A recent survey has suggested that up to 25 000 km of UK sewerage networks may be affected by in‐pipe sediment deposits. Field evidence indicates that, typically, up to 90% of the pollution load discharged from storm sewage overflows may be derived from the erosion of accumulated in‐system sediment, commonly referred to as silt. A sewer flow quality simulation model is being developed under the aegis of the WRc/Water Industry Collaborative River Basin Management research programme. Such a model will enable sewerage engineers and water quality planners to produce more effective designs for sewerage rehabilitation schemes to control river pollution. In order to produce this model it is necessary to understand the nature, characteristics and controlling mechanisms of in‐pipe sediment deposits. Field observations, coupled with sampling and analysis of combined sewer sediment deposits, have produced a five category classification for such sediments. Each category has distinctive characteristics in terms of appearance, composition and polluting potential.

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The origins of sediments and associated pollutants found in sewers are clear, and demonstrable temporal and spatial variations have been found. It is likely that these variations are such that it is not possible to formulate general rules for the rates and nature of surface wash-in to sewers. Sanitary sources, however, are more amenable to deterministic assessment. The rates of build-up of sediments in sewers vary widely, and whilst it is possible to estimate the build-up in small-sized collector sewers, no general rules are yet available to predict the rate of deposition in larger sewers. There are broad similarities in the nature of sediments found in sewers in terms of particle size and pollutant characteristics. It is possible to formulate a pragmatic taxonomy for distinct classes of these sediments in order to facilitate understanding of sediment effects and to assist engineers with developing control strategies.

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