Dissolved air flotation (DAF) thickening has been a fairly common process for thickening of municipal sludges since the late 1950’s. This process, however, has been applied most often only to waste activated sludges (WAS) which are difficult to thicken by gravity. One of the principal reasons for this limited focus has been the fact that properly designed and operated primary sedimentation tanks do a good job of thickening primary sludges. The only difficulty experienced with gravity thickening of primary sludge involves the successful removal of the thickened sludge from the sedimentation tanks. The thixotropic characteristic of primary sludges makes operation of primary sludge pumps with negative suction heads or long suction piping difficult, if not impossible.