The GRAF Retention cistern combines rain retention with rainwater harvesting.

To guarantee retention of rainwater, a delayed drain is fitted in the channel with 0.05 to 2.0 l/sec. depending on the customer’s specifications and the retention or harvest volumes. In the event of heavy rainfall, the water level in the cistern rises above the max. water level (short term) and is drained into the channel at delayed speed using a floating drain choke.
This relieves the channel and the treatment works.
The water level can rise to the emergency overflow and sinks slowly back down again after the rain to the long-term max. water level. This rainwater can then be harvested.

We also construct and supply individual, special solutions.
Sizes between 3,750 – 100,000 l with corresponding retention volumes are possible.

This special type of rainwater harvesting is prescribed or subsidised in many communities. 

  • For rainwater retention
  • For rainwater harvesting in the house and garden
  • For frost-proof installation in the ground
  • The entire tank volume can be used for rainwater retention
  • "USA Patent 7854338"


Volume [l]Length [mm]Width [mm]Height [mm]Weight [kg]order no.add to enquiry

calculated size Request for information

Technical details


max. earth covering [mm] (without groundwater, vehicle loading) 2000


Earth covering with loading capacity [mm] 800 - 2000
max. axle load [t] 8
max. vehicle weight [t] 12
max. vehicle weight with reinforced concrete slab [t] 60

Groundwater installation

Earth covering with groundwater installation [mm] 800 - 1800