collects in the channel 142 and flows to the clearwell during filter backwashing however backwash water is pumped into the channel 142 from the clearwell and is evenly distributed upward across the area of the filter media 130 the backwash flow flushes out gases trapped between the particles of sand 132 and gravel 134 and carries these gases out to the atmosphere above the water level 137 138 of the filtrate preferably a freeboard 136 above the lower water level 137 is maintained so that the combined in flows of influent and backwash water do not overflow the weir 116 a preferred freeboard height is from six to eight feet with this preferred method of backwashing any biomass suspended during the gas release remains in the filter 100 as does any additives to the filtrate such as methanol if the backwash flow does cause the maximum water level 138 to rise above the distribution weir 116 the surcharge will enter the influent piping and be directed to other filters 100 so as not to be lost from the system in a preferred method of degassing biological filters 100 in a water treatment system a plurality of downflow filters 100 are selected.