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Water cistern: what it is and how to keep the water in it usable

By the Plain Prepper editors. Updated 2026-10-15.

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Quick answer

A water cistern is a large container that stores water for a home, either treated water from a public system or untreated water from rain or another catchment, CDC explains. For rain-fed cisterns, the inflow matters most. A first flush diverter, such as Rain Harvesting's First Flush Carrot (first 5 gallons), sends the dirtiest roof runoff away. A fine screen like the Leaf Eater Advanced (955 micron) keeps leaves and mosquitoes out. CDC still advises against drinking rainwater unless you test and treat it, and gives specific steps for before and after a disaster.

Three kinds of cistern

According to CDC, a cistern can sit on the roof, under the ground or beside the home, and it stores one of two kinds of water:

Location	Usually filled by	What CDC notes
Rooftop	Public water system or groundwater	Common in Puerto Rico; filling it before a storm adds weight against wind
Underground	Catchment (rain, ground, spring or surface water)	Common in the U.S. Virgin Islands; untreated water
Beside the home	Either public water or catchment	Treatment depends on the source

The difference matters: treated public water in a sealed cistern is drinking water, while catchment water is untreated until you test and treat it. Check your state's rules too; CDC notes that some states regulate rainwater collection.

Keep the water cleaner on the way in

CDC says rainwater can pick up dust, smoke, roofing chemicals and bird droppings, especially when it rains after several dry days. Two devices address the inflow:

• First flush diverter: CDC describes it as a simple device that directs the first rainwater of a storm, which carries the most dirt, germs and chemicals, away from the system. The right amount to divert depends on the roof size.
• Inlet screen: CDC recommends a screen where water enters to keep out bugs, debris and mosquitoes.

Rain Harvesting's First Flush Carrot diverts about 20 L (5 gal) with no chamber, and an Extra Volume version doubles that to 40 L (10 gal). The First Flush Max uses a chamber that widens to 100 mm (4 in), which the maker says holds about 30% more water per meter of pipe, and adds an electronic release valve.

Setting how often the diverter resets

The Max's electronic valve lets you choose how often the diverter empties and resets. Rain Harvesting's guidance:

• 3 days or less: for the best water quality.
• 2 weeks or more: to capture the most volume.
• 5 days: a starting compromise between quality and quantity.

Both diverters use a Catch-All Tee, which the maker says stops water skipping past the diverter before it fills, a problem it says standard tees can have at flows under 1 L per second.

Before and after a disaster

CDC's cistern guidance for storms and floods:

• Before: fill the cistern, then close off the water source or divert the catchment away so storm water cannot enter. Keep a separate emergency supply of at least 1 gallon per person per day for 3 days, and 2 weeks if possible. For a family of four that is 12 gallons for 3 days or 56 gallons for 2 weeks.
• Fuel or chemicals: if gasoline, oil or other toxic chemicals get in, treatment cannot make the water safe. Drain it fully.
• Treated public-water cisterns: CDC gives a disinfection method (3 cups of unscented 5% to 9% bleach per 100 gallons, held at least 12 hours, then drained to a sanitary sewer and refilled), for use when officials advise it, after an evacuation or after weeks away. Follow CDC's full steps.
• Untreated catchment cisterns with open connections that took in storm debris or floodwater: CDC says not to use the water, to ask the health department, and that disinfecting such cisterns does not work well. Use your stored emergency water instead.

Everyday use and upkeep

• Match use to quality: CDC suggests limiting untreated rainwater to uses where you are unlikely to swallow it, such as watering plants you do not eat, and using public or bottled water for drinking, cooking and brushing teeth.
• Test if you drink it: if you drink, cook or bathe with rainwater, CDC says to test it regularly and ask your health department what to test for and how to treat it.
• Keep systems separate: rainwater must not be able to enter pipes carrying safe drinking water.
• Maintain: keep the system clean and fix broken parts. Rain Harvesting says the Leaf Eater Advanced's angled screen lets leaves slide off, reducing cleaning.

What to look for when you buy

Questions people ask

What is a water cistern?

CDC defines cisterns as large containers that store drinking water, either treated public water or untreated catchment water from rain, ground, spring or surface sources.

Is cistern water safe to drink?

Treated public water in an unaffected cistern is. CDC says rainwater is not necessarily safe without removing germs and chemicals, so test and treat it with health department advice.

What does a first flush diverter do?

It sends the first, dirtiest runoff of a storm away from the cistern. CDC says the right amount depends on roof size.

What should I do with my cistern before a hurricane?

CDC says fill it, close off the source or divert the catchment, and keep a separate emergency supply of at least 1 gallon per person per day.

Sources

• Cistern safety and disasters: before, during, and after, Centers for Disease Control and Prevention
• Collecting rainwater and your health: an overview, Centers for Disease Control and Prevention
• First Flush Carrot, Rain Harvesting (BMCo)
• First Flush Max with Catch-All Tee and Electronic Valve, Rain Harvesting (BMCo)
• Leaf Eater Advanced rain head, Rain Harvesting (BMCo)

Sources checked 2026-10-15.