A mathematical model to assess the heterotrophic growth supported by soluble microbial products in a sulfur-based autotrophic denitrification system

Nitrate represents one of the most common pollutants for groundwater and industrial wastewaters. In recent years, many studies demonstrated that sulfur-based autotrophic denitrification with elemental sulfur represents a valid solution for the removal of nitrate in case of lack of organic substances. However, in presence of organic carbon the autotrophic species compete with heterotrophic denitrifying bacteria for nitrate and nitrite. Moreover, the formation of sulfate induced by the autotrophic denitrification may lead to the growth of sulfate reducing bacteria, which compete with denitrifying bacteria for organic carbon. These microbial interactions result in both beneficial and controversial effects. For instance, heterotrophic denitrification could induce a further reduction of nitrate in the effluent; while sulfate reducing bacteria could contribute to the reduction of sulfate concentration in the effluent and the consequent recovery of elemental sulfur. The aim of the work is to introduce a mathematical model to assess the competition between the autotrophic denitrifying bacteria, the heterotrophic denitrifying bacteria and the sulfate reducing bacteria. The heterotrophic ones are supposed to grow on soluble microbial products, released during the mentioned biological processes. The mathematical model is defined based on mass balance considerations and leads to consider a system of impulsive differential equations, which are solved numerically by implementing an original software on the Matlab platform. Numerical simulations are performed to assess process efficiency by varying different operational parameters.