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Winter check
These measures ensure that your rain tanks are ready for winter!

Rainwater harvesting: Questions & Answers


  • What components should my rainwater harvesting system comprise for garden irrigation?

    Often, simple and good value systems are sufficient to irrigate gardens with rainwater. If you have a large garden with a high water requirement and corresponding collection surfaces, we recommend that you install an overground or underground plastic tank due to the easy handling.The tank should be fitted with a course filter. You will need a garden pump with suction hose (intake set) to remove the water from the tank, if you do not choose a submersible pressure pump.The tanks and filters must be emptied in winter or removed, unless they are installed at frost-proof depths. The pumps must also be removed.

  • What components should my rainwater harvesting system comprise for toilet flushing, washing machine and garden?

    Plants that provide rainwater for toilet flushing and washing machines all year round are more expensive to plan and install than garden plants. As you can save up to 50 % of your drinking water consumption with these plants, the greater expenditure is completely justified.

    Modern rainwater harvesting plants for domestic use comprise the following
    components in addition to the connection pipes:

    • Prefilter
    • Rainwater tank with steady inflow and overflow siphon
    • Floating extraction with intake filter
    • Domestic waterworks
    • Drinking water intake
    • Reversible flow filter (optional), domestic waterworks and drinking water intake can also be integrated into a compact drinking water intake module.

    To ensure these plants operate all year round, the underground tank and filter and the water-carrying pipes in the ground must be installed at frost-proof depths.

  • Can I wash my clothes with rainwater?

    Extensive bacteriological tests have shown that using rainwater has no negative effect on the washing result and does not increase the germ content of the washed clothes. Experts have proven that pathogens do not get into the washing machine through rainwater, but rather from dirty clothes for the most part.

    Compared to the mostly harder tap water, the soft rainwater has the advantage for washing clothes that you need less detergent, no fabric softener or descaler and you get better washing results overall.

  • Do I need a rainwater filter?

    At least one prefilter is built into every rainwater harvesting plant. It cleans the water of impurities even before it gets into the tank (e.g. leaves, sand and moss). Prefilters are the 1st cleaning stage in the rainwater harvesting plant and are very important for the quality of the collected water and the function of the plant as a whole. For this reason, only use filters that have been specially developed for rainwater harvesting, which meet the following requirements: good filter performance, high water yield, easy to clean, low maintenance and warranty in line with DIN 1986.

    The basic choice is between filters integrated in the down pipe, underground, or in the tank. The choice of suitable variations depends on the structural features and the area of use for the rainwater.

    GARANTIA® offers you different filter alternatives that satisfy different demands.

    All our underground filters can be installed at frost-proof depths before a rainwater tank. As they have several inflow options, they can filter all the water flowing from the roof surfaces centrally. An overflow connection also enables surplus water (from heavy rainfall) to flow into the sewer network.

    The self-cleaning SUPRA-Filter is also low maintenance. The underground fine filter achieves a 100 % water yield, but does need to be cleaned from time to time. In difficult installation situations we recommend also the SUPRA-Filter, as it offers the lowest height offset (just 10 mm) between the inlet and outlet.

    If you also want to use the collected rainwater for toilet flushing and the washing machine, we recommend that you also fit a floating extraction with intake filter to your tank. This means that water extraction is always effected at the cleanest point, approx. 10 cm below the water surface.

    It is also sensible to install a reversible flow filter after the pump in the pressure hose, if you want to use the rainwater to wash clothes. With a mesh width of 0.1 mm, this filter also keeps out the smallest dirt particles. Installation is optional.

  • Does the collected rainwater in the tank stay fresh?

    Underground tanks are ideal for storing rainwater, as the earth protects the tank from light and heat. The temperature of water from underground tanks is only 6 to 8 °C all year round.

    Because it is dark, no algae can form either so the water cannot "get agitated".
    Equally, no microorganisms or bacteria can survive in cold water, let alone multiply, as apart from a lack of heat, there is nothing for them to eat.

    Where installation of an underground rainwater tank is not possible, cellar tanks are preferred, which should be impervious to light in principle. To prevent the water heating and, thus, to prevent algae and germs from forming and multiplying, cellar tanks should installed in cool, dark places.

    Chemical treatment of the rainwater is not necessary and should not be performed!

  • How large does by rainwater tank need to be?

    The size of your tank depends mainly on what you want to use your rainwater for (garden irrigation and/or toilet flushing and washing machine) and how many people live in your household.

    You can calculate the size of your tank interactively using the “Calculation Form”.

  • How do I connect several tanks together?

    Smaller rainwater tanks can easily be connected to a battery with greater storage volume using tank connectors, which are mounted on the bottom of the tank.This is always sensible if narrow access routes and/or the intended location prevent the transport and installation of a larger tank (e.g. in the cellar).

  • Why a GARANTIA® rainwater tank?

    We have been developing and producing plastic tanks and containers for more than 40 years. Over this period, we have amassed vast experience and built up expertise, which we are happy to pass on to you.
    Our aim is to supply our customers with innovative products of the highest quality. Our newly developed Cristall- and Columbus tank range for example has been specially developed for rainwater harvesting. It presents a future-oriented, ecological solution, with which you can save valuable drinking water on a long-term basis. Its features:

    • Completely recyclable, as made from polyethylene
    • Maximum stability due to heavy ribbing
    • Very long service life
    • 15-year warranty on materials
    • Checked quality due to individual inspection in the factory
    • Best water hygiene thanks to smooth interior surfaces and use of food-safe materials (DVGW tested)
    • Easy to transport and install due to low weight and carry handles
    • Our 3,700 litre tank for example only weighs 140 kg
    • Easy and safe to install due to ready-to-plug connections

    Our plastic tanks can be transported and installed “by hand” by 4 people; no crane is required. Therefore, they are ideal for retrofitting.
    Where it is not possible to install an underground tank, we recommend our space-saving cellar tanks, which also have the advantage that, with a width of 80 cm, they fit through almost all doors and stairwells.

    Our Herkules Tank is a rainwater tank with many uses, as it can be installed overground in the garden, in the cellar, or – with a support pipe – underground.
    It consists of two shells, which can be permanently sealed with patented fast connectors and a solid profile packing.
    As each shell only weighs 30 kg, and as 78 cm high fits through almost all doors, transport and installation is very easy. It is TÜV safety-tested and was awarded “Product of the Year 1999” by the German Trade Association for Plastics Processing (FVKK).

  • How thick are the walls of the GARANTIA® rainwater tanks?

    Underground tanks in the Columbus range: 9 to 21 mm (depending on tank size)Herkules Tank: approx. 6.5 mm

  • What is the warranty period on GARANTIA® rainwater tanks?

    We give a 15-year warranty on our Cristall and Columbus range underground tanks. Within this period, we provide material replacements free of charge. Services beyond this are excluded. Prerequisites for the warranty are proper handling, assembly and installation of the tank in line with the instructions.

    However, the life expectancy of the tank is much higher. When installed properly a tank can be used for decades. Naturally, you also receive warranty periods for an array of other components, such as pumps and plant controls, which we will be happy to give to you.

  • Do I need to empty a GARANTIA® rainwater tank in the winter?

    If you would like to operate your rainwater harvesting plant in winter too, or if you connect toilet flushes and washing machines to the plant, the underground tank and filters and water-carrying pipes in the ground must be installed in the ground at a depth of 60 to 100 cm – depending on the local frost line – to protect against frost.

    Please ask your building authority for the applicable local frost-free line!

    Like cellar tanks, frost-proof installed underground tanks do not need to be emptied in winter.

    Butts and tanks installed overground in the open air must be emptied in winter.

  • Why do I need a dome shaft or a dome shaft extension?

    Underground tanks and filters must have a sufficient opening or a sufficiently large shaft, so that for example they can be cleaned as needed. As underground tanks and filters are normally installed at frost-proof depths, this opening not only needs to be large enough, but also high enough.

    Depending on the installation depth the height of the standard shaft may be insufficient. For this reason, we supply suitable dome shafts and shaft extensions for all our underground tanks and filters, which can be reduced or built up on top of one another as needed.

  • Are underground rainwater tanks capable of bearing vehicles?

    The Cristall and Columbus underground tank ranges are available in car-bearing variations.

    • e.g. Cristall car-bearing
      Size 1,600 to 2,650 l
    • e.g. Columbus car-bearing
      Size 3,700 to 9,000 l
      max. axle load 2.2 t combined with telescope and cast cover class B or BEGU telescope dome shaft

    Follow the instruction manual!

  • What happens when the tank is full?

    Underground and cellar tanks must be fitted with an overflow (see instruction manual), through which the surplus water can either flow directly into the sewer network or into the groundwater through percolation.

    The simplest solution, though not the best from an ecological perspective, is for the water to flow into the sewer network through an interface. When connecting your tank to the sewer network you must pay particular attention to the backpressure level of the sewer. The overflow must be above the backpressure level. A backpressure flap should be installed if necessary.

  • When do I need a steady inflow and an overflow siphon?

    You always require a steady inflow and an overflow siphon in the tank if you are also using the rainwater for toilet flushing and washing machines.

    The steady inflow facilitates a reduction of fine dirt particles on the bottom of the tank where they form a sediment layer. It also protects this layer, which is of the greatest significance for the quality of the stored water, from slowly being swirled back up into the water.

    The overflow siphon ensures that particles floating on the surface are removed from the tank and that no odours or gasses from the sewers can get into the tank through the siphon.

    This means that the water in the tank remains clear and odour-neutral.
    Together with the prefilter, these two measures are very significant for the quality of the water and the function of the rainwater harvesting plant and, therefore, are designated as cleaning stages 2 and 3 of a rainwater harvesting plant after the prefilter.

  • Why do I need floating extraction?

    If you also want to use the collected rainwater for toilet flushing and the washing machine, we recommend that you also fit a floating extraction to extract water from the tank.

    It comprises floating balloon, siphon pipe, return valve and fine filter basket.
    The floating balloon holds the intake with the fine filter approx. 10 cm under the water surface. so that water extraction is always effected at the cleanest point.

    The floating extraction is the 4th cleaning stage after the prefilter, the steady inflow and the overflow siphon, in a plant, which is also connected to toilet flushes and washing machines.

  • Why do I need a drinking water intake or a fill level display?

    During longer dry periods it is possible that the stored rainwater will not cover consumption. So that the consumption points connected to the rainwater harvesting plant can still be supplied during such times, drinking water from the public network must be drawn in.

    There is no way to prevent drinking water intake if toilet flushes and washing machines are connected to the rainwater harvesting plant.

    It is advisable to use an automatic control unit for drinking water intake, so that only as much drinking water as needed is drawn in to the tank.

    For this reason, there is a sensor in the tank that sends a signal to the control unit if the water drops below the minimum level, which opens a magnetic valve for drinking water intake.

    You can check the water level in your tank in 1 % steps at any time on the user-friendly display of our drinking water control unit. You can also enter the intake volume manually, so that expensive drinking water is not used unnecessarily. The unit automatically switches the drinking water intake off if there is a water shortage. The unit also has integrated dry run protection for the pump.

    The drinking water control unit guarantees smooth operation of the entire plant and is quick and safe to install thanks to its compact design.

  • Why do I need the drinking water feeding module?

    The drinking water feeding module is a completely prefabricated drinking water intake module, which integrates system control, drinking water intake and domestic waterworks. The drinking water is drawn freely into a small, compact, in-house reservoir as needed in accordance with DIN and from there is fed directly to the integrated pressure increase station when there is a rainwater shortage.

    The drinking water feeding module provides very safe monitoring, regulation and control for your entire rainwater harvesting plant. It represents the cutting edge of technology for rainwater harvesting. The advantage of the prefitted module on the one hand is that only so much drinking water is drawn in as is absolutely necessary and, on the other, that it is very safe, easy and cost-effective to install.

  • How often do I need to clean my rainwater harvesting plant?

    Rainwater tanks that have a filter, steady inflow and an overflow siphon only need to be cleaned every 10 to 15 years.
    Otherwise, the layer of sediment on the bottom of the tank, which is very important for the water quality, would be destroyed by frequent cleaning. The sediment layer only “grows” by a few millimetres a year.
    The tank is regularly cleaned of floating matter (e.g. pollen), which forms a thin film on the water surface, thanks to the overflow siphon (three to five times a year when the tank is dimensioned correctly).
    Additionally, tanks that are connected to the sewer network should be protected from harmful influences by an odour trap and a small animal guard.

    How often you need to clean the filter depends mainly on the type of filter and the local conditions.
    Our self-cleaning SUPRA-Filter for example only needs to be checked once or twice a year as a rule.

  • Can tanks be installed underground in spite of raised groundwater level?

    The Cristall underground tanks can be installed up to 50 cm (Columbus 80 cm) above the tank bottom in the groundwater. This requires an earth covering of at least 80 cm.

    Please see the instruction manual for more detailed information.

  • Who installs my rainwater harvesting plant?

    You should task this work to a landscape gardener, a heating and plumbing fitter, or a construction company.

    Our instruction manual should always be followed.

  • Do I need to get permission for my plant?

    You do need to consider some approval procedures and a few legal by acceptance of work and setting up a rainwater harvesting system.

    As the building regulations are state law, but drinking water supply and sewage removal are subject to local self-government, we can only make general statements by acceptance of work and set-up. You will have to contact the local building authorities, water supply and sewage disposal companies personally to find out which actual provisions apply to you.

    In general operating a rainwater harvesting system cannot be forbidden in principle if the plant complies with the relevant provisions (e.g. DIN standards).
    The basis for installing a plant are the standards laid down in DIN 1988 “Drinking Water Conduits” and DIN 1986 “Draining Plants for Buildings and Property”.
    Using a GARANTIA® plant guarantees compliance with the listed provisions.

  • Where can I buy GARANTIA® rainwater tanks and plants?

    We will of course give you a list of retailers in your area on request. You can also send us an e-mail via the “Contact” tab, which we will answer as quickly as possible.

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