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Construction site with cistern made of cement filled with gravel.

What Is a Cistern?

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Author Icon Written by This Old House Reviews Team Updated 03/22/2024

A cistern is an underground tank that captures and stores rainwater that you can use for irrigation, emergencies, or daily water needs. Cisterns are especially helpful in areas with unreliable or expensive municipal water systems. We’ll explore the different types of cisterns, describe the benefits each offers, and explain how cisterns differ from wells in the guide below.

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Definition of a Cistern

A cistern is a watertight tank that collects and stores water for later use and is typically made of reinforced concrete, cinder block, precast concrete, fiberglass, and steel. Your home’s gutters and downspouts usually funnel rainwater into your cistern, and you can use that water as a supplemental supply, an emergency supply, or for irrigation. Cisterns can also hold groundwater, household greywater, spring water, well water, or treated city water.

Professionals can install cisterns above ground, underground, or into building structures. Burying cisterns underground might be the best option, though, as underground cisterns keep water cool during warm months and prevent it from freezing during colder weather.

Cisterns intended for domestic use typically hold at least 5,000 gallons, but sizes and setups vary according to homeowners’ needs.

Types of Cisterns

Most cisterns fall into one of three basic categories: rainwater cisterns, water storage cisterns, and greywater reuse cisterns.

Rainwater cisterns are the most common type of cistern and connect to your home’s gutter system. Water that falls onto your roof flows into the gutters, through the downspouts, and into the cistern.
You can use the collected runoff to irrigate your lawn or garden, or if you treat the water, you can use it for indoor use and even for drinking. Roof catchment systems that collect rainwater typically include a roof washer and a filter box to minimize potential water contaminants that reach the underground cistern.
Water storage cisterns are common in rural areas where public water is unreliable, inadequate, or unavailable. Homeowners can use water storage cisterns to tap into a nearby spring or groundwater in these areas.
A water storage tank provides a buffer against drought or service interruptions and has the potential to collect enough water to cover your daily needs. Installing a residential well and water tank can also save you money and help you eliminate your monthly water bill.
You can recycle wastewater from domestic activities such as laundry, dishwashing, and bathing with a greywater reuse cistern. These tanks collect and treat greywater instead of using rainwater or water storage systems.
A greywater cistern will not provide potable water, but it can help reduce your overall water consumption. This cistern can help you collect water from your shower, sink, or washing machine and use it to flush toilets, wash your car, or irrigate your lawn.

Why Have a Cistern?

Cisterns help homeowners tap into additional water sources, reduce their water consumption, and prepare for emergencies.

Emergency Water Supply

Ground cisterns provide backup if disaster strikes or service disruptions affect your primary water supply. Having this water source can be lifesaving if you live in an area prone to flooding or hurricane damage, as you can fill your cistern with city water ahead of potential natural disasters. 

Irrigation Systems

Cisterns make it possible to keep your lawn lush and green without wasting water. You can use your cistern’s rainwater or greywater to water crops and ornamental plants or offer it to livestock. Every drop of water you save is a step toward self-sufficiency and environmental conservation.

Supplemental Water Supply

Many homeowners use cisterns to reduce their dependence on municipal water. Having your own water supply can help you lower your water bills. This is especially advantageous if you live in an area with high water rates or one prone to drought, hurricanes, or service interruptions. 

Fighting Fires

The water stored in your cistern can be a helpful resource for firefighters. This is especially true if you live in a remote area far from ponds or fire hydrants. Quick access to plenty of water can help protect your family and property.

Cisterns and wells both help homeowners reduce dependence on municipal water systems, but there are key differences between them:
Depth: Underground cisterns are shallower than wells and typically go no deeper than 40 feet. Wells, meanwhile, are sometimes hundreds or thousands of feet deep so that they can extend past the water table and tap into abundant water sources.
Location: Cisterns can function above or below ground and are typically near your home. Wells should be further away from potential contamination sources, such as buildings or septic tanks.
Storage and capacity: Cisterns are essentially water storage tanks and can hold between a few hundred gallons and several thousand gallons. Wells draw water directly from aquifers, offering a potentially unlimited supply of water.
Water quality: The rainwater that cisterns collect typically requires filtration and treatment to ensure potability. The quality of well water varies and requires testing but may not require any treatment.
Water source: Most cisterns collect and store rainwater, while wells draw up groundwater from deep in the earth.

Water Quality

Cistern water can harbor various contaminants, from harmless sediment to deadly bacteria. Test water from your cistern regularly to ensure it’s safe for drinking, bathing, and cooking.

Potential Contaminants

Leaves, debris, and animal droppings can wind up in your rainwater cistern, affecting taste and exposing you to harmful bacteria, such as E. coli. Naturally occurring minerals such as iron and manganese can also affect the taste and color of water, while heavy metals pose a more serious health risk.

Testing and Monitoring

Be sure to test your cistern water for bacteria, heavy metals, and other contaminants at least once a year. Monitor the acidity and hardness of your water to protect your home’s plumbing system. Local water authorities or private labs can provide testing services. 

Treatment Options

You can employ several cistern treatment options to remedy issues or eliminate contaminants. Filtration systems remove sediment and larger particles, while UV lights or bleach can help disinfect the water. A roof washer or flush-first diverter can prevent rooftop debris from reaching the cistern. Consider installing an acid neutralizer, water softener, or reverse osmosis system to address specific water quality concerns.

Our Conclusion

Cisterns can keep your garden green, help you to conserve water, and lower your monthly expenses. Rainwater requires treatment to be safe for drinking, but it can be a literal lifesaver during emergencies. 

Cisterns are not a one-size-fits-all solution. Carefully weigh your specific needs and the initial investment against the potential long-term benefits to determine whether installing a cistern makes sense.

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FAQ About Cisterns

What is the purpose of a cistern?

The primary purpose of a cistern is to store water for various uses, such as irrigation or emergency drinking water. Cisterns collect rainwater or water from other sources and store it until you need it.

How long does water last in a cistern?

Cistern water can last for months or even years depending on the tank size, water use, water quality, and maintenance required.

Is cistern water safe to drink?

Cistern water is not usually safe to drink due to potential contamination. Test and treat your water regularly to ensure safety.

What is the difference between a well and a cistern?

Cisterns typically capture rainwater or surface water, while wells tap into groundwater. Cisterns are also shallower than wells and offer limited storage capacity.

Can I install a cistern myself?

You may be able to install a cistern yourself, but this will depend on your skill level and the type of system you require. However, we recommend consulting a professional to ensure proper installation.

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