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

Abstract

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 OD600 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.