At the bottom of the filter bed is the under-drainage system. It consists of porous or perforated pipes. These pipes provide an outlet for filtered water and support the filter medium. Once the filter bed has been laid, the under-drainage system will not be visible.
Filter box: The first 3 elements (e.g. supernatant water, sand bed and under-drainage system) are contained in the filter box. It is a rectangular open box, 2.5 to 4 metres deep and built partly or wholly below the ground. The walls may be made of stone, brick or cement.