Abstract: Apparatus for and method of cultivating micro-organisms.The apparatus includes a tank 10 in which the cultivation medium flows, for example sewage or trade effluent to be treated. Within the tank are hollow plates 11 into which flows an oxygenating gas. The plates have walls of gas-permeable material to the outer face of which the micro organisms adhere. Air or oxygen within the plates pass through the wall and through the layer of micro organisms, to provide an aerobic biomass 20 (FIG. 3). This biomass is thicker or more active than the aerobic layer 21 of apparatus where the biomass is acted upon only from the outer faces such as in fuller beds (FIG. 1) or rotary discs (FIG. 2).Tubes or troughs may be used instead of plates; the tubes may be upright or horizontal with the air or oxygen passing therethrough. The troughs may be open topped.The plate, tube or trough consists of a frame 14, a net 15 mounted on the frame and supporting the gas-permeable walls, which may be of micro porous material 16.