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

Elemental sulfur (S 0 ) 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 ( q ˆ ) 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 ( µ ˆ ) 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 q ˆ for NO 2 − relative to that for NO 3 − . The high q ˆ 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.