Detention tanks have a restricted flow and direct the rain water gradually into the sewer system. This relieves the load on the public sewer system. This often brings financial relief in the calculation of the impermeable area and thus in the stormwater fee. Detention systems are frequently required in new residential areas. Both cisterns with total retention volume in which no rainwater harvesting is possible, as well as detention tanks, which have a usable volume of (for instance) 50 % rainwater utilization can be installed. Half of the tank contents can then be used, the other half is discharged at a slower rate.