These systems come mainly in the basic S.F.I.A design of individual deep layer bins. Due to the large volumes of water which is re-circulated, more efficient stacking systems which are being used all over Europe are gaining in popularity and will replace the older space depriving systems. Stack systems are usually based on a stack of 3 bins containing a total of 1 tonne of bi-valves on an area of approx. 1.25 sq metres. The water cascades through the bins and is much more energy efficient in that only half the volume required by the old system is used.