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 (N_rd, 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 N_rd) and HRT. This study first utilizes nonlinear models to quantitatively understand the relationship between NRR, NRE, N_rd, 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 (HRT_O). Overall, the model could be beneficial for designers and practitioners to optimize nitrate removal. © 2023 The Authors