Rainwater harvesting is economically advantageous and helps preserve a resource that is becoming scarce: water. Here is some useful information on choosing a water tank for a rainwater harvesting system.

Concept of rainwater harvesting

Water that falls from the clouds during precipitation is commonly called rainwater. However, water from melting mountains and glaciers, hail or snowfall is also defined as rainwater. Suppliers of rainwater harvesting systems usually only use water that flows directly from the roof into the cistern. Rainfall from streets and impervious surfaces is rarely used for a rainwater harvesting system. After being stored in a PE tank or a concrete cistern, the collected rainwater is conveyed by a submersible pump or a water treatment plant to different users, such as the toilet, the washing machine, the cleaning water point or the garden watering.

The rainwater cistern

The water tank is under the house or now buried in the garden collects and stores rainwater, which saves you from using clean and expensive drinking water.

While in the past, in ancient times for example, entire villages depended on such tanks due to irregular rainfall, we almost only know these tanks for private use on a small scale. But here too, they can be very useful. Indeed, a cistern not only saves drinking water, but also money!

You can choose between a concrete cistern and a plastic one. The advantage of a concrete cistern is that it is very robust and durable. Thanks to its solid construction, it is very stable and has a very long minimum service life.


citerne à eau de pluie béton

Plastic tanks are made of polyethylene and are therefore environmentally friendly. The low weight of this rainwater tank allows you to bury it independently.

citerne à eaux de pluie

Rainwater tanks store produced rainwater so that there is enough rainwater available for the rainwater harvesting system, even in times of drought. The size of the tank is an important factor. A rainwater harvesting system that has to be regularly supplied with drinking water would be much less profitable. This is why the calculation of the rainwater cistern is the object of considerable attention.

How does a cistern work?

When it rains, precipitation falls into the cistern via downspouts and feeder pipes. Before reaching the cistern’s reservoir, the water passes through a rainwater filter that cleans the rainwater of large dirt, leaves or stones. The water then collects in the tank and settles there. This means that the remaining suspended solids fall to the bottom, this is called sedimentation. The water can then be stored in the tank until it is needed.

If a period of heavy rainfall occurs, the tank can quickly become full, depending on the size of the tank. The excess water can then flow into the sewer system through an overflow similar to that of a bathtub. On the other hand, if the weather remains dry for a long period of time, the cistern must be supplied with drinking water. Otherwise, it risks drying out. For small quantities of water, it is therefore preferable to use a rain cistern as an alternative. A large rainwater installation must be well thought out

How to calculate the volume of a rainwater cistern

A rainwater cistern should normally be able to cover the water needs for about 25 days. However, in order to be able to cope with longer periods of drought, 30 or 35 days is considered a storage period for the calculation.
However, the tank should not be too large, as the tank should overflow at least once a year, if not twice.

A rainwater tank holds about 5,000 to 6,000 liters for a house with a family of three or four and a little garden. However, this size is not automatically correct, but must be calculated individually according to the project.

Filtering rainwater before the cistern

Before flowing into a cistern or underground PE tank, the rainwater produced during a rain event must first be filtered. For professional rainwater use, a rainwater filter is almost indispensable, because without the filter, the input of coarser materials, such as leaves or sand, becomes considerably too high over the years and the water quality suffers.
In practice, filters are often installed in the discharge pipe after the rainwater pump, but this only brings a limited improvement of the water quality at the withdrawal points. But in no case an improvement of the quality of the rainwater in the tank. To obtain this quality in the cistern, it is therefore essential to filter as much organic matter as possible upstream, before storing the rainwater in the tank. If this first filtration step is not implemented, the organic matter will degrade over time at the bottom of the rainwater cistern.
There are generally two types of rainwater filters. Filters that first collect all of the debris during a rainfall event and release the rainwater into the cistern as it falls. The remaining debris must then be regularly removed from these filter baskets or bags.
Similarly, rainwater filters directly separate incoming rainwater from the accompanying debris and send the clean rainwater directly into the cistern. Both systems have advantages and disadvantages. The “basket” variant, for example, achieves near optimal efficiency. However, the residues in the basket or cartridge carry some of the waste into the cistern. Especially if the basket/cartridge is not cleaned regularly.
In case of direct separation, the efficiency can be affected depending on the conditions. In addition, there must be a connection to an overflow or a gutter. However, the cleaning intervals are usually much shorter in this case.

How much does a rainwater tank cost?

The cost of cisterns varies considerably, of course, depending on the material, size and capacity. The price of a rather small rainwater harvesting tank, with a plastic tank and a capacity of 1,500 liters, is less than 1,000 euros. The large version, with a capacity of 13,000 liters, is available from about 4,000 euros. This price does not include the pump, filter system, pipes and fittings. In addition, the pit must also be dug. If you are not able to do this yourself, you must call in a craftsman. This involves additional costs for the professional or fees from the planning department. However, the cost of the tank is a good thing and usually pays for itself in just a few years.

Installing a rain watertank properly

The capacity and size of the cistern should of course be determined according to the space available in your garden. After the size, you also have to choose the material: You can choose between concrete and plastic, but the installation of a plastic cistern is easier for do-it-yourselfers and amateur gardeners, as it is lighter. When choosing the material, you should also make sure that the rainwater cistern has a sufficient thickness. Indeed, nothing is more annoying than a leak in the cistern, because the work to get it out of the ground is still very important. You can save this effort and money by opting directly for a thicker material.

Conclusion: saving money with a cistern

A cistern may cost money, but it is worth it: the benefits are obvious. A four-person household saves 75,000 liters of drinking water per year: 33 liters per day for flushing the toilet, 15 liters for laundry, 10 liters for watering the garden or cleaning the house. This is not only good for the environment and the drinking water supply, but it also saves money! In fact, rainwater is free. Of course, the cistern has to be purchased, installed and maintained, but it is still much cheaper than tap water. Thus, we can all contribute to ensuring that clean drinking water remains available for a long time to come for the entire world population.