Using kinetics and modeling to predict denitrification fluxes in elemental-sulfur-based biofilms

Elemental sulfur (S⁰ ) can serve as an electron donor for water and wastewater denitrification, but few researchers have addressed the kinetics of S ⁰ -based reduction of nitrate (NO ₃^- ), nitrite (NO ₂^- ), and nitrous oxide (N ₂ O). In addition, S ⁰ -based denitrifying biofilms are counter-diffusional. This is because the electron donor (S ⁰ ) is supplied from the biofilm attachment surface while the acceptor, for example, NO ₃^- , is supplied from the bulk liquid. No existing mathematical model for S ⁰ -based denitrification considers this behavior. In this study, batch tests were used to determine the kinetic parameters for the reduction of NO ₃^- , NO ₂^- , and N ₂ O. Additionally, a biofilm model was developed to explore the effects of counter-diffusion on overall fluxes, that is, the mass of NO ₃^- or NO ₂^- removed per unit biofilm support area per unit time. The maximum specific substrate utilization rates ( $$ ) for NO ₃^- , NO ₂^- , and N ₂ O were 3.54, 1.98, and 6.28 g N g COD ^-1 ·d ^-1 , respectively. The maximum specific growth rates ( $$ ) were 0.71, 1.21, and 1.67 d ^-1 for NO ₃^- to NO ₂^- , NO ₂^- to N ₂ O, and N ₂ O to N ₂ , respectively. Results suggest that the observed NO ₂^- accumulation during S ⁰ -based denitrification results from a low $$ for NO ₂^- relative to that for NO ₃^- . The high $$ for N ₂ O, relative to that for NO ₃^- and NO ₂^- , suggest that little N ₂ O accumulation occurs during denitrification. A counter-diffusional biofilm model was used to predict trends for NO ₃^- fluxes, and confirmed NO ₂^- accumulation in S ⁰ -based denitrification biofilms. It also explains the observed detrimental effects of biofilm thickness on denitrification fluxes. This study allows a more accurate prediction of NO ₃^- , NO ₂^- , and N ₂ O transformations in S ⁰ -based denitrification.