INTERNATIONAL WORKSHOP ON ORIGIN, OCCURRENCE AND BEHAVIOR OF SEDIMENTS IN SEWER SYSTEMS - SUMMARY OF CONCLUSIONS

An international workshop held at the Universite Libre de Bruxelles (4-6 Sept. 1991) on the ''origin, occurrence and behaviour of sediments in sewer systems'' has shown that much progress has been made in the last 5 years in the general understanding of sewer sediments. Comprehensive research programmes, including field surveys, have indicated that solids found in sewers, either in suspension or in deposition, cannot be considered as having a unique entity. On the basis of sewerage and sources, a categorization can be made between three broad types of solids: sewer grits, sanitary solids and (runoff-generated) fine material. Although some other categorizations are also possible, it is apparent that each of these three types has very specific physico-chemical characteristics and this has important implications both from the environmental and the operational points of view. The diversity of the types of sewer solids is not infinite (from place to place, sediments will differ only within relatively narrow bands), nonetheless the differences are the main reason why the development of a ''universal'' sewer sediment transport model is a remote prospect. For some elementary processes, such as the movement of non-cohesive grit in drainage conduits, detailed engineering solutions are beginning to emerge. It is also apparent that bed shear stress is a more appropriate parameter than flow velocity for defining erosive or depositional criteria in sewers. There is no doubt that in the future, the various problems of sewer sediments, washout of pollutants during storm overflows, reduction of hydraulic capacity, recurrent costs for cleaning sewerage lines and tanks, will have to be considered intrinsically in all urban drainage applications (design and analysis). It is also likely that wastewater treatment technology (and terminology) will intrude more and more in the apparent unchanging traditions of sewer network operation.