Effects of HRT on the efficiency of denitrification and carbon source release in constructed wetland filled with bark

Constructed wetland is widely used to treat municipal sewage. However, lack of carbon source always constraints the application of constructed wetland in advanced tailwater treatment process. Bark was used as the filler and external carbon source of constructed wetland in the study, and the effects of hydraulic retention time (HRT) on NO₃^--N removal efficiency and carbon release velocity were explored. Results showed that the NO₃^--N removal process was steady in the constructed wetland filled with bark without additional carbon source. The NO₃^--N removal efficiency and NO₃^--N concentration presented a first-order reaction. The reaction rate constant k was 0.4 day^-1. The relationship between NO₃^--N removal efficiency (η) and HRT (t) was η = 1-e^-0.4t, and η was increased with increasing of HRT. η reached a maximum of 77% at HRT of 4.48 days. η obtained the minimum of 20% at HRT of 0.75 days. The relationship between the carbon source releasing velocity (v) by bark and HRT was v = 0.53(1.62/t-1/t²) + 0.32. v increased first and then decreased with HRT increasing. The maximum v was detected at t = 1.12 days.