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

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 188mgN₂g^-1VSSd^-1. Free ammonia did not show any effect on denitrification at concentrations up to 53mg NH₃-NL^-1. Different sulphide concentrations were also tested with stoichiometric nitrite and nitrate concentrations. Sulphide inhibited denitrification at concentrations higher than 200mgS^2-L^-1. A 50% inhibition was also found at nitrite concentrations above 48mg NO2--NL^-1. The maximum specific activity decreased until a value of 25mgN₂g^-1 VSSd^-1 at 232mg NO2--NL^-1. The Haldane model was used to describe denitrification inhibition caused by nitrite. Kinetic parameters determined from the fitting of experimental data were r_max=176mgN₂g^-1VSSd^-1, K_s=10.7mg NO2--NL^-1 and K_i=34.7mg NO2--NL^-1. The obtained model allowed optimising an autotrophic denitrification process by avoiding situations of inhibition and thus obtaining higher denitrification efficiencies.