AdminWater scarcity and environmental sustainability are pressing concerns in today’s world. Installing a rainwater harvesting system is an effective way to address these issues while also reducing your household water bills. This eco-friendly practice involves collecting, storing, and using rainwater for various purposes such as irrigation, toilet flushing, and even laundry. Not only does this help conserve valuable drinking water resources, but it also provides a sustainable solution for managing stormwater runoff.
In this guide, we will walk you through the steps to install a rainwater harvesting system, from selecting the appropriate components to setting up and maintaining the system. Whether you are a DIY enthusiast or planning to hire a professional, this comprehensive overview will equip you with the knowledge needed to successfully implement a rainwater harvesting solution in your home. Let’s get started on your journey towards water conservation and sustainable living.
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.
Plastic tanks are made of polyethylene and are therefore environmentally friendly. The low weight of this rainwater tank allows you to bury it independently.
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
Calculating the volume of a rainwater tank involves a few key steps to ensure you meet your household’s water needs while maximizing efficiency. First, determine the catchment area by measuring the surface area of your roof (length x width). Next, find the average annual rainfall for your region, typically measured in millimeters. Then, consider the runoff coefficient based on your roof type: 0.6 for flat roofs with gravel, 0.8 for concrete or slate roofs, and 0.9 for tile roofs.
Use the formula: Volume (liters)=Roof Area (m2)×Annual Rainfall (mm)×Runoff Coefficient
For example, a 150 m² roof in an area with 800 mm of annual rainfall and a coefficient of 0.8 would yield: 150 m2×800 mm×0.8=96,000 liters/year
Finally, account for the desired storage period (e.g., 21 days) to determine the tank size. Proper calculation ensures efficient water use and sustainable resource management.
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 rainwater tanks varies considerably depending on the material, size, and capacity. Here is a detailed breakdown of the costs associated with different types of rainwater tanks:
Concrete Tanks
- Cost: A 5000 liters concrete rainwater tank is available from around 1.500 euros. This price excludes additional components such as the pump, filter system, pipes, and fittings.
- Additional Costs: The installation of a concrete tank typically requires excavation. If you are unable to perform this task yourself, hiring a professional will incur extra costs. There may also be fees from the planning department.
Plastic (Polyethylene) Tanks
- A plastic rainwater tank with a capacity of around 5.000 liters costs 5,000 euros.
- Larger plastic tanks can cost significantly more, depending on the specific size and features.
- Additional Costs: Similar to concrete tanks, the price does not include the pump, filter system, pipes, and fittings. Installation is generally easier and may not require professional assistance, reducing additional costs.
Metal Tanks
- Cost: Metal tanks, particularly those made from galvanized steel, can vary in price based on size and finish. Small to medium-sized metal tanks generally start at a few hundred euros and can go up to several thousand euros for larger capacities.
- Additional Costs: These may include installation costs, as metal tanks can be heavy and may require a sturdy base or professional setup.
Flexible PVC Tanks
- Cost: Flexible PVC tanks are relatively affordable, with prices starting at a few hundred euros for smaller capacities. Larger, high-capacity models can cost more, but they are typically less expensive than rigid tanks of the same capacity.
- Additional Costs: These tanks are easier to install, potentially reducing the need for professional installation services.
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.
