Division of Environmental Health Drinking Water Program

Cleaning & Disinfection of Water Holding Tanks

Cleaning & Disinfection of Small Water Holding Tanks for both Residential and Public Water Systems (approximately 3,000 gallons or less)

Household water storage tanks frequently test positive for high levels of bacteria even if they have not been subject to sewage contamination. Even if your water is coming from an approved source, you may need to clean and sanitize your household system periodically.

If you have access to the inside of your tank

This method uses a smaller volume of water that can be disposed of in a septic system; the chlorine concentration is higher with a shorter disinfection time.

1. Plan to clean your tank when the water level in it is low.
   • Add 1 or 2 cups of unscented household bleach per 25 gallons of water remaining in the tank.
   • Make sure that if you have a sewage holding tank, it has been pumped to prevent overflow and allow venting during the water tank disinfection procedure.
2. Turn on the faucets in the house to allow bleach solution to be distributed throughout the water system.
   • You may need to do them one at a time, starting with the one farthest (longest pipe) from the water tank.
   • Once you smell bleach coming from the tap, the sanitizing solution has filled your system and you can turn off faucets. Allow the chlorinated water to sit in your pipes for at least 2 hours but preferably overnight. Remember that the water will not be usable during the entire procedure.
3. Make a spray bottle of bleach water by mixing 1 teaspoon of household bleach per quart of water.
   • Spray down the insides of tank thoroughly with this solution.
   • Make sure that the room is well ventilated so that you are not breathing in too many bleach fumes.
   • If there is a lot of debris settled at the bottom of the tank, try to dip or siphon the last of the water out of the tank instead of sending it through the system.
4. At end of the “sit time” of at least 2 hours, open faucets to flush solution through again. If the tank is not already empty, run your taps until tank is drained.
5. Fill tank with fresh water. Open taps until fresh water flushes through.

If you do not have access to the inside of your tank

This method uses a large volume water that should not be disposed of into a septic system; the chlorine concentration level is lower and disinfection time is longer.

1. Add 1/2 cup of unscented household bleach for each 100 gallons of tank volume (i.e. a 500 gallon tank would need 2 1/2 cups of bleach).
2. Fill the water tank with clean water.
3. Run each faucet in the distribution system until you can smell bleach.
4. Connect a clean hose to a faucet and run water back into the tank through the tank fill pipe.
5. Run the water back into the tank for 15 to 30 minutes to ensure good mixing in the tank.
6. Turn off the water and let it sit for at least 6 hours but preferably overnight.
7. Run the water to drain the chlorinated water out of the tank. You may want to run the water using a hose out onto a gravel or area of soil so you do not overload your wastewater system or kill the vegetation. Be careful that you do not run the chlorinated water into any river, stream, or lake. Chlorine is toxic to fish!

If you have any questions please contact your local DEC office for additional information.

Cleaning & Disinfection of Large Water Holding Tanks (greater than 3,000 gallons)

Disinfect and flush per AWWA Standard C652 or contact DEC for assistance.