Kinetic limitations during the simultaneous removal of <Emphasis Type="Italic">p</Emphasis>-cresol and sulfide in a denitrifying process

The aim of this study was to evaluate the capacity of a denitrifying consortium to achieve the simultaneous removal of nitrate, sulfide and p-cresol and elucidate the rate-limiting steps in the mixotrophic process. Nitrite reduction appeared as the most evident rate-limiting step in the denitrifying respiratory process. The nitrite reduction rate achieved was up to 57 times lower than the nitrate reduction rate during the simultaneous removal of sulfide and p-cresol. Negligible accumulation of N₂O occurred in the denitrifying cultures corroborating that nitrite reduction was the main rate-limiting step of the respiratory process. A synergistic effect of nitrate and sulfide is proposed to explain the accumulation of nitrite. The study also points at the oxidation of S⁰ as another rate-limiting step in the denitrifying process. Different respiratory rates were achieved with the distinct electron donors provided (p-cresol and sulfide). The oxidation rate of p-cresol (q_CRES) was generally higher (up to 2.6-fold in terms of reducing equivalents) than the sulfide oxidation rate (q_S2−), except for the experiments performed at 100 mg S²⁻ L⁻¹ in which q_S2− was slightly (~1.4-fold in terms of reducing equivalents) higher than q_CRES. The present study provides kinetic information, which should be considered when designing and operating denitrifying reactors to treat industrial wastewaters containing large amounts of sulfurous, nitrogenous and phenolic contaminants such as those generated from petrochemical refineries.