a method and apparatus for retrofitting existing waste water treatment facilities having at least one existing basin including installing generally vertical partitions at spaced locations in the existing basin in order to divide the existing basin into a plurality of treatment stage regions , installing at least one air lift in each of the plurality of treatment stage regions , loading each treatment stage regions with a quantity of floatable porous particles , supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water , but generally not the particles , to flow from the plurality of treatment stage regions to at least another of the plurality of treatment stage regions and operating the air lift in each of the plurality of treatment stage regions to provide aerobic waste water flow therein in operative engagement with the floatable porous particles . a biomass support including a plastic biomass support element having a maximum dimension which does not exceed 50 mm and having a specific gravity of between approximately 0 . 70 - 0 . 91 , a method of manufacture of a biomass support and a waste water treatment system employing the biomass support are also disclosed .