KINETIC-MODEL FOR AUTOTROPHIC DENITRIFICATION USING ELEMENTAL SULFUR
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Increasing prices for methanol and other petrochemicals have decreased the attractiveness of biological dentrification processes which require such compounds. An alternative to such treatment systems is an autotrophic process using enrichment cultures of Thiobacillus denitrificans fed reduced sulfur compounds. Elemental sulfur appears to be the most promising substrate for this system due to its low cost and ease of handling. However, present models for microbial growth are inadequate to describe growth at high biomass density on a water-insoluble solid substrate such as elemental sulfur. This paper presents a model to describe such a system, and experimental evidence of its validity. The model recognizes three steps which could limit the observed rate of denitrification. (1) Sulfur must be solubilized in the attached biofilm and be transported through the film while being simultaneously removed by microbial reaction. (2) Nitrate must be transported from the bulk liquid to the biofilm surface. (3) Nitrate must be transported through the biofilm where it is microbially reduced to nitrogen gas. The model predicts and experimental evidence verifies that the unit rate of denitrification will be proportional to the ratio of the sulfur concentration to the biomass concentration when this ratio is low and nitrate is in excess. At high values of this ratio, the unit rate is observed to approach a maximum as predicted by the model. The accuracy of the model's prediction for the dependence of the unit rate of denitrification on the concentration of nitrate could not be evaluated due to very low concentrations of nitrate measured in the steady-state reactors. Available evidence also supports the prediction of the model that, at low values of the sulfur to biomass ratio, the activation energy of the reaction is about half the value that would be observed at higher sulfur to biomass ratios. 1978.
