Cisterns Before and After a Disaster
Cisterns are large containers used to store drinking water. They can store either:
- Treated water that comes from a public water system, or
- Untreated water from a catchment (collection) system that gathers rain, groundwater, spring water, or surface water.
Cisterns can be on rooftops, underground, or standalone structures beside houses. Cisterns on rooftops are often connected to public water systems or to groundwater systems. Underground water cisterns often use catchment systems. Standalone cisterns can be filled by public water or catchment systems.
Natural disasters—like floods, heavy rains, or hurricanes—can impact the quality of cistern water. See below for more information on what to do with your cistern before and after a natural disaster.
Before a Natural Disaster
- If possible, fill your cistern to the maximum capacity. The additional water can increase the amount you have available after a disaster. If your cistern is on the roof, the extra water will help to weigh down the cistern in the event of strong winds.
- If possible, close off the water source or divert the catchment system away from the cistern to prevent water from entering the cistern during the disaster.
- Store an emergency supply of safe water to use, in addition to the water in your cistern. Store at least 1 gallon of water per person per day for 3 days, for drinking and sanitation. Try to store a 2-week supply, if possible.
- Store a bottle of unscented liquid household chlorine bleach (label should say it contains between 5% and 9% sodium hypochlorite). You can use this bleach to treat your water, if necessary, and for general cleaning and disinfecting.
After a Natural Disaster
IMPORTANT: Use only clean, treated water for drinking, cooking, making ice, washing dishes, showering/bathing, brushing teeth, and household cleaning. You may use contaminated water to flush toilets.
IMPORTANT: If water is contaminated with fuel (gasoline or oil) or other toxic chemicals, you cannot make it safe by chemical disinfection. Water contaminated in this way will need to be fully drained from the cistern.
- If your cistern uses treated water from a public system and was not impacted by the disaster, you can use the water.
- If you are unable to access your cistern water or the water was impacted by the disaster:
- If your cistern uses untreated water and has open connections (for example, gutters and overflows), your cistern may fill with storm debris or floodwater during a disaster. Disinfecting the water in cisterns filled with storm debris or floodwaters is not effective. It is very difficult to determine the exact amount of chlorine needed for proper disinfection, and correctly mixing it with the water is also an issue. In addition, these cisterns might get re-contaminated by germs in the environment because of their open connections. Instead, you can do one of the following:
Cleaning and Disinfecting Cisterns
If your cistern uses treated public water, your water system or government authorities may tell you to disinfect the cistern after a disaster. You also may need to disinfect your cistern if you evacuated your home or have been away from your home for weeks or months and think it may have become contaminated.
You can disinfect the cistern using the following steps:
- If the cistern is connected to any water treatment units, close valves to those units. Follow manufacturer recommendations to disinfect the treatment system.
- Add 3 cups (a little less than 1 liter) of 5%–9% unscented liquid household bleach for every 100 gallons (about 380 liters) of water in the holding cistern.
|Tank is ¼ full||Tank is ½ full||Tank is ¾ full||Full Tank|
|200||1.5 cups||3 cups||4.5 cups||6 cups|
|450||3.5 cups||7 cups||10.5 cups||14 cups|
|600||4.5 cups||9 cups||13.5 cups||18 cups|
|1,000||7.5 cups||15 cups||22.5 cups||30 cups|
- If the cistern is connected to indoor plumbing, open each faucet and run the water until you smell chlorine (bleach).
- Turn off all faucets and allow the solution to remain in the cistern and plumbing for at least 12 hours.
- Drain all water from the cistern.
- Refill the cistern with clean, safe water.
- Open each faucet and run the water until you do not smell chlorine (bleach).
If your cistern uses untreated water and has open connections (for example, gutters and overflows), you might need to drain and clean your cistern. Ask your health department whether this is needed. If so, use the following steps:
- Remove all debris from the catchment area.
- Remove all debris and water from the cistern.
- If you can do it safely, scrub the inside of the cistern with a stiff brush and a solution of 1 cup (about 0.25 liter) of unscented liquid household bleach (5%–9% sodium hypochlorite) mixed with 10 gallons (about 38 liters) of water.
- Rinse the cistern with clean water; then drain.
- Refill the cistern.