How Do You Tackle Mold In An RV Holding Tank?

Mold in RV holding tanks is a fairly common occurrence. The fact that tanks are dark, moist environments, susceptible to frequent temperature changes creates an excellent recipe for mold development. Since this is the situation, it is important to know how to sanitize your holding tanks and plumbing. 

The two most common holding tanks that accumulate mold are the freshwater and gray water tanks. (The black water holding tank is actually less susceptible to mold because of the waste that goes into it and the holding tank treatments that are used in it). Due to the nature of mold growth, it is recommended the freshwater tank and plumbing, along with the gray tank, are sanitized about every six months if you are a weekend warrior or once every three to four months if you are a full timer.

The best solution for sanitizing the freshwater holding tank and plumbing is diluted household bleach. The recommended ratio of bleach to water is one quarter cup of bleach per fifteen gallons of capacity in your tank.

To get the solution into the freshwater holding tank, we recommend mixing the bleach into a five gallon bucket of water and using a funnel to pour the solution into the RV holding tank. From there, fill the tank with water.

Be sure that the water heater is in the bypass mode. This is the same setting that you use for winterizing the RV.

Once the freshwater holding tank is full, run the cold and hot water faucet in each of the sinks and shower(s) until you smell bleach. This will pump the water through all of the plumbing and water lines, filling them with the solution. 

This process should be done in the evening, as you will want the bleach to sit in the tank and water lines for around eight hours. We do not recommend leaving the bleach in the RV plumbing system for longer than twelve hours. Bleach can be corrosive on the gaskets and fittings around the RV. 

Once enough time has passed, run all of the sinks and shower(s) to drain the freshwater holding tank. Then refill the freshwater holding tank with fresh water. Continue running the water at the sinks and shower(s) until there is no longer a smell of bleach. By now the gray water holding tank should be relatively full with the bleach and water solution. Allow this to sit for between four to eight hours to help sanitize the tank.

Dump the gray water holding tank solution at a dump station or in the sewer port at your campsite. If the gray water holding tank filled before you were able to remove the bleach smell from the fresh water taps around the RV, leave the gray water valve open and continue to flush the plumbing with fresh water until there is no longer a bleach smell. 

The diluted bleach solution is by far the most effective way to sanitize an RV freshwater and gray water system of mold. Other remedies such as undiluted vinegar or baking soda and water are far less effective at eliminating mold and other bacteria. 

Note: We do not recommend using the bleach solution in the black water holding tank. A healthy black water holding tank uses bacteria and enzymes to break down waste and eliminate odors. As bleach will kill bacteria, it must be kept out of the black water holding tank.

Got a question you’d like answered by our certified RV technicians? Submit your question here!