A rotating drum filter assembly for thickening lime sludge includes a liquid jet distributor for directing liquid jets against a strip of lime sludge filter cake on the ascending side of the drum. The distributor is positioned on the ascending side of the drum and within 45.degree. of a vertical plane tangential to the drum peripheral surface. The liquid jets are directed so as to disperse the strip in a manner such that substantially all of the lime sludge comprising the strip remains on the drum. Cake wash water formed by water distributed from shower heads immediately refills the cleaned strip thereby minimizing loss of vacuum within the hollow drum interior.