Several methods are ...
Published by Frank Scovill
Several methods are appropriate for cleaning water storage tank.
physically clean the interior of the tank by any available method, wet vacuum cleaners are very effective for smaller tanks, but for larger tanks, you may need to flood, agitate and rinse the debris out an access port or bulkhead fitting.
sanitizing the interior of the tank can be accomplished by mixing a solution of chlorine and water keeping the free chlorine level at approximately 2ppm for 24 to 48 hrs to kill bacteria. Then thoroughly rinse the tank with clean potable water before returning to service.
Sanitizing the water collected can be done by installing an ozone generator that is functional while the tank contains water. If the collected water is intended for consumption, installation of physical filtration followed by a sufficiently powerful Ultraviolet treatment product will make the water potable.