Addition of external carbon sources to postdenitrification biofilters (DNFs) is frequently used in municipal wastewater treatment plants to enhance dissolved inorganic nitrogen removal. However, little is known about its influence on the removal of dissolved organic nitrogen (DON). This study investigated the effect of the carbon-to-nitrogen (C/N) ratio (3, 4, 5, and 6) on the removal characteristics of DON and bioavailable DON (ABDON) in the pilot-scale DNFs treating real secondary effluent. Results showed that DNFs effluent DON accounted for 31.2-39.8% of the effluent total nitrogen. The maximum effluent DON and ABDON concentrations both occurred in DNF operated at a C/N ratio of 3. There was no significant difference in effluent DON concentrations in DNFs at C/N ratios of 4, 5, and 6; however, effluent ABDON and DON bioavailability significantly decreased with C/N ratios (p < 0.05, t-test). According to the chemical composition analysis, effluent DON at high C/N ratios tends to contain less % molecular weight < 1 kDa nitrogenous organic compounds and proteins/amino sugars-like nitrogenous organic formulas, which is likely responsible for its low bioavailability. Overall, this study indicates the benefit of a high C/N ratio during the DNF process in terms of controlling the DON forms that readily stimulate algal growth.

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