Abstract: A method for manufacturing preassembled drainage line units comprises feeding a length of perforated vent pipe along a direction of manufacture longitudinally through a tubular mandrel, feeding a continuous sleeve of netting concentrically over the vent pipe along the direction of manufacture from a front end of the mandrel and continuously supplying quantities of artificial aggregate into the space between the sleeve of netting and the vent pipe. A low friction coating is applied to the inside surface of the mandrel to reduce the surface friction of the artificial aggregate caused by compaction, and to permit high speed manufacture.

Abstract: A method for manufacturing preassembled drainage line units comprises feeding a length of perforated vent pipe along a direction of manufacture longitudinally through a tubular mandrel, feeding a continuous sleeve of netting concentrically over the vent pipe along the direction of manufacture from a front end of the mandrel and continuously supplying quantities of artificial aggregate into the space between the sleeve of netting and the vent pipe. A low friction coating is applied to the inside surface of the mandrel to reduce the surface friction of the artificial aggregate caused by compaction, and to permit high speed manufacture.

Abstract: Preassembled drainage line units for a septic tank nitrification field employ a predetermined length of perforated conduit having an aggregation of discrete, crush resistant, light-weight elements surrounding the length of conduit and a perforated sleeve member enveloping the aggregation and bounding the same relative to the conduit with the ends of the conduit extending beyond the sleeve. The drainage line unit is provided with connecting means at each extending end, for end-to-end interconnection with an adjacent drainage line unit.