Even though the water delivered by pipe or tanker has been treated, cleaning out the sediment and disinfecting the household water storage tank at least once a year is necessary to maintain family health. This applies whether you have an attic mounted tank containing an emergency supply of potable water, or an external water storage tank. Increase the cleaning frequency if you happen to live in a high sediment area, or if the tap water becomes cloudy or smelly.
Close the tank inlet shut-off valve. Attach a hose to the wash-out valve that draws water from the base of the tank. Lay the end of the hose in an area away from the house, preferably on a hard surface below the level of the storage tank. Open the wash-out valve to drain the tank completely.
Use a pressure washer and laundry detergent to clean the sediment from the bottom of the tank while the wash-out valve is open. Alternatively, use a long-handled stiff-bristle broom to scrub the base of the tank with a mixture of clean water and a liberal amount of laundry detergent. Pay particular attention to corners or joints. After scrubbing or pressure washing, rinse all the inside surfaces of the tank with clean water until the water draining from the hose runs clear. If this is impossible because the tank is buried with no access hatch, go to Step 3.
Fill the tank to with clean water. Add 2 pints of household bleach for every 5 gallons of water, or 50g HTH pool chlorine per 5 gallons of water in the tank. Note: premixing and stirring the HTH solution is recommended.
Open all the taps in the household and garden. Run the water until you can smell bleach, then turn the taps off. Top up the tank and replenish the used bleach or HTH. Close the access hatch. Allow 24 hours to disinfect the tank. If this is inconvenient, double the amount of bleach or HTH and allow 8 hours for the tank to become disinfected.
Drain the tank as described in Step 1. Refill the tank with clean drinking water.