Revealing the overlooking role of polysulfides in sulfide-based denitrification biofilms through mathematical...

Sulfide-based denitrification produces several intermediates, and the subsequent oxidation of elemental sulfur (S⁰)/polysulfides (S_n^2-) is the rate-limiting step. However, the factors affecting S⁰/ S_n^2- utilization in sulfide-based denitrification biofilms are not entirely clear. In this study, a one-dimensional (1-D) biofilm model was developed in AQUASIM, incorporating the kinetics of mass transfer and the four-step denitrification process. Experimental data obtained in a moving bed biofilm reactor were used to calibrate and validate the model. Simulations were performed with different diffusion coefficients of S⁰/ S_n^2- and biofilm thicknesses, which were also compared with the simulation performed in an ideal mixed reactor. Modeling results show that the decrease in the diffusion coefficient of S⁰/ S_n^2- leads to exacerbated S⁰/ S_n^2- accumulation in the outer biofilms and reduced active biomass concentration. Polysulfides with better water solubility should be the actual electron donors in the biofilm system. The increase in biofilm thickness not only results in higher mass transfer resistance but also forms separated layers. Sulfide and S_n^2- function as dual electron donors in the outer biofilm layer, while only S_n^2- is available in the deeper layer. The difference in sulfide and S_n^2- utilization rates further leads to the imbalanced electron consumption rates of four-step denitrification throughout the biofilm. Compared to the suspended system, the biofilm system is less effective in enhancing S_n^2- utilization, but shows a kinetic advantage in S_n^2- recovery. The findings of the study shed light on the role of S_n^2- in sulfide-based denitrification biofilm systems and offer guidance for process optimization.