Abstract: A transportable self-contained sewage treatment plant includes an outer shell containing a plurality of adjacent chambers having pipes or lines providing a serial flow of fluid therebetween. An aeration chamber delivers biologically treated wastewater to a clarifier chamber which returns both heavy as well as lighter, floating particles to the aeration chamber while passing the remaining wastewater to a filter chamber for passage through a filter bed. The resultant treated fluid next passes into a backwash chamber before discharging as clean water from the plant. The filter bed in the filter chamber is periodically backflushed by applying a pressure head upon the clean water in the backwash chamber to direct the water in a counterflow manner through the filter bed while at the same time this pressure head closes check valves both to preclude discharge of clean water from the plant and to direct backflushed water to by-pass the clarifier chamber and enter the aeration chamber.