C/N ratios inform sustainable aerobic denitrification for nitrogen pollution control: Insights into the key parameter from a view of metabolic division.

Aerobic denitrification is a feasible and sustainable process for nitrogen removal in environmental protection. The C/N ratio was the vital one that has a significant impact on the performance. However, comprehension of the C/N ratio effect on this bioprocess was not well clarified. In this study, the response of an aerobic denitrifier (Pseudomonas balearica strain RAD-17) to the C/N ratio was investigated from a view of metabolism. It was found that a high total nitrogen (TN) removal efficiency of about 95% was achieved with the dosing C/N ratio of 7.5, while only a slight increase when the dosing C/N ratios were further increased to a high range of 10–30. The carbon validity was out of sync with TN removal and a balance between the two was realizable at a dosing C/N ratio of 7.5 based on the economic consideration. Moreover, weakened assimilation while enhanced respiration along with the increased C/N ratios was revealed, evidenced by the decreased net intracellular nitrogen and increased nitrogenous gas proportions in nitrogen balance analysis. The growth characteristics in terms of cell number and OD 600 showed a division for the high flux of nitrate and organics directed to assimilation and respiration, respectively. The expression levels of carbon and nitrate metabolism-related function genes in P. balearica supported the results above. The C/N ratio of 5.0–7.5 was supposed to be the turning region for the metabolism division between assimilation and respiration in aerobic denitrifying. The results are expected to offer a guideline for the decision made for organics supply for the aerobic denitrification process and support its sustainable application. [Display omitted] • C/N ratio of 7.5 was the threshold value to achieve TN removal of over 95%. • The carbon validity and nitrogen removal efficiency had an inverse relationship. • Nitrogen balance and growth property of P. balearica showed a metabolic division. • Expression levels of nitrate and carbon related functional genes were investigated. • Turning region of C/N ratio for microbial assimilation and respiration was 5.0–7.5. [ABSTRACT FROM AUTHOR]