Described is an apparatus for the removal of organic substances and/or nitrogen sources from an aqueous medium by exposing the aqueous medium to the action of microorganisms supported on a gas permeable, water impermeable layer, or on and/or in a three dimensional microbial support layer, that is proximate a gas delivery layer through which gas, preferably, oxygen-containing gas, is supplied to the gas permeable layer such that it diffuses through the gas permeable layer and is supplied to the microorganisms.