The influence of near bed solids transport on first foul flush in combined sewers

The problems associated with deposited sediments in sewers, and their transport through sewer systems have been the subject of detailed fieldwork programmes in the UK, and elsewhere in Europe. Existing laboratory, and some field based research exercises have focused on the relatively small, discrete particles. It is clear, however, that combined sewer systems have inputs which comprise of a significant proportion of large organic solids (faecal and food wastes), as well as the finer range of particle sizes. The increased concern regarding CSO spills into the environment has fuelled the recent development of sewer flow quality models, such as HYDROWORKS QM and MOUSETRAP, some of which make no attempt to represent the transport of these larger organic particles. Herein, the results of a collaborative research programme undertaken between three UK universities and a water authority are discussed. Transport at the bed in sewers, as "near bed solids", is defined. Based on a comprehensive data collection program undertaken in the Dundee combined sewerage system, a method is presented which may be used to estimate the rate of sediment transport near the bed in sewers. The influence that solids in transport near the bed have on first foul flush in combined sewers is discussed. A methodology is proposed which may be used to estimate the extent to which sediment in transport near the bed in sewers contributes to first. foul flush phenomena, by describing the movement of a storm wave along a conceptual sewer length. (C) 1998 IAWQ. Published by Elsevier Science Ltd.