Frequently asked questions - Rainwater harvesting

An underground tank is ideal for storing rainwater because the soil protects the tank from light and heat. The temperature of water from the underground tank only reaches 6 to 8° C all year round. Algae and microorganisms cannot form due to the darkness and temperature, ensuring consistent water quality.

You should also ensure that an above-ground rainwater tank is as opaque as possible. To avoid the water warming up too much, it should installed out of direct sunlight.

The size of a rainwater tank is basically determined by three factors:

1. Local rainfall

2. Rain catchment area

3. Demand for rainwater

You can use the GRAF product advisor to work out the ideal tank size.

The rainwater contains no lime whatsoever, which offers major benefits over tap water which is usually harder.Using soft rainwater requires less detergent - softeners and descalers become a thing of the past.Washing results are improved overall and you help protect the environment. Most germs enter your washing machine via your dirty laundry and not the rainwater. Some washing machine manufacturers even recommend using rainwater for the best results.

Rainwater tanks with a filter fitted upstream and which have a steady inflow and overflow siphon should be cleaned every 10 to 15 years. A layer of sediment forms on the base of the rainwater tank over time. This is very important for the water quality. Cleaning too often would destroy it.

Overflowing the rainwater tank via the overflow siphon regularly cleans the rainwater tank of floating matter (e.g. pollen).

An odour trap and rodent guard should also be fitted on rainwater tanks which are connected to the sewage system to protect them.

The frequency at which you need to clean the filters depends on the filter types and local circumstances. The self-cleaning GRAF Optimax-Pro Filter usually only has to be checked once or twice a year.

In principle building regulations are covered by federal state law - the supply of drinking water and disposal of wastewater are subject to communal self-administration. We are therefore only able to provide general information about construction and commissioning. At the end of the day, you need to contact your local building and water authorities for the specific regulations of relevance to you.

The standards laid down in DIN 1988 "Drinking water supply systems" and DIN 1986 "Drainage systems on private ground" form the basis for installing a system.

As a rule, construction and operation of a rainwater harvesting system cannot be prevented if the system conforms to the relevant specifications (e.g. DIN standards). When you buy a GRAF system, you can be sure of compliance with the relevant requirements.

If you want to use rainwater in the house (e.g. for flushing toilets and in your washing machine), you will need to apply to your water supplier for partial exemption from the water supply.

Underground and cellar tanks have to be fitted with an overflow through which the water can overflow into the sewage system or an infiltration system.

If connected to the sewage system, the overflow should be above the system's backed-up water level. If necessary, a backpressure flap should be installed. Above-ground tanks should always be connected to a downpipe filter. This prevents the water from flowing in when the rainwater tank is full and instead it flows through the downpipe into the sewer.

If you want to use your rainwater to flush toilets and in your washing machine, the underground tank and filter and any pipes carrying water which are laid in the ground must be installed 60 to 100 cm below ground level to protect against frost. Please contact your building authority for the local frost-proof limit! Underground tanks with frost protection and cellar tanks do not have to be emptied in the winter.

Containers installed above ground always have to be emptied in the winter.

These measures will prepare your rainwater butts for winter!

When combined with a cast or concrete cover, many GRAF underground tanks are suitable for vehicle or HGV loading. There are therefore virtually no restrictions on how the ground above them is used.

Many ministries, towns and cities, states and communities have recognised the need for a sustainable approach to water and are encouraging the use, infiltration or retention of rainwater.

GRAF rainwater harvesting systems use a multi-stage cleaning process to ensure a consistently high water quality:

1st cleaning stage

First the rainwater is cleaned by a patented filter system. The cleaned water flows into the tank and dirt particles are rinsed into the sewage system along with a small amount of residual water. GRAF provides various filter systems for every installation situation.

2nd cleaning stage

To allow very fine dirt particles (< 0.35 mm) to settle, the filtered water is routed through a calmed inlet on the base of the tank. This prevents the water from being stirred up continually and also adds oxygen to the water at the bottom of the tank. This keeps the water clear and fresh.

3rd cleaning stage

Particles that are lighter than water (such as pollen) form a floating layer on the surface. The floating layer is removed through the overflow siphon installed in the tank when the tank overflows. So it's important to let the tank overflow regularly to ensure consistent water quality.

4th cleaning stage

The collected rainwater is removed by means of a floating extraction unit near the top of the tank. This is where the best water quality can be found (around 10 cm below the surface).