Contaminated water is chemically treated and aerated while being gravity fed through a trough. A series of slats are positioned in the trough to interrupt the flow and induce mixing and aeration. An array of aeration edges is removably installed over the slats. A holder for containing chemical treatment matter therein is positioned adjacent each slat. The aeration edges consist of upper and lower plates stacked one on top of the other. Each plate is provided with horizontal fingers extending in a direction downstream relative to the flow direction of the treated water. Each finger terminates in a baffle, which baffle is positioned substantially perpendicular to its finger.