The Erosion and Movement of Sediments and Associated Pollutants in Combined Sewers

The complex and inhomogeneous nature of sediments in sewers, and the variability of the particles and dissolved substances flowing in dry and wet weather combined sewage, make the prediction of sediment erosion and pollutant release a difficult proposition. It is apparent that the erosion of sediments in sewers can release pollutants in concentrations which exceed by many times the levels found in the various contributing sources of the sediments and pollutants, and whilst this release is normally in the form of an initial highly polluting foul flush at the start of wet weather flow, the occurrence of foul flushes has not been found to be ubiquitous, even from event to event in the same sewer. The origins of foul flushes may be attributable to the speedy erosion of a weak layer of highly concentrated surficial sediments (or bed-load) at the start of wet weather flows. Various models have been proposed, and some successfully applied to field data, to simulate the erosion and movement of sediments and associated pollutants in combined sewer systems.

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Origins and characteristics of urban wet weather pollution in combined sewer systems: the experimental urban catchment “Le Marais” in Paris

Water Science & Technology ◽

10.2166/wst.1998.0010 ◽

1998 ◽

Vol 37 (1) ◽

pp. 35-43 ◽

Cited By ~ 13

Author(s):

Marie-Christine Gromaire-Mertz ◽

Ghassan Chebbo ◽

Mohamed Saad

Keyword(s):

Waste Water ◽

Sewer System ◽

Urban Catchment ◽

Rainfall Events ◽

Sources Of Pollution ◽

Sewer Systems ◽

Combined Sewer ◽

Wet Weather ◽

Wet Weather Flow ◽

Different Sources

An experimental urban catchment has been created in the centre of Paris, in order to obtain a description of the pollution of urban wet weather flows at different levels of the combined sewer system, and to estimate the contribution of runoff, waste water and sewer sediments to this pollution. Twenty-two rainfall events were studied from May to October 1996. Dry weather flow was monitored for one week. Roof, street and yard runoff, total flow at the catchment outlet and waste water were analysed for SS, VSS, COD and BOD5, on both total and dissolved fraction. Results show an evolution in the characteristics of wet weather flow from up to downstream: concentrations increase from the catchment entry to the outlet, as well as the proportion of particle-bound pollutants and the part of organic matter. A first evaluation of the different sources of pollution establishes that a major part of wet weather flow pollution originates from inside the combined sewer, probably through erosion of sewer sediments.

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Separate and combined sewer systems: a long-term modelling approach

Water Science & Technology ◽

10.2166/wst.2009.376 ◽

2009 ◽

Vol 60 (3) ◽

pp. 555-565 ◽

Cited By ~ 32

Author(s):

Giorgio Mannina ◽

Gaspare Viviani

Keyword(s):

Cost Benefit Analysis ◽

Treatment Plant ◽

Cost Benefit ◽

Sewer System ◽

Sewer Systems ◽

System Versus ◽

Combined Sewer ◽

Wet Weather ◽

Wet Weather Flow

Sewer systems convey mostly dry weather flow, coming from domestic and industrial sanitary sewage as well as infiltration flow, and stormwater due to meteoric precipitations. Traditionally, in urban drainage two types of sewer systems are adopted: separate and combined sewers. The former convey dry and wet weather flow separately into two different networks, while the latter convey dry and wet weather flow together. Which is the best solution in terms of cost-benefit analysis still remains a controversial subject. The present study was aimed at comparing the pollution loads discharged to receiving bodies by Wastewater Treatment Plant (WWTP) and Combined Sewer Overflow (CSO) for different kinds of sewer systems (combined and separate). To accomplish this objective, a comparison between the two systems was carried out using results from simulations of catchments characterised by different dimensions, population densities and water supply rate. The analysis was based on a parsimonious mathematical model able to simulate the sewer system as well as the WWTP during both dry and wet weather. The rain series employed for the simulations was six years long. Several pollutants, both dissolved and particulate, were modelled. The results confirmed the uncertainties in the choice of one system versus the other, emphasising the concept that case-by-case solutions have to be undertaken. Further, the compared systems showed different responses in terms of effectiveness in reducing the discharged mass to the RWB in relation to the particular pollutant taken into account.

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Technical solutions envisaged in managing solids in combined sewer networks

Water Science & Technology ◽

10.2166/wst.1996.0220 ◽

1996 ◽

Vol 33 (9) ◽

pp. 237-244 ◽

Cited By ~ 5

Author(s):

Ghassan Chebbo ◽

Dominique Laplace ◽

André Bachoc ◽

Yves Sanchez ◽

Benoit Le Guennec

Keyword(s):

Large Scale ◽

Suspended Solids ◽

Drainage Area ◽

Bed Load ◽

Urban Drainage ◽

Combined Sewer ◽

Wet Weather ◽

Technical Solutions ◽

Dry Weather Flow ◽

Sewer Networks

Solids in combined sewer networks represent two important technical questions: - the clogging of man-entry sewers, and - pollution in urban wet weather discharges, whose main vectors are generally suspended solids. In this paper, we shall present first, curative technical solutions which avoid or remove deposits in man-entry sewers. We shall discuss the partial extraction of the largest solids; selective trapping of bed load solids, which form deposits; and the displacement of deposits using dry weather flow flushing waves. We shall then examine technical solutions to control pollution in urban wet weather discharges. This will show that decantation is an efficient means of fighting pollution. However, it is not always feasible because it involves large scale investments. Complementary methods should, therefore, be developed and used at different points in the water's passage through an urban drainage area.

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Sediment transport under dry weather conditions in a small sewer system

Water Science & Technology ◽

10.2166/wst.1998.0038 ◽

1998 ◽

Vol 37 (1) ◽

pp. 155-162

Author(s):

Flemming Schlütter ◽

Kjeld Schaarup-Jensen

Keyword(s):

Sediment Transport ◽

Field Data ◽

Field Measurements ◽

Weather Conditions ◽

Sewer System ◽

Transport Models ◽

Sewer Systems ◽

Combined Sewer ◽

Initial Results

Increased knowledge of the processes which govern the transport of solids in sewers is necessary in order to develop more reliable and applicable sediment transport models for sewer systems. Proper validation of these are essential. For that purpose thorough field measurements are imperative. This paper renders initial results obtained in an ongoing case study of a Danish combined sewer system in Frejlev, a small town southwest of Aalborg, Denmark. Field data are presented concerning estimation of the sediment transport during dry weather. Finally, considerations on how to approach numerical modelling is made based on numerical simulations using MOUSE TRAP (DHI 1993).

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The influence of biodegradability of sewer solids for the management of CSOs

Water Science & Technology ◽

10.2166/wst.2005.0036 ◽

2005 ◽

Vol 51 (2) ◽

pp. 89-97 ◽

Cited By ~ 5

Author(s):

R. Sakrabani ◽

R.M. Ashley ◽

J. Vollertsen

Keyword(s):

Weather Conditions ◽

Bulk Water ◽

Combined Sewer Overflows ◽

Sediment Erosion ◽

Combined Sewer ◽

Utilisation Rate ◽

Laboratory Device ◽

Receiving Waters ◽

Oxygen Utilisation ◽

Wet Weather

The re-suspension of sediments in combined sewers and the associated pollutants into the bulk water during wet weather flows can cause pollutants to be carried further downstream to receiving waters or discharged via Combined Sewer Overflows (CSO). A typical pollutograph shows the trend of released bulk pollutants with time but does not consider information on the biodegradability of these pollutants. A new prediction methodology based on Oxygen Utilisation Rate (respirometric method) and Erosionmeter (laboratory device replicating in-sewer erosion) experiments is proposed which is able to predict the trends in biodegradability during in-sewer sediment erosion in wet weather conditions. The proposed new prediction methodology is also based on COD fractionation techniques.

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Prediction of locations with sediment deposits in sewers

Water Science & Technology ◽

10.2166/wst.1996.0198 ◽

1996 ◽

Vol 33 (9) ◽

pp. 147-154 ◽

Cited By ~ 1

Author(s):

Ole Mark ◽

Uros Cerar ◽

Gustavo Perrusquía

Keyword(s):

Sediment Transport ◽

Field Data ◽

Transport Model ◽

Combined Sewer Overflows ◽

Sewer System ◽

Sewer Systems ◽

Sediment Transport Model ◽

Sediment Deposits ◽

Combined Sewer ◽

Movable Bed

The present paper presents an application of MOUSE ST, a general deterministic sediment transport model for sewer systems. MOUSE ST is used to predict the locations subjected to sedimentation in the sewer system of Ljubljana, Slovenia. The prediction is made by means of a sediment transport model with a movable bed. This model is run in parallel with the hydrodynamic MOUSE model. The results, in terms of locations with sediment deposits, are compared with field data from the sewer system in Ljubljana. Further, the model is used to predict the effect of the removal of the sediment deposits on the combined sewer overflows.

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