A preassembled unit for a sewage nitrification field has minimum outward dimensions of loose, lightweight plastic aggregate material enveloped and bound by a plastic sleeve around a horizontal conduit. In one embodiment, the unit takes a bag form with an aggregate-filled netting sleeve concentrically surrounding and fastened to the ends of corrugated plastic vent pipe. In another form, the sleeve is a plastic extrusion of rectangular cross-section and the conduit can be formed integrally with the sleeve. Cross-webbing is added for strength. In ground installation, conduitless units make up insufficiency of aggregate dimension. Mating male and female end caps are used to interconnect the conduit of adjacent units end-to-end. Apparatus for manufacturing the concentric netting sleeve/vent pipe version has a hopper that discharges aggregate into the top of a mandrel and an annular disc or helical auger blade that pushes the aggregate forward into the sleeve.