An apparatus and method produces nitrate ions on-site from water, natural gas and air extracted in proximity to the apparatus. The apparatus generates nitrate ions and brings the nitrate ions into contact with an aqueous system. Hydrogen sulfide present in the aqueous system is removed and the production of hydrogen sulfide by sulfate-reducing bacteria (SRB) is eliminated by introducing into the system nitrate ions, whereby denitrifying microorganisms, using nitrate, outcompete the sulfate-reducing bacteria for the available carbon nutrients, thus preventing the SRB from producing hydrogen sulfide. Nitrate ions generated by the apparatus and added to the aqueous system which contains the denitrifying microorganisms can enhance oil recovery by means of microbial enhanced oil recovery mechanisms.