Abstract: The treatment system is for use with septic tank effluent. Water is passed over polyurethane foam blocks, arranged in a heap so that air can circulate freely around the blocks. The size of the foam cells is small enough that water soaks into the blocks and is retained therein. The foam is soft and easily deformed. At the points of block-to-block engagement, the blocks deflect and conform to each other, giving rise to a large contact area through which water can pass between blocks.