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 ( 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.