Typically, the water level in the storage tank of the system is managed by a float valve. For the RO system to work as intended, the float valve must remain open. When the water in the storage tank is no longer needed because of the float valve, the water supply is cut off. Overflow and water waste can be avoided with this method. Inside the storage tank is where you'll find the float valve, a tiny plastic or metal piece. The valve stem is attached to a float inside the container. The float is designed to rise in tandem with the water level in the tank, at which point it will engage the valve stem and cut off the water supply. Some reverse osmosis systems have a float valve already built in, while others call for a separate float valve to be installed.