A buffer system for rainwater/water comprising a water tank T for collecting rainwater. The tank has one or more outer walls 1 made from non-porous concrete which define an inner volume 2 for storing water. The tank has an inlet 4, an outlet 5, a lid 6 and a manhole 7. The tank also has a control system 5, 8 comprising at least one porous concrete vertical inner 8 wall extending into the inner volume, wherein the vertical inner wall splits the inner volume into a storage chamber 3 and a buffer space 9 having a buffer volume to collect water flowing through the porous concrete vertical inner wall. The porous vertical inner wall is made from porous water draining concrete that is manufactured from a mixture of cement, aggregates having a size range of 6-14mm and water to obtain an open pore volume of 8-12% in the hardened porous water draining concrete and which has a permeability of 0.05 to 5litres/m2/s. The permeability is measured with an almost completely water filled storage chamber and empty buffer space. A later embodiment relates to the use of said buffer systems.
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