Regenton © Martha de Jong-Lantink

Rainwater usage


Using precipitation for household or lower-grade purposes can greatly help efforts to optimise the urban water balance. In doing this, the load on the combined sewers and treatment systems is reduced, the number of overflows is diminished and the amount of drinking water consumed is lowered. Limiting drinking water use is especially important in times of drought. For these reasons, use of precipitation is recommended in many countries, such as Germany, and in some cases is even mandatory, such as in Belgium. Because the use of rainwater is mandatory or encouraged, products are available in the market in those countries that enable rainwater to be used profitably, and the installation sector has become familiar with these technologies. Do-it-yourself chain stores offer complete systems for self-installation, including manuals.

Using precipitation is a simple, tried and true method. The system required can easily be integrated in new constructions, and the impact on the volume of drinking water saved and the reduction in the volume of wastewater is significant.

Rainwater can be used in buildings for flushing toilets and in towns and cities for irrigation, cleaning and other low-grade purposes for which drinking water would otherwise be used. Precipitation can also be used as process water in industry and for fire-fighting, offering an alternative to using drinking water.

Use rainwater

Use rainwater © atelier GROENBLAUW

Rainwater tank

Rainwater tank © atelier GROENBLAUW

The most efficient use of precipitation is in new constructions. In existing situations, it is often a complicated and therefore expensive undertaking to realise a second water grid and the reservoir required. It is important that the dimensions of systems for using precipitation find a balance between supply, retention and consumption. Constant use, as that of households, makes the system more efficient to use. Rainwater systems affect not only the consumption of household drinking water but also the urban water system. Rainwater buffers that have an electronic interface with weather forecasts and are emptied before heavy rainfall have a positive impact on the urban water system, as they increase the town or city’s retention capacity. These types of systems are already being used and tested in Singapore.

Quality of precipitation

The quality of rainwater is determined largely by the surface from which the water runs off. As a rule, the runoff from roofs is of a higher quality than rainwater from roads.

Although the quality of rainwater running off from roofs is generally good, flat roofs are more polluted than sloping roofs, since dust and leaves have a greater tendency to settle there.

Precipitation running off from green roofs or bituminous roofs sometimes carries a slight colour or odour. Roofing materials themselves can also become a source of pollution. Roofing materials with no or very little emissions include glass roofs, roofs with fired tiles, roofs with concrete tiles and roofs with an HDPE film (synthetic rubber). Rainwater running off from roofs contains little bacterial pollution or nutrition, and for purposes of hygiene presents no problems when used for flushing toilets. Retaining the water in concrete reservoirs neutralises the influence of acid rain, making it suitable for watering plants. Rainwater from copper roofs is not suitable for watering plants. Rainwater is also suitable for use in water art.

Precipitation running off from paved areas is more polluted than runoff from roofs. The various pollutants, for example PAHs (Polycyclic Aromatic Hydrocarbons) and heavy metals, render precipitation from roads unsuitable if it does not undergo an additional treatment stage. As such, the focus here is on the use of precipitation running off from roofs.

Rainwater systems are technically simple and reliable in their operation. In the correct dimensions, and if used for new construction, they are highly effective.