Method and apparatus in which a biomass is carried on a substrate that is fluidised. The fluidised substrate has a layer of distributor particles below it, through which the fluidizing medium passes prior to contact with the fluidised substrate and biomass associated therewith. The effect of the distributor layer is that the particles damp out excessive turbulence in the fluidizing medium, thus preventing undue turbulence of the medium within the fluidised layer. The particles may be recycled through the distributor layer to strip excess biofilm from the substrate particles prior to returning the substrate particles to the fluidised bed.