Drinking water is one of the most precious raw materials in our daily life. Made into food in complex processes, it will continue to increase in value and become more expensive. A good reason to replace as much drinking water as possible with rainwater in the house and garden – with a collecting tank in the garden this can be done in every house.
Every year countless cubic meters of rainwater seep into gardens unused or are drained unnoticed into the rainwater channel via the gutter. A huge waste, actually. Every cubic meter of drinking water costs at least around 1.50 dollars and, depending on the disposal company, costs around 1.60 to 2.60 dollars as wastewater, since it has to be reprocessed. It is particularly incomprehensible when the garden is watered, the toilet cistern is fed or the laundry is washed with the expensive water.
A rainwater collection tank with a volume of several cubic meters saves these expenses and at the same time helps to save valuable drinking water. Today there are suitable tanks for every need, every house and garden size. Even in a small front yard you can bury a voluminous tank and connect it in the house.
Where is rainwater collection worthwhile?
“Various model calculations show a clear trend: In this century we will experience milder and wetter winters and drier summers due to global warming,” says Gerhard Müller-Westermeier, the qualified meteorologist responsible for national climate monitoring. “Due to the rise in temperature from the middle of the century, there will often only be snow at high altitudes. But there are already harbingers of the future climate, as the last two very mild winters have shown.
Even today, at the beginning of the predicted climate change, depending on the region, an average of between 500 and 1500 liters / sqm of rain fall worldwide each year. Anyone who collects part of it with a usable roof area of 50 square meters collects at least 25,000 liters or 25 cbm per year, which can be used in the house and garden instead of expensive drinking water – at the same time, you often save on wastewater fees if these are coupled with the drinking water consumption .
What is my need?
According to the consumer advice centers, the average water consumption worldwide is now 127 liters per person per day. A family of four uses one cubic meter of drinking water every two days. That can be cut in half! Up to 50% of the consumption can be covered by the use of rainwater. In addition to bathing or showering, where drinking water cannot be replaced for hygienic reasons, the toilet flush and washing machine swallow most of the water in the household with a combined share of around 38%. Here, when watering the garden and cleaning, rainwater can replace the expensive water.
But how much water do you have to collect and how big does the collection tank have to be so that everyone in the house has enough rainwater available? There is a simple calculation formula that takes into account the amount of rain, roof area and a safety reserve. It is important to choose the tank size so that the tank overflows several times a year in order to keep the surface water clean. Deposits on the water surface are flushed out by themselves.
Rainwater requirement in liters per year
- Toilet flush: 9000 liters per person / year x number of people
- Washing clothes: 4500 liters per person / year x number of people
- Cleaning: 1000 liters per person / year x number of people
- Garden irrigation: 60 liters per person / year x number of people
Which tank is suitable?
Tanks for collecting rainwater are now either made of concrete or plastic, mostly polyethylene (PE), which is also used in the food industry for packaging.
Concrete tanks have a huge disadvantage: They are heavy and can only be brought into place with large construction equipment. Plastic tanks, on the other hand, weigh between 100 and 300 kilos, depending on their size. The smaller models can even be carried with a few helpers, while the larger models can be easily lifted on transport trolleys in pairs and then pushed.
Tip: Tack the tension belts to the roller boards, which you then put around the tank and lash down. In this way, the transport rollers stay in place even on uneven terrain.
What is important is a seamless tank (no weak point in the tank shell) and an entry dome that is high enough that the tank can be installed at least 60 cm below the surface of the terrain. In this way, the water will remain frost-free and stored in a cool and dark place all year round, which largely prevents the formation of algae and germs.
A fine filter must be installed in the water supply to filter out coarse dirt (leaves, twigs, etc.). In tanks that are only used for watering the garden, a suspended plastic strainer is sufficient, which thanks to its fine mesh retains even the smallest seeds and allows almost 100% of the water to enter the tank. When rainwater is used at home, a permanently installed cartridge filter in a closed housing is usually used.
It has the advantage that the dirt is flushed out again via the overflow. This saves frequent checks as with suspended sieve filters, but around 5% of the water is required that does not end up in the tank. In addition, it has a larger mesh size that allows even the smallest impurities to pass through. A so-called calmed inlet ensures that the water does not splash freely into the tank and stir up all dirt particles, but rather oozes out of a pipe at the bottom of the tank. This means that any remaining dirt particles quickly settle on the bottom of the tank.
It is ideal for use if the water is sucked in a few centimeters below the surface. This is where the water is cleanest, because heavy particles sink to the bottom, while light ones float on the surface. A heavy, metal suction strainer, which is mounted on the end of the hose and hangs on a floating ball, guarantees that water is drawn just below the surface, and the suction level remains the same regardless of the water level.
How clean is rainwater?
Thanks to a good filter in the tank inlet, the collected service water is largely free of coarse impurities. It looks different with bacterial or germ load. Here, an effective treatment of the water is extremely complex, so that this is not worthwhile for private use according to the current state of the technology offered. This is also the reason for its exclusive use in areas that do not affect the human body. This also justifies the requirement that a separate pipe network is necessary for using rainwater in the house. This must be strictly separated from the drinking water network and all pipes and tapping points – even in the cistern – must be marked with the notice “No drinking water”.
Experts differ on the question of whether to wash clothes with rainwater. The Federal Environment Agency warns of this because rainwater is contaminated with faecal bacteria, for example through bird droppings, and the last rinse is carried out with cold water, so that the full number of bacteria in the rainwater is transferred to the laundry. In contrast, studies by the Hamburg environmental authority showed as early as the 1990s that the germ load of the washed laundry was far below that of the unwashed laundry. Practical experience from rainwater users rather confirm the harmlessness.
You protect the environment, they say, because less washing powder is required due to the low hardness of the rainwater and fabric softener can be omitted entirely. So it is not surprising that the washing machine manufacturer Miele, for example, offers an “all-water washing machine” that has two water inlets, so you can wash with rainwater, but the final rinse is made with drinking water. Ultimately, opponents and supporters have good arguments and everyone has to decide for themselves whether the washing machine should be connected or not. At most, a few water disposal companies who are hardly interested in the use of rainwater, because after all, an increasing number of cisterns initially reduce their income.
What cannot be completely dismissed, however, are the relatively high investment costs, which averaged around 2500 and 6000 dollars and in many regions stand against comparatively low water and sewage costs. Model calculations by the consumer advice centers result in around 20 to 25 years before the investment is amortized. The effective, annual operating costs for the house will fall, however, and whoever uses a building society savings contract for the purchase, for example, finances with low monthly costs, which ideally do not exceed the savings.
Can you install the collection tank and domestic waterworks yourself?
If you buy a complete package from a manufacturer consisting of a plastic tank, submersible pump (garden use) or domestic water machine (house use), dirt filter, suction set and tank cap, you can largely install your rainwater system yourself.
Before you buy, however, you should inquire at the building authority and, in the case of use in the house, also at the health department, whether there are restrictions and whether permits are required. As a rule, however, this is not the case as long as all regulations are observed. It is worth asking the water supplier for a possible subsidy or a reduction in fees. And realize that for a 3000 liter tank you have to dig a pit 2 m deep, 2.5 m wide and 3.5 m long. That is more than 17 cbm of earth moving plus the soil that has to be excavated for a proper embankment. Renting a mini excavator or professional help is inevitable here. In addition, large construction equipment helps lower the tank into the pit.
We buried a 7 cbm tank and connected it to the washing machine and toilet cisterns via a domestic water machine. Before the connection is made in the house, however, a KG pipe must be placed in the basement using a core drill. The suction hose and, if applicable, the cable for the water level indicator are fed through here. In the basement, the KG pipe ends flush with the wall and a so-called wall duct is used. These are two multiple perforated stainless steel disks with a solid rubber body in between, held together by screws.
Cable and hose are fed through the holes. The whole thing is inserted into the core hole or into the KG pipe, the screws are tightened so that the rubber body is compressed. This tightly encloses the lines and completely seals the wall opening.
In the tank, the suction strainer hangs under the floating ball and the suction hose ascends from the tank to the pump in the cellar. It is connected to a water storage tank that maintains the water supply for toilet flushing and washing machine when the tank is empty. Incidentally, this is the only point at which there is a connection to the drinking water network, because when the collecting tank is empty, drinking water is automatically replenished here.
In order to prevent rainwater and drinking water from mixing, according to DIN 1988 the inlet must run freely, for example via a filling funnel, so that rainwater cannot flow back or press back into the drinking water network.
In any case, a professional is required here to lay the connection and usually also to install the lines to the consumption points. You can install all other water connections yourself. Of course, you leave the electrical connection to the professional.
Rainwater for the garden
If you only use your collected rainwater in the garden, you do not need complex building services. A submersible pump and a so-called water socket are sufficient here. The water socket is connected to the tank via a KG pipe through which the pressure hose from the pump runs.
It ends at a hose connector, ideally with a shut-off valve, onto which a standard hose coupling can be screwed. The pump should have a hose with a suction basket and floating ball on the suction side to suck in the water in the clean zone below the surface – this is gentle on the pump.