron donors to drive denitrification, but their biodegradation releases ammonium nitrogen that must be nitrified and denitrified to maintain total-nitrogen removal. We evaluated glutamate, isoleucine, and methionine as example amino acids to explore the fate of nitrogen when they are used as electron donor to drive denitrification during two stages of alternating denitrification and nitrification. The experimental results documented that each amino acid enabled complete removal of exogenous NO₃⁻ in the first stage of denitrification and complete NO₃⁻ removal in the second stage. After two alternations of denitrification and nitrification, the TN concentration in effluent was less than 5 mgN/L for all amino acids, and COD in the effluent was less than 25 mg/L. Based on stoichiometry and the ratio of chemical oxygen demand (COD) to organic N in each amino acid, 57%–66% of the COD from the amino acids had to be oxidized to reduce the endogenous NO₃⁻–N in the first stage. N from the amino acids was nitrified and denitrified in the subsequent nitrification and denitrification stages, and the percentages of COD used for denitrification from both stages were 72%–85%. The residual NH₄⁺-N concentrations were slightly higher with methionine, possibly due to inhibition from sulfide released from methionine.