Characterizing the role of hydraulic retention time on nitrate removal indices in denitrifying bioreactors by nonlinear models

Denitrifying bioreactors (DNBRs) are a sustainable and cost-effective practice commonly used at the edge of fields to reduce nitrate from agricultural runoff. The hydraulic retention time (HRT) is a crucial variable that affects nitrate removal rate (NRR, g N m-3 d-1), nitrate removal efficiency (NRE, %), and nitrate concentration reduction per length (Nrd, mg N L-1 m-1). In this study, two nonlinear models, the developed Michaelis-Menten (MM) model and the Mitscherlich (MT) model, were developed to characterize the relationship between nitrate removal indices (NRR, NRE, and Nrd) and HRT. This study first utilizes nonlinear models to quantitatively understand the relationship between NRR, NRE, Nrd, and HRT. To verify the models, eight experiments were conducted under different conditions, including different scales (laboratory and field), media (woodchip, woodchip+biochar, woodchip+silage leachate, woodchip+biochar+silage leachate), and influent nitrate concentrations (6.8-70 mg N L-1). The results showed that the MT model outperformed the MM model and MT could accurately characterize the nitrate removal changes with HRT and provide the optimal HRT (HRTO). Overall, the model could be beneficial for designers and practitioners to optimize nitrate removal.