Abstract: A fluidized bed waste water treatment reactor which includes a reactor tank containing a particulate media, an array of perforated distribution pipes disposed in a horizontal, parallel array adjacent the bottom of the reactor tank for introducing fluidizing liquid substantially uniformly over the cross-sectional area of the reactor tank. The arrangement is characterised in that an inlet end of each of the distribution pipes extends externally of the reactor tank and the distribution pipes are connected together by a header tank positioned externally of the reactor tank. This arrangement allows access externally of the reactor tank to individual distribution pipes which allows blockages of the pipes to be readily cleared.