Abstract: An apparatus for treating fluid provides a main flow path for fluid. The apparatus includes a plurality of pairs of basins arranged along the main flow path which define a series of enlargements and constrictions. Each pair of basins is constituted by a basin on each side of the main flow path opposite one another, both basins being in fluid communication with the main flow path. The walls and floors of the basins may be flat or may be curved or have some other configuration. The size of the pairs of basins may vary from one end of the flow path to the other. In operation, liquid flowing through the apparatus executes an oscillatory motion about a median longitudinal axis defined by the flow path.