Abstract: A method of degassing biological filters utilizing a backwash system wherein the steps of opening and closing valves are reduced, flow interruption through the filter is minimized, and the process of draining down a downflow filter is eliminated. A preferred method of degassing a plurality of downflow biological filters in a water treatment system comprises the steps of maintaining the influent flow throughout the degassing process, ceasing the effluent flow out of a filter selected to be degassed to prevent draining of the selected filter, introducing a backwash water flow into the bottom, and flushing of the filter until entrapped gases are released out of the filter. The backwash flow is then ceased and the effluent flow is recommenced to return the filter to service. The steps are successively repeated and the backwash pump remains on until all filters are degassed.