Solids transport by flushing of combined sewers

For several reasons field observations of solids transport processes in combined sewage flows are very problematic. To avoid some of these difficulties a case study in a real sewer under controlled conditions was carried out to examine solids movement at high flow rates by means of flushing experiments. The investigations illustrate the complexity of solids transport. Corresponding behaviour of the bed deposits and the suspended solids was recognized. An abrupt increase of the SS load was caused entirely by volatile solids. Additional factors influencing the transport processes, such as the prehistory of the event are demonstrated. With regard to the question if the procedure of flush cleaning may be used as an alternative measure to the cost intensive high pressure jetting it has to be stressed that complete cleaning of sewer lengths with sediment beds cannot be expected by flushing. However, keeping sewers clean over prolonged periods by regular flushing may be an economic way of sewer operation.