The paper is authored by David Cueto, Mabel Mora and David Gabriel

Biological treatment of effluents containing H2S and ammonium are of great interest as both compounds can trigger serious environmental problems when disposed of. The aim of this study was to optimize the production of biosulfur from the partial oxidation of sulfide in sulfide- and ammonium-containing streams. Biological performance was evaluated under different aerating conditions and key kinetic parameters were adjusted based on an existing mathematical model adapted to this system. An optimal conversion of sulfide to S0 of 86% (w/w) was found at an oxidation-reduction potential (ORP) of -380 ± 10 mV and at an O2/S2- molar ratio of 0.44. Partial nitrification was observed at ORP higher than -200 mV and in excess of oxygen supply. Sulfide-oxidizing bacteria (SOB) outcompeted ammonium-oxidizing bacteria (AOB) in the competition for dissolved oxygen. In a modelling effort, the maximum specific growth rate for SOB, the sulfur shrinking kinetic constant, the maximum specific growth rate for AOB and the AOB oxygen half saturation constants were adjusted to 10.1 d-1, 0.3 mg2/3 VSS mg-2/3 S, 1.75 d-1 and 1.5 mg L-1, respectively, during model calibration. Optimal S0 production was found under limiting O2 conditions in which AOB were not able to outcompete SOB. The mathematical model described satisfactorily the experimental profiles for ammonium, nitrite, sulfide and sulfate as a function of the aeration flowrate.