Abstract: A wastewater treatment installation includes at least two tank units having portions located in juxtaposed relation, the units being secured in such position. An inlet to the first tank unit communicates with a drop pipe connecting the inlet to a liquid delivery structure intermediate the top and bottom of the first tank unit, and located diametrically opposite the respective portion. The liquid delivery structure ejects liquid tangentially in opposite directions along the side wall of the first tank unit. An outlet from the second tank unit (or the last of a series of units) is located opposite the respective portion, and an outlet riser pipe connects the outlet to an intake opening, which is located intermediate the top and bottom of the second tank unit.