Sulfur-based denitrification: Effect of biofilm development on denitrification fluxes

Elemental sulfur (So) can serve as an electron donor for denitrification. However, the mechanisms and rates of So-based denitrification, which depend on a biofilm development on a solid So surface, are not well understood. We used completely-mixed reactors packed with So chips to systematically explore the behavior of So-based denitrification as a function of the bulk nitrate (NO3−) concentration and biofilm development. High-purity (99.5%) and agricultural-grade (90% purity) So chips were tested to explore differences in performance. NO3− fluxes followed a Monod-type relationship with the bulk NO3− concentration. For high-purity So, the maximum NO3− flux increased from 0.4 gN/m2-d at 21 days to 0.9 g N/m2-d at around 100 days, but then decreased to 0.65 gN/m2-d at 161 days. The apparent (extant) half-saturation constant for NO3− KSapp, based on the bulk NO3− concentration and NO3− fluxes into the biofilm, increased from 0.1 mgN/L at 21 days to 0.8 mgN/L at 161 days, reflecting the increasing mass transfer resistance as the biofilm thickness increased. Nitrite (NO2−) accumulation became significant at bulk NO3− concentration above 0.2 mgN/L. The behavior of the agricultural-grade So was very similar to the high-purity So. The kinetic behavior of So-based denitrification was consistent with substrate counter-diffusion, where the soluble sulfur species diffuse from the So particle into the base of the biofilm, while NO3− diffuses into the biofilm from the bulk. Initially, the fluxes were low due to biomass limitation (thin biofilms). As the biofilm thickness increased with time, the fluxes first increased, stabilized, and then decreased. The decrease was probably due to increasing diffusional resistance in the thick biofilm. Results suggest that fluxes comparable to heterotrophic biofilm processes can be achieved, but careful management of biofilm accumulation is important to maintain high fluxes.