1. Perforated pipe drain installed in crushed stone under a slab to intercept ground water and drain it away from the structure. 2. The distribution system at the bottom of a sand filter to collect the filtered water during a filter run and to distribute the backwash water during backwash.
Also call filter laterals or lower collection system - Slotted, finger-like tubes that are attached to a sand filter manifold. The slots are on the bottom side to prevent the sand from passing through. Water comes into the filter tank, through the sand, into the underdrain, and then back to the pool.