A backwash distribution apparatus for a filter having a filter tank and a plurality of underdrain laterals includes a first and second separator device positioned in a common main conduit to define first and second auxiliary flumes. The first and second auxiliary flumes are in fluid communication with the common main conduit and the underdrain laterals. The first separator device has at least one flume liquid metering orifice preferably located near the bottom of the first separator device so that it may be below a gas liquid interface in the first auxiliary flume. The second separator device has either a weir edge or a plurality of metering orifices (or both) for passing backwash liquid from the common main conduit into the second auxiliary flume. The second separator device creates an adequate liquid pool in the common main conduit to prevent short circuiting of gas into a liquid opening for the common main conduit and also to provide low velocity flow of backwash liquid therein, while also providing uninhibited flow of backwash gas. The second separator also controls the distribution of backwash liquid and/or gas inside the common main conduit. A method for gas backwash in the presence of liquid according to the invention is also disclosed.