The transfer of solids in combined sewer networks

Abstract

Suspended solids are the main vectors of pollution in combined sewer wet weather flows. In spite of being very fine, they decant rather quickly except in the case of light rainfall events rich in organic matter. The part originating from sewer sediment deposits provides a large proportion of organic matter mass. By analysing the content and the volume of sewer deposits at different points from upstream to downstream, it emerges that the main source of this contribution is probably located in man-entry sewers. In one of these, sewer trunk no. 13 in Marseilles, five hundred metres of which were monitored continuously for sediment build-up over three years, the deposit volume grows mainly during certain rainfall events and its surface slope tends to an equilibrium with the (millimetric) particle size. Theoretical bed-load and suspension thresholds have been validated, then analysed for various situations and sections. A typology of the vulnerable areas, in several networks, has been drawn up, as well as a study of shear stress variations during a rainfall event. The sediment deposits seem to occur in particular hydraulic discontinuity zones where transport capacities can drop sharply.

Sediment transport in sewers: the Cesarina combined sewer network

“…Therefore, alternative solutions have been studied aimed at using the online storage capacity of the drainage system and at reducing runoff pollution by sedimentation [2][3][4][5]. In particular, the selected option points at separating solid matter of natural origin from the urban one by building silt traps at the main inlets of the local streams.…”

“…As well as holding polluting potential in itself, solid material in combined wastewater also represents a means for the conveyance of pollutants attached to the solid particles [1][2][3][4][5]. A portion of approximately 30% of settleable solids consists of organic matter, biomass growing on pipe walls (slime), of which 80% is characterised by an average settling velocity of 0.0028 cm/s.…”

Microbial Contamination Detection in Water Resources: Interest of Current Optical Methods, Trends and Needs in the Context of Climate Change

“…In a constant environment, the respect for strict procedures gives precise and reliable turbidity data, in the laboratory as well as in situ [ 75 ], with a high frequency acquisition (minute order). Moreover, if this parameter is known as a good indicator of suspended matters in sewage networks [ 76 , 77 , 78 ], the turbidity measurement of fresh waters is influenced by the particle size and shape of SS, the presence of plankton and the presence of dissolved humic substances. Some authors have tried to establish correlations between turbidity and parameters like water level, flow rate, or rain intensity [ 73 , 74 ] without success, excepted in the case of a dilution phenomenon following a short dry period before raining [ 79 ].…”

Hacia un cambio en el paradigma del manejo y modelación de sistemas de drenaje urbano en países en desarrollo

“…Sewer sediments are of major importance for wastewater quality processes in urban drainage systems because of the solids provide a transport matrix for different pollutants [60]. It has been observed that TSS is the main vector for many pollutants such as COD, hydrocarbon, heavy metals, micro-pollutants, etc [61,62,63]. As a consequence, the management of sewer solids is a key component in developing a holistic approach to the design and operation of wastewater systems [64].…”