Cross effect of temperature, pH and free ammonia on autotrophic denitrification process with sulphide as electron donor

• NO2- accumulation associated to NO3- reduction was not influenced by the initial S/N ratio.
• Nitrous-oxide reductase enzyme was inhibited by sulphide above 200 mg S2− L−1.
• Inhibition of sulphide oxidation by nitrite was described with a Haldane model.
• Autotrophic denitrification was strongly influenced by T and pH.
• Free ammonia up to 53 mg NH3–N L−1 did not influence autotrophic denitrification.

Autotrophic denitrification is a suitable technology to simultaneously remove oxidised nitrogen compounds and reduced sulphur compounds yielding nitrogen gas, sulphur and sulphate as the main products. In this work, several batch tests were conducted to investigate the cross effect of temperature, pH and free ammonia on the autotrophic denitrification. Denitrification efficiencies above 95% were achieved at 35 °C and pH 7.5–8.0 with maximum specific autotrophic denitrifying activities up to 188 mg N2 g−1 VSS d−1. Free ammonia did not show any effect on denitrification at concentrations up to 53 mg NH3–N L−1. Different sulphide concentrations were also tested with stoichiometric nitrite and nitrate concentrations. Sulphide inhibited denitrification at concentrations higher than 200 mg S2− L−1. A 50% inhibition was also found at nitrite concentrations above 48 mg NO2-–N L−1. The maximum specific activity decreased until a value of 25 mg N2 g−1 VSS d−1 at 232 mg NO2-–N L−1. The Haldane model was used to describe denitrification inhibition caused by nitrite. Kinetic parameters determined from the fitting of experimental data were rmax = 176 mg N2 g−1 VSS d−1, Ks = 10.7 mg NO2-–N L−1 and Ki = 34.7 mg NO2-–N L−1. The obtained model allowed optimising an autotrophic denitrification process by avoiding situations of inhibition and thus obtaining higher denitrification efficiencies.