Abstract: The upflow filter assembly includes a primary chamber situated above a secondary chamber within an enclosure and having a filter element spanning the primary chamber towards the upper end thereof. Airlocks are situated near the dividing floor of the two chambers so that when in the liquid treating or filtering mode, they operate to maintain the air/liquid interface between the chambers and prevent liquid from passing into the secondary chamber assisted by compressed air introduced into the secondary chamber. Liquid containing solids and/or flocculents, is fed into the primary chamber adjacent the lower side thereof and gradually fills the chamber with much of the flocculents and solids settling out by gravity. The partially deflocculated liquid then passes up through the filter element to a height above the filter element with excess, relatively clear effluent flowing out through an outlet spaced above the filter element to provide a hydraulic head above the filter element.