Mix one tablespoon of bleach to a gallon of water. Wear rubber gloves anytime you work with bleach to prevent skin irritation.
Unplug the water cooler to make sure it is not receiving power.
Pull the water jug out of the dispenser. It is easiest to do this when you are about to change the tank or if the tank is nearly empty.
Scrub the inside of the water cooler reservoir with a scrubbing sponge soaked in the bleach solution. Let the solution sit for two to five minutes. Avoid using any bleach solution in the hot water tank area if you have a hot water option on the cooler. It is not needed since the hot water disinfects its reservoir by itself.
Drain the bleach solution that had accumulated from the scrubbing through the cold water faucet and into a container.
Rinse the reservoir multiple times with tap water to clean out the bleach solution. At least four cycles are needed to ensure no bleach remains in the system. If your water cooler has a "room temperature" tap in addition to cold water, rinse the room temperature tap as well.
Pull out the drip tray and scrub it with a mild detergent.
Wash your hands and wipe off the top of a new water bottle before adding the new bottle to the cooler.