A loose fill lightweight plastic aggregate for a preassembled drainage line unit. The bead comprises a central portion, at least three legs extending from each of the top and bottom of said central portion, a plurality of channels each defined by adjacent ones of said legs, the width of said legs being greater than the width of said channels. A rib protrides laterally from each of the sides of the central portion. In the drainage unit, the beads maintain a substantial void space for flow of waste water and also provide maximum surface area for bacterial breakdown of sewage wastes.