An Experimental Approach to the Efficiency of a First-Flush Tank

First flush tanks are supposed to be useful to control the impact of the pollutants contained in combined sewer overflows over the receiving waters. Their design is based upon the idea that the first water in a storm event contains most of pollution caused by the cleaning of the catchment and the conduits. The design of these structures is often controlled by dilution rules which give retention volumes. These rules do not take into account the hydraulic behaviour of the structure itself, which is not well known so far, as these tanks include some hydraulic phenomena which are difficult to evaluate and even to measure. An experimental facility has been developed in the CITEEC Hydraulics Laboratory in A Coruña University in order to measure how the hyetographs and the pollutographs vary at the inlet and at the outlets of the structure. Discharge, accumulated volume and pollutant taxes have been measured in unsteady conditions for different specific volumes (m3/Ha) in the structure. The removal of the pollutant mass has been evaluated, as well as the mixing phenomena inside the tank. The experimental facility consists of a scale model of a first-flush tank similar to those used by the Northern Water Authority in Spain. The hydrographs and the pollutographs have been obtained by direct field measurements in a combined catchment in Santiago de Compostela (Spain). A colorimetric tracer has been used to simulate pollution (neither settling nor reaction has been measured) and different volumes of the structure have been simulated by varying the discharge scales, according with a Freudian model. The result consists of a series of graphs which show the amount of pollutant removed as a function of the tank volume, for some first-flush hydrograph-pollutograph peak distance, and a discussion of the actual efficiency in the removal of pollution in the structures installed in Spain and world-wide.