A system and method for treating water containing at least one contaminant. The system and method include a water treatment module such as a reverse osmosis unit. An electrochemical contaminant detection system is positioned in the waste water stream of the water treatment module. The contaminant detection system includes a contaminant sensor and a water quality sensor module. The contaminant sensor measures the concentration of the contaminant in the waste water stream and the water quality sensor module measures one or more water quality parameters of the waste water stream. A processor uses an algorithm to determine the concentration of the contaminant in the feed water based on the measurements of the contaminant sensor and water quality sensor module.