During dry weather, organic and inorganic sediments increasingly settle on the sewer base in combined sewer systems due to slow flow velocities. Under stormwater conditions, these sediments float up and arrive along with the sewer flow at the sewage treatment plant or stormwater overflow structure.
If the combined sewer volume exceeds the capacity of the overflow structure or sewage treatment plant, the combined water is frequently discharged to a receiving water course, impairing water quality with the result of increased costs for maintenance and cleaning. As a preventive measure, coarse material retention systems are installed to retain unsightly coarse matter. DWA, the German Water, Wastewater and Waste Association, recommends in its Guideline A 166 the use of screening plants to retain coarse material. Scum boards are considered as unsuitable for this purpose. This position corresponds with the experience made in practice. Even though scum boards are able to retain the majority of the floating solids, the suspended matter can pass unhindered.