Abstract: Granules of plastics material have grains of an inert mineral such as sand coated thereon to provide a habitat for microorganisms effective in waste water treatment, the granules having a predetermined particle size range, and the grains having a predetermined particle size range and being disposed at a predetermined packing density range on the granules. The particles have an average density of approximately 1.0 g/cc such that a proportion tend to float and a proportion tend to sink in the waste water. The particles arc manufactured by contacting the granules of plastics material with a mixture of grains of the inert mineral and grains of a soluble substance such as salt, at an elevated temperature, to coat the granules with the mixture, and subsequently dissolving the soluble substance grains from the coating to provide the granules coated with the grains of inert mineral in the predetermined packing density range.

Abstract: Particulate material for use in waste water treatment. The material comprises granules of a plastic material (e.g. poly-ethylene), each of which contains a plurality of grains of a substantially inert mineral (e.g. sand) coated thereon to provide a habitat for microorganisms effective in waste water treatment. The granules will have a predetermined particle size which is governed by the particle size range of the granules, e.g 3-10 mm and the grains will have a predetermined particle size range, e.g. 0.1-3.5 mm.