Abstract: The water treatment device is an AOP (advanced oxidization process) apparatus that provides effective sanitation of pool water. The device includes couplings that can adapt to fit various standard pool equipment. Inside the reaction chamber of the main body, there are a nozzle and a metal catalyst. The nozzle creates water jet impinging the metal catalyst, which subsequently creates bubbles. Being entrained in the water, the bubbles substantially collapse and create high temperature and pressure changes. These changes cause water to go through chemical dissociation, thus creating hydroxyl radicals, a highly reactive oxidizer. In a fast rate, the hydroxyls attach to and destroy organic contaminants in the water. The metal catalyst remains chemically unchanged. The apparatus has a long service life and substantially reduces the use of chemicals in a pool.