Abstract: Apparatus for controlling liquid from a site comprising at least three liquid retention cells which are arranged in series with respect to the flow path of the liquid through the apparatus, the liquid retention cells being arranged in at least one group which comprises at least three of said cells disposed adjacent one another, an inlet for delivering liquid to an upstream cell of the apparatus and an outlet for discharging liquid from a downstream cell of the apparatus to a discharge line, and transfer orifices which provide liquid communication between adjacent cells in the group, the or each transfer orifice being configured so as to control the liquid flow between adjacent cells and create a condition whereby there is a dissipation of energy of the liquid between each cell apparatus.