A cold water storage tank can prove invaluable, especially in emergency situations when fresh, drinkable water is in short supply. Of course, in order to provide drinkable water, the tank must be free of sediment and other foreign material. If the water supplied from the tank appears cloudy or simply doesn't taste right, it is imperative that you clean the tank as soon as possible. For regular maintenance and peace of mind, flush and clean the storage tank at least once per year.
Shut off the valve or valves that connect the cold water storage tank to your water lines. Open the drainage valve, which is found on the bottom of most storage tanks. If your tank isn't directly connected to any water lines, simply open the drainage valve in order to drain the water from the tank.
Allow all water to drain from the tank. Remove the access lid or hatch on the storage tank, usually located on top of the tank. If necessary, use a ladder in order to reach the lid.
Use a hose with a high-pressure spray attachment to wash down the inside of the storage tank with clean water. A pressure washer also will work, if you have access to one. Be sure to spray the entire interior surface of the tank.
Scrub the interior of the tank with a long-handled brush. Rinse the tank with a high-pressure spray of clean, fresh water. Allow the tank to drain completely.
Close the drainage valve on the storage tank and begin filling it with clean water. As the tank fills, add 5ml (0.17 fl oz) of household bleach for every 1 litre (1.76 pint) of water the tank is capable of holding, or about 1 tbsp per 4.5 litres (1 gallon). Once the tank is full and you've added the necessary amount of bleach, allow the mixture to remain in the tank for 24 hours. This will provide the necessary time for the tank to disinfect.
Open the valve to the water lines. This will allow the chlorinated water to drain from the tank, through the water lines and into the sewer. If there are no water lines connected to the tank, connect a hose to the drainage valve on the tank. The hose will need to be long enough to reach a nearby drain. Place the end of the hose into the drain. Allow all of the chlorinated water to empty from the storage tank. Disconnect the hose and close the drainage valve once the water has drained.
Refill the tank with clean, fresh water. Close the access hatch or lid on the tank.