The highest rate of oxidation occurs in the top section of percolating filters, where the limiting factor is usually the amount of oxygen provided by natural ventilation. An investigation was carried out to ascertain whether the loading to a conventional single pass filter could be increased by replacing the surface layer of mineral medium with a 750mm layer of random plastic medium, which has greater surface area and voidage. This would allow greater film accumulation and subsequent removal of organic matter, at the same time avoiding ponding and anaerobic conditions normally associated with excessively loaded single pass mineral filters. A pilot plant was designed and three identical filters construc-ted, one containing 2 m[3] of blast furnace slag and another 2 m[3] of random plastic medium and the third 0.8 m[3] of plastic mediumupon 1.2 m[3] of slag. The comparative treatment efficienciesof the various packings were studied at three different loadings,for three months during maturation at 5.72 m[3] m[-3] d[-1] (0.85 kgBOD m[-3]d[-1]) and then for 12 months at 1.68 m[3]m[-3]d[-1] (0.28 kgBOD m[-3]d[-1]) and a further 12 months at 3.37 (0.63 kg BOD m[-3] d[-1]). The ecology was studied both qualitatively and quantitatively throughout the depth of the filters, during the two longer loading periods. The film accumulation, temperature and retention time were all recorded and directly compared with the biological and chemical results. Medium replacement was shown to be a viable system for upratingfilters, providing the operator with a more versatile filter, less susceptible to ponding, with less variable retention times and capable of treating greater organic loadings thanconventional filters in excess of 0.2 kg BOD m[-3] d[-1]. The cost of the system is dependent upon specific requirements and availability of medium. In the mixed filter the slag portion regulated the loss of animals from the plastic layer, retaining greater numbers of micro- and macro-grazers in the lower mineral portion, resulting in an increase in film control, and lower film accumulation at both the interface and slag portion of the mixed filter.