A backwash system and method for a waste treatment system employ a waste inlet duct extending into an accumulation of microbial growth media particles in a lower region of a tank, with a vertical duct having a duct outlet in an upper region of the tank. A downwardly convergent strainer extends around the vertical duct beneath the duct outlet, with a liquid collector beneath the liquid leakage openings and a liquid outlet from the liquid collector communicating with the exterior of the tank. Compressed air is supplied into the vertical duct to impel the waste water up to the duct outlet and onto the strainer. A solids return duct has an upper end opening through the downwardly convergent strainer and a lower end opening above the accumulation of microbial growth media particles.