Assessment of erosion and sedimentation dynamic in a
combined sewer network using online turbidity
monitoring
T. Bersinger, I. Le Hécho, G. Bareille and T. Pigot
ABSTRACT
Eroded sewer sediments are a significant source of organic matter discharge by combined sewer
overflows. Many authors have studied the erosion and sedimentation processes at the scale of a
section of sewer pipe and over short time periods. The objective of this study was to assess these
processes at the scale of an entire sewer network and over 1 month, to understand whether
phenomena observed on a small scale of space and time are still valid on a larger scale. To achieve
this objective the continuous monitoring of turbidity was used. First, the study of successive rain
events allows observation of the reduction of the available sediment and highlights the widely
different erosion resistance for the different sediment layers. Secondly, calculation of daily chemical
oxygen demand (COD) fluxes during the entire month was performed showing that sediment storage
in the sewer pipe after a rain period is important and stops after 5 days. Nevertheless, during rainfall
events, the eroded fluxes are more important than the whole sewer sediment accumulated during a
dry weather period. This means that the COD fluxes promoted by runoff are substantial. This work
confirms, with online monitoring, most of the conclusions from other studies on a smaller scale.
T. Bersinger
I. Le Hécho (corresponding author)
G. Bareille
Laboratoire de Chimie Analytique Bio-Inorganique
et Environnement,
UMR 5254 CNRS/UPPA, IPREM, Hélioparc Pau
Pyrénées,
2 avenue du Président Angot,
64053 PAU Cedex,
France
E-mail: isabelle.lehecho@univ-pau.fr
T. Bersinger
T. Pigot
Equipe de Chimie Physique,
UMR 5254 CNRS/UPPA, IPREM, Hélioparc Pau
Pyrénées,
2 avenue du Président Angot,
64053 PAU Cedex,
France
Key words | combined sewer, continuous monitoring, erosion, sedimentation, turbidity, wastewater
INTRODUCTION
The European Water Framework Directive has the aim of
achieving ‘good ecological and chemical status’ for all of Eur-
ope’s aquatic environments by 2015 (WFD ). In this
context, reduction of the impact of wastewater effluents of
urban areas from combined sewer overflows (CSOs) and
wastewater treatment plants (WWTPs) has become a priority.
This directive implies that sanitation managers must take
measures to decrease pollutant transfer into the receiving
environments. This challenge requires a perfect knowledge
of pollutant concentrations and fluxes, as well as the dynamics
of the wastewater sewer system. Continuous monitoring of
wastewater thus becomes a key issue for wastewater treatment
management and for protection of the receiving environment.
Recent studies have demonstrated the usefulness of
measuring turbidity as a substitute for conventional analysis
(Lacour et al. a; Hannouche et al. ; Métadier &
Bertrand-Krajewski ). Because good correlations are
found between turbidity and total suspended solids (TSS)
or chemical oxygen demand (COD), online turbidity
measurements allow instantaneous monitoring of global
pollutant loads such as TSS and COD (Lawler et al. ;
Lacour et al. b; Métadier & Bertrand-Krajewski ).
The dynamics of the studied systems can then be obtained
with a better temporal resolution than with discrete analyses
at laboratory scale. Better management of CSOs and
WWTPs can be achieved (Boutayacht et al. ; Blumen-
saat et al. ) in this way.
Many studies point out the importance of sedimentation
in combined sewers during dry weather periods and then
erosion processes during wet weather periods (Ahyerre
et al. ; Oms et al. ). Most of them have shown
that between 30 and 65% of the wet weather fluxes of TSS
and COD come from sewer sediments (Chebbo et al. ;
Gasperi et al. ). Deposition/erosion processes contrib-
ute to increasing the sewer pollutant fluxes and the total
pollution load discharged by CSOs during rain events.
Indeed, 40–70% of the total pollution load discharged by
CSOs comes from sewer deposits (Laplace et al. ).
Because many big cities in the world use a combined
sewer, it is important to have more information about
1375 © IWA Publishing 2015 Water Science & Technology | 72.8 | 2015
doi: 10.2166/wst.2015.350
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