Abstract: Methods for treating contaminated water from water cooling systems are provided. The inventive methods include treating water contaminated with anti-biofouling quaternary amine compounds with a bentonite clay slurry which has been formed on-site in a mix tank. The slurry is formed on-site by mixing a metered amount of bentonite clay particles with mix water in a mix tank to produce the desired slurry concentration and directly transferring the formed slurry from the mix tank into a flow of the contaminated water, where it mixes with the quaternary ammonium compound and detoxifies it.