A method of controlling addition rate of an odor control chemical to a wastewater line by adding odor control chemical to the wastewater. The method includes measuring a level of dissolved sulfides in the wastewater line using a sulfide probe placed in said wastewater line, transferring data on the level of dissolved sulfides to a computing unit, determining a required odor control chemical addition rate in said computing unit based on said level of dissolved sulfides, and adding the odor control chemical at said required addition rate. The method allows for accurate on-line control of the level of H2S in the wastewater line.