Abstract: A process and an apparatus is provided for growing biomass from a supply of nutrient in a manner which permits precise control of the biomass growth environment. A reaction vessel includes a media-free compartment and a media compartment which retains biomass support bodies. A screen separates the two compartments. Nutrient is circulated through the media compartment, passed through the screen and is oxygenated and/or re-circulated between said compartments. As shown in FIG. 3, additional pairs of media-free and media compartments can be connected in series to provide a single reaction vessel. Suspended or packed biomass support bodies can be used and can be continuously or intermittently removed for cleaning.