Among the sustainable water management strategies, water saving has prime priority. In the German town of Giessen, local authorities promote water saving through the domestic use of rainwater. Instead of flowing into the sewer, rainwater replaces drinking water in several domestic operations such as WC, washing of clothes, house cleaning and watering with considerable saving effects:
Possibilities of domestic rainwater use with corresponding drinking water saving 
- WC: 8.000 lt water saving per year and person
- Washing of clothes: 6.000 lt water saving per year and person
- House cleaning: 2.000 lt water saving per year and person
- Watering: 6.000 lt water saving per year and 100 m2
A household can replace more than half of the drinking water with rainwater. In addition, pressure on the drainage system and the biological wastewater treatment is reduced.
In Giessen, citizens are encouraged to collect rainwater on the roof and use modern storage tanks for storing rainwater. Storage tanks can be installed in earth or in the basement of a building. A system of pumps brings water into the house. Plant price and installation costs amount to 4.450-5.250 €.
Advantages of rainwater storage tanks are:
* simple installation
* saving of drinking water
* retention of intensive rainfalls
* acceptable construction and operation expenses, low maintenance expenses
* low power consumption
* good water quality
* high operation-security guarantee
* long life
* positive ecological and economical balance
* environmentally friendly materials
* complete separation of rain- and drinking water flow
* economy per m3 of used rainwater (2006 values):
- water bill: 2,20 € (incl. VAT 7%)
- sewer bill: 1,79 € per m3 waste water.
In the urban sustainable development, energy saving should go along with water saving. After a century-gathered experience on the Greek islands, it is time for the urban use of the rainwater saving technology in its modern version. Local authorities should promote this and other water saving strategies, since protection and management of water resources is their formal responsibility.
3. Law 3463 «Ratification of Municipalities and Communities ». FEK 114, issue Α’, 8/6/2006