The combined effect of dissolved oxygen and COD/N on nitrogen removal and the corresponding mechanisms in intermittent aeration constructed wetlands

In this study, the combined effect of dissolved oxygen (DO) and COD/N on nitrogen (N) removal as well as the corresponding mechanisms were investigated in aerated constructed wetlands (CWs). At each investigated COD/N level, the ammonium removal efficiency increased as DO concentration increased. However, the highest total N removal efficiency occurred at different DO concentration at each COD/N level. The results of functional gene analysis and cyclic N profile studies indicated that DO supply and COD/N influence the N removal performance, which is not only exert a direct effect on nitrification-denitrification process, but also change N removal pathway in intermittent aerated CWs. At a relatively high influent COD/N of 20, the simultaneous nitrification and denitrification (SND) via nitrite was almost the exclusive N removal pathway at all investigated DO concentration. With the decrease of COD/N from 20 to 2 at DO of ∼1.8, ∼3.5 and ∼6.0 mg/L, SND efficiency all decreased, however, its decreasing rate was much higher at relatively high DO level of ∼6.0 mg/L than that at DO levels of ∼1.8 and ∼3.5 mg/L. In comparison, a simultaneously partial nitrification, anammox and denitrification was established at DO of ∼0.8 mg/L along with reducing influent COD/N.