Emergency Disinfection of Cisterns and Other Rain-Catchment Systems

Either of two methods can be used to disinfect cisterns and other rain-catchment systems in disaster situations depending on whether another source of drinkable water is available while the cistern or catchment system is being disinfected (Method 1) or whether the cistern or rain-catchment system needs to be used more quickly as a drinking water source (Method 2). If fuel products or other chemical contaminants have come in contact with the cistern or rain-catchment system, or if there is a smell of fuel or other chemicals near the system, use only Method 1 for disinfection.

Method 1

Use this method if clean drinking water IS available from another source while the cistern or catchment system is being disinfected. This method is a better method, but it takes longer because the cistern/system must be drained and refilled twice.

1. Thoroughly clean the surface catchment area (for example, rooftop) and remove all debris.
2. Drain the cistern or storage tank completely.
3. Remove all debris from the tank.
4. Without entering the tank, scrub the inside as well as you can with a stiff brush or broom and a solution of 1 cup (8 ounces; approximately 0.25 liter) of 5.25% unscented chlorine bleach (sodium hypochlorite) in 10 gallons (approximately 38 liters) of water.
5. Refill the tank with rainwater or drinkable water.
6. If you don’t know the capacity of the tank, use the following formula to determine the amount of water it will hold.
   • For a rectangular tank: multiply the depth of the tank (in feet) times the length (feet) times the width (feet), and multiply the result by 7.5 to get the number of gallons the tank will hold.
     METRIC: To get metric units, multiply length (meters) X width (meters) X height (meters) X 1,000 = capacity in liters.
   • For a round tank: multiply the length (feet) by the square of the tank radius, multiply the result by 3.14, and then multiply that result by 7.5 to get gallons.
     METRIC: To get metric units, multiply height X the radius squared (meters) X 3,140 = capacity in liters.
   • If neither of these methods is practical, you can estimate the capacity in gallons by making an "educated guess," and then double the amount of bleach to be used in the disinfection solution.
7. Add 4 cups (32 ounces; approximately 1 liter) of 5.25% unscented household bleach for every 100 gallons (approximately 380 liters) of water in the holding tank. This should give you a chlorine concentration of approximately 100 parts per million (ppm).
8. Run water through all plumbing used for drinking water in your home until you notice a strong chlorine odor.
9. Let the solution sit undisturbed for 24 hours.
10. Drain the whole tank.
11. Let the tank refill with rainwater, or fill the tank with drinkable water.
12. Flush all piping in your home until the odor of chlorine is completely gone before you use the system again.

Method 2

Use this method if clean drinking water is NOT available from another source while the cistern/system is being disinfected. This method is not as good as Method 1, but it takes less time because the cistern/system needs to be drained and refilled only once.

1. Thoroughly clean the surface catchment area (rooftop) and remove all debris.
2. If you don’t know the capacity of the tank, use the following formula to determine how much water it will hold.
   • For a rectangular tank: multiply the depth of the tank (in feet) times the length (feet) times the width (feet), and multiply the result by 7.5 to get the number of gallons the tank will hold.
     METRIC: To get metric units, multiply length (meters) X width (meters) X height (meters) X 1,000 = capacity in liters.
   • For a round tank: multiply the length (feet) by the square of the tank radius, multiplying the result by 3.14, and then multiply by 7.5 to get the number of gallons the tank will hold.
     METRIC: To get metric units, multiply height X the radius squared (meters) X 3,140 = capacity in liters.
   • If neither of these methods is practical, you can estimate the capacity by making an "educated guess," and then double the amount of bleach to be used in the disinfection solution.
3. Add 4 cups (32 ounces; approximately 1 liter) of 5.25% unscented household bleach for every 100 gallons (approximately 380 liters) of water in the holding tank. This should give you a chlorine concentration of approximately 100 parts per million (ppm).
4. Run water through all plumbing used for drinking water in your home until you notice a strong chlorine odor.
5. Let the solution sit undisturbed for 24 hours.
6. Drain the whole tank.
7. Without entering the tank, remove all debris from the interior of the tank.
8. Without entering the tank, scrub the inside as well as you can with a stiff brush or broom and a solution of 1 cup (8 ounces; approximately 0.25 liter) of 5.25% unscented chlorine bleach (sodium hypochlorite) in 10 gallons (approximately 380 liters) of water.
9. Drain all of the disinfectant solution from the tank.
10. Let the tank refill with rainwater, or fill the tank with drinkable water.
11. Flush all piping in your home until the odor of chlorine is completely gone before you use the system again.

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