A method and apparatus for controlling water flow in a wastewater treatment plant is provided. The method comprises the steps of providing a trickling filter effluent water basin having filter media provided over a storage space for water. Water outlet flow from the basin is controlled via a flow weir or other suitable structure, thereby enabling the trickling filter effluent water basin to fill with water and store water when desired. Such storage is particularly useful during wet weather and/or overflow events. A trickling filter effluent basin modified so as to accommodate water storage and methods of modifying existing trickling filter effluent basins are also provided.