October 10, 2024, 10:43 am | Read time: 4 minutes
The drought in 2018 has taught us to question our water consumption. But especially on hot summer days, many people shower more often, and the plants in the garden also need replenishment. Storing rainwater can be a solution. How does it work? And is it worth it?
The summer of 2018 will be remembered by many for its extremes. Some regions of Germany experienced weeks of drought, while others experienced heavy rain and flooding at times. In the face of such natural events, some people ask themselves: How can I take precautions at home? Because neither too much nor too little water is good. So, what ways are there to save drinking water and still have enough water for the house and garden?
“One answer to these questions can be the intelligent and sustainable use of rainwater,” says Dietmar Sperfeld from the Fachvereinigung Betriebs- und Regenwasser-Nutzung in Darmstadt. With a rainwater harvesting system, rain can be collected and used for flushing toilets, cleaning, and washing clothes, for example.
“This saves around 50 percent of domestic water consumption,” explains Sperfeld. These systems are also helpful during heavy rainfall – because the cistern can collect a lot of water.
Experts’ opinions differ
The Federal Environment Agency also sees climate change and man-made influences as the reasons for the heavy pollution of groundwater. In particular, the sealing of surfaces reduces natural groundwater recharge. Environmental policy measures would, therefore, be to unseal surfaces. In addition, rainwater should be managed locally in built-up areas.
However, Germany is still a country rich in water, and its water reserves are distributed regionally. Experts are therefore divided on the ecological and economic benefits of rainwater harvesting systems. According to the Federal Environment Agency, increased use of rainwater cannot generally be justified on the grounds of water scarcity.
How does a rainwater harvesting system work?
“The rain from the roof surface is collected and fed through a filter via the downpipes. It then flows into an underground storage tank, also known as a cistern,” explains Andreas Braun from the Zentralverband Heizung Sanitär Klima. These storage tanks are available in various sizes. They can be placed inconspicuously under the garage driveway or underground in the garden.
It is important that the rainwater is cleaned well. “In addition to the upstream filter, a calmed rainwater inlet serves this purpose.” It prevents sediment from being stirred up in the storage tank. “In order to extract the water from the cleanest layers in the tank, the extraction hose is laid floating,” explains Braun.
Can anyone use a system like this?
Not quite. “Some roof materials are not suitable, or only suitable to a limited extent, for collecting and using water in the garden,” says Bernd Kirschbaum from the Federal Environment Agency. For example, metal compounds can be washed off copper and zinc roofs, and the bitumen sealant on roofs with tar paper can also release biocides. Rainwater harvesting should be avoided here.
Anyone cleaning laundry with rainwater should be careful if people with a weak immune system or babies live in the household. “When washing laundry with rainwater, germs that are harmful to health are usually killed off by the temperature and detergent,” says Kirschbaum. “However, this is not guaranteed with subsequent rinses in cold water, which means that germs can be transferred to the laundry.” This risk can only be eliminated by treating the water appropriately or by ironing the laundry afterward.
Do I need a separate rainwater pipe in the house for this?
Yes, “If you use rainwater, you need a second pipe system in addition to the drinking water pipe,” says Braun. According to the Drinking Water Ordinance, you must ensure that there is a safety device on the rainwater harvesting system that prevents rainwater from mixing with drinking water. These pipes must be color-coded so that it is obvious that they do not carry drinking water.
How long does a rainwater storage tank last in dry weather?
That depends on consumption. “The water should last for three to four weeks if the tank is full,” explains Sperfeld. In longer dry periods, however, it may be necessary to top up with drinking water.
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Cost savings with a rainwater harvesting system?
With such a system, a four-person household can replace around 40 cubic meters of drinking water per year, saving between 160 and 200 euros. If no wastewater charges are levied for the rainwater used, savings of up to 300 euros are possible.
The costs of purchasing and maintaining a rainwater harvesting system are comparatively high. The Federal Environment Agency calculates that the construction costs for storage tanks, pipes, filters, and pumps are between 2,500 and 5,000 euros.
In addition, there are annual maintenance costs of around 100 euros. And some federal states require water meters and their calibration, which incurs further costs. All in all, according to the Federal Environment Agency, the payback period is well over ten years – without taking interest effects into account.