A wastewater treatment system and process utilizing a trickling filter with a rotary distributor extending thereover for the distribution of wastewater over the filtering medium. A sealed insulating dome overlays the filter to provide a closed environment. An underdrain system extends underneath the filtering medium covering the floor of the filter and includes perforated sections into which the applied wastewater flows. The perforated sections extend into an underdrain gallery which collects the applied wastewater to be recycled by the pump through the distributor or expended out of the filter. Controllable air recirculation system for selectively moving air under the dome through the filtering medium includes a blower in communication with the gallery which directs air through the gallery from under the dome, through discharge conduits and into the filtering medium. The direction of air flow is reversible and the introduction of atmospheric air is provided after passing through a controllable heater. The rate of introduction of atmospheric air is controllable in response to a decreased dissolved oxygen level in the filter effluent and a vent discharges existing air within the system.