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 . an outlet channel is located in an upper portion of the outlet chamber and has an outlet weir on each side of the outlet channel . the outlet channel and outlet weirs extend for the length of the separation chamber , and a surface layer of water in the outlet chamber overflows the outlet weirs into the outlet channel to create an outflow . an overflow rate of water per unit length of the outlet weirs is minimized for a given tank flow rate .