Underground in plastic or concrete, starting from 2000 to 20,000 litres of possible, but for a normal household is a sufficient stock of 3 to 5000 litres.
Before the rain water collects in the tank, the purified so that clean water is stored and can be used.
You can use it for toilet flushing, washing machine, and the garden.
Rain water use goes fully automatically; through a separate manual, flushes the toilet or does the washing machine once there is rain water is needed. The pump will for several seconds and fills the reservoir or machine. It is clear and clean water and contains no lime, leaving ookminder detergent necessary.
If the tank is almost empty, by long drought, then it automatically suplleert 10%, so that one always have sufficient water.