Evaluating simultaneous chromate and nitrate reduction during microbial denitrification processes

Sulfur-based autotrophic denitrification and heterotrophic denitrification have been demonstrated to be promising technological processes for simultaneous removal of nitrate (NO₃ ^-) and chromate (Cr (VI)), two common contaminants in surface and ground waters. In this work, a mathematical model was developed to describe and evaluate the microbial and substrate interactions among sulfur oxidizing denitrifying organism, methanol-based heterotrophic denitrifiers and chromate reducing bacteria in the biofilm systems for simultaneous nitrate and chromate removal. The concomitant multiple chromate reduction pathways by these microbes were taken into account in this model. The validity of the model was tested using experimental data from three independent biofilm reactors under autotrophic, heterotrophic and mixotrophic conditions. The model sufficiently described the nitrate, chromate, methanol, and sulfate dynamics under varying conditions. The modeling results demonstrated the coexistence of sulfur-oxidizing denitrifying bacteria and heterotrophic denitrifying bacteria in the biofilm under mixotrophic conditions, with chromate reducing bacteria being outcompeted. The sulfur-oxidizing denitrifying bacteria substantially contributed to both nitrate and chromate reductions although heterotrophic denitrifying bacteria dominated in the biofilm. The mixotrophic denitrification could improve the tolerance of autotrophic denitrifying bacteria to Cr (VI) toxicity. Furthermore, HRT would play an important role in affecting the microbial distribution and system performance, with HRT of higher than 0.15 day being critical for a high level removal of nitrate and chromate (over 90%).