Water holding tanks can be found in boats, recreational vehicles or homes or businesses. Their purpose is to collect and store water for future use. Such tanks should be sterilised at least annually so bacteria don't contaminate the water. This article describes the sterilisation of water tanks on a boat, but the method is applicable to other types of water tanks.
Drain the water tank until it is empty by running the faucet or other water outlet.
Mix bleach and water in a sink or large bucket (1 tbsp of bleach per 1 gallon of water).
Sterilise the funnel, hose and measuring cup by soaking the items completely in the mixture. Put the funnel, hose and measuring cup aside in a clean place after sterilising.
Remove the cap on the water tank and place it aside.
Fill the water tank approximately halfway with fresh water, using the sterilised hose.
Calculate how much bleach you need based on the size of the water tank. According to Coleman's guide, use 1 cup of bleach for every 4 gallons of water when sterilising a water tank system that is new or has been sitting for a prolonged period. If you sterilise your water tank on a regular, seasonal basis, according to Paul Esterle's "Water System Prep," you can use 1 cup of bleach for every 10 gallons of water.
Measure the bleach with a sterilised measuring cup.
Insert the sterilised funnel into the opening of the water tank. Pour the bleach through the funnel into the water tank.
Fill the water tank completely with water, using the sterilised hose. The hose will overflow at the opening once the tank is overfull.
Turn on one or more faucets or taps throughout the boat to allow the bleach and water mixture to move from the water tank through the water pipes and fixtures. When you smell bleach in the running water, turn off the faucet or tap immediately.
Let the bleach and water mixture sit in the water tank for at least 12 hours. Drain the water tank by running a faucet or other water outlet.
Flush the holding tank with regular water. Turn on a tap or faucet and sniff the water to check for the smell of bleach. Flush the tank again, if necessary, until the smell of bleach is gone.
If your water tank has been sitting partially full for a prolonged period, take the extra precaution of sterilising the tank two or three times.
When pouring bleach, avoid splashing it onto your eyes, hands or clothing.