Mildew is a growth caused by mold, or fungi. According to the University of Missouri Extension's website, mildew-causing molds thrive especially well in areas that are warm, damp and have poor air circulation. This characteristic of mildew makes water coolers an ideal breeding ground for it, and mildew can begin to form in the spouts or tubing. If your water cooler has developed an off taste, it's time to give it a thorough deep cleaning.
Drain or pour out the water from the water cooler. Make sure to get all the old water out so that you can start the cleaning process with fresh water that has not been sitting in the cooler.
Wash the cooler with soap and water. Because you will be using bleach later, it is very important that the soap you use does not contain any ammonia. When mixed, ammonia and bleach create a toxic reaction that is dangerous to breathe.
Rinse the cooler thoroughly with fresh water. If the cooler has spigots, you may want to drain both the soapy water and the rinse water through them so they can get washed and rinsed also.
Mix the household bleach with water. The Centers for Disease Control and Prevention recommend mixing 1 tsp. of bleach for every cup of water used when sanitizing water storage containers.
Wear rubber gloves for the following steps. Use the teaspoon measure and measuring cup to get an accurate ratio of bleach to water. Use as many cups of water as you need to fill the container, with 1 tsp. of bleach for each cup.
Stir the water and bleach mixture with a long, metal spoon. You can also cover the cooler and shake it, if you prefer. Make sure the bleach is thoroughly mixed in and reaches all parts of the cooler.
Let the bleach and water mixture soak in your water cooler. The CDC recommends letting it rest for at least 30 minutes.
Drain the bleach mixture from the cooler, through the spigots, if any.
Rinse the cooler with fresh water thoroughly. Drain and rinse your water cooler several times to ensure you remove all the bleach. Be sure to let the rinse water drain through the spigots.