Abstract: A water treatment tank for separating particles from water, has a separation chamber with at least one set of inclined parallel plates, with a slot between each pair of adjacent plates for substantial horizontal flow of water between the plates. A sludge chamber is located beneath the separation chamber for collecting sludge including the particles separated from the water during passage of the water through the separation chamber. An inlet chamber on one side of the separation chamber introduces the substantially horizontal flow of water into the separation chamber, and an outlet chamber on an opposite side of the separation chamber receives the flow in the substantially horizontal direction from the separation chamber. The outlet chamber is located above and communicates with the sludge chamber and further separates particles from the water in the outlet chamber, the particles descending into the sludge chamber.