Contaminants are removed from fluid in a system where the fluid is sprayed into an air head at the top of a tank through a diffuser and withdrawn through a pick-up tube. A solenoid valve operates to allow oxygen-containing gas from a compressed gas source to flow into the tank through a shuttle valve, which is opened by the gas pressure. Simultaneously, a drain valve connected to the shuttle valve opens a drain, venting water and air from the tank. The solenoid valve also operates to close the gas supply line and connect the shuttle valve through a vent to atmosphere. Pressure within the tank closes the shuttle valve, which in turn closes the drain valve and disconnects the shuttle valve from the tank. A one-way-flow valve prevents fluid or gas from flowing from the tank backwards through the shuttle valve to the solenoid valve and out the vent.