Determination of critical operational conditions favoring sulfide production from domestic wastewater treated by a sulfur-utilizing denitrification process

The aim of this study was to determine the critical operational conditions leading to the generation of sulfide in a domestic wastewater treated by a sulfur-utilizing denitrification process. The influence of various important parameters on the reduction of the sulfates present in denitrified domestic wastewaters to sulfide was studied. Experiments were carried out in batch mode with denitrified domestic wastewaters containing various amounts of both organic matter and sulfates. Preliminary results showed that aqueous sulfide was generated for DOC and sulfate contents higher than 56 mg/L and 371 mg/L, respectively, while DOC and sulfate contents of 77 mg/L and 412 mg/L, respectively, were required to allow the release of gaseous H2S. Good correlations were also observed between gaseous sulfide production and the values of ORP and DOC, while the amounts of dissolved sulfide produced seemed to be correlated with the ORP values and the concentration of sulfates. Additional experiments were conducted using a Box-Behnken methodology to determine if the production of aqueous or gaseous sulfide can be predicted depending on the DOC (from 50 to 90 mg/L) and sulfate contents (from 160 to 380 mg/L) at various temperatures ranging from 5 to 25 °C. The highest sulfide generation (H2S(g) = 84.8 ppm and H2S(aq) = 2.42 mg/L) occurred at 25 °C with DOC and sulfate concentrations starting from 90 mg/L and 270 mg/L, respectively, indicating that the production of sulfides from denitrified domestic wastewaters required conditions not likely to occur at the effluent of a sulfur-based denitrification unit following secondary treatment.