equipped with a combined sewer network, the water flow in the sewers significantly
increases during rain events due to runoff waters, and the transport capacity of the
sewer system can be insufficient to allow all the water flow to reach the WWTP, or
the treatment capacity of the WWTP can be insufficient to treat all the water flow. In
such cases, CSOs can occur, resulting in the direct release of untreated waters into
surface waters. The content of pathogenic microorganisms and FIBs in CSO is
slightly lower compared to raw sewage entering the WWTPs during dry weather
(Fig. 7). This is due to the dilution of domestic waters by runoff waters that are less
contaminated by fecal microorganisms. Studies conducted after intense rain events
in the Seine River downstream of one of the major CSO outfalls in Clichy have
shown that the FIBs brought by the CSO can result in FIB concentrations in the
Seine River exceeding the usual dry weather concentrations by two orders of
magnitude [39].
In the last few years, the SIAAP (Syndicat Interdépartemental pour
l’Assainissement de l’Agglomération Parisienne, in charge of the management of
the sewer network and the WWTPs in the Paris agglomeration) has made a significant attempt to reduce the frequency and the volume of the CSO released into the
Seine River (e.g., by building storage tunnels and detention basins). It resulted in a
significant decrease of the annual volume of CSOs in the Paris area from
80~100 Â 10
6 m
3 (volume discharged by the three main CSO outfalls located
downstream from Paris at Clichy, La Briche, and La Frette) during the 2000–2002
period to around 15 Â 10
6 m
3 in 2015 [40]. With knowledge of the annual CSO
volume and the annual volume treated in the Paris WWTPs (800 Â 10
6 m
3 ) and the
FIB concentrations in CSOs and treated effluents (Fig. 7), the annual flux of FIBs
brought to the Seine River by CSOs and WWTPs was calculated. For E. coli, an
annual flux value of 1 Â 10
17 for the WWTPs and 8 Â 10
17 for CSOs were estimated
for the very recent years. Even if these data are approximate values, they clearly
demonstrate that CSOs are nowadays the major source of microbiological contamination of the Seine River in the Paris area.
6 The EU Bathing Water Directive
In the 1970s, the World Health Organization (WHO) stated the existence of a link
between the microbiological quality of waters and the health of swimmers. Specific
European regulations for the management of the water quality of bathing areas
appeared in 1975 (76/160/EU). Based on WHO recommendations, the bathing
directive provided guidelines for grading the water quality of bathing sites using
FIB count data. Its main purpose was to safeguard public health against microbial
pollution.
The directive was modified in 2006 (2006/7/EC) to consider recent advances in
risk evaluation for bathers. In addition to previously existing epidemiological data
[41, 42], a more recent epidemiological study was available for fresh water in
Germany [43]. These epidemiological studies explore the most likely illnesses that
336
J.-M. Mouchel et al.
increases during rain events due to runoff waters, and the transport capacity of the
sewer system can be insufficient to allow all the water flow to reach the WWTP, or
the treatment capacity of the WWTP can be insufficient to treat all the water flow. In
such cases, CSOs can occur, resulting in the direct release of untreated waters into
surface waters. The content of pathogenic microorganisms and FIBs in CSO is
slightly lower compared to raw sewage entering the WWTPs during dry weather
(Fig. 7). This is due to the dilution of domestic waters by runoff waters that are less
contaminated by fecal microorganisms. Studies conducted after intense rain events
in the Seine River downstream of one of the major CSO outfalls in Clichy have
shown that the FIBs brought by the CSO can result in FIB concentrations in the
Seine River exceeding the usual dry weather concentrations by two orders of
magnitude [39].
In the last few years, the SIAAP (Syndicat Interdépartemental pour
l’Assainissement de l’Agglomération Parisienne, in charge of the management of
the sewer network and the WWTPs in the Paris agglomeration) has made a significant attempt to reduce the frequency and the volume of the CSO released into the
Seine River (e.g., by building storage tunnels and detention basins). It resulted in a
significant decrease of the annual volume of CSOs in the Paris area from
80~100 Â 10
6 m
3 (volume discharged by the three main CSO outfalls located
downstream from Paris at Clichy, La Briche, and La Frette) during the 2000–2002
period to around 15 Â 10
6 m
3 in 2015 [40]. With knowledge of the annual CSO
volume and the annual volume treated in the Paris WWTPs (800 Â 10
6 m
3 ) and the
FIB concentrations in CSOs and treated effluents (Fig. 7), the annual flux of FIBs
brought to the Seine River by CSOs and WWTPs was calculated. For E. coli, an
annual flux value of 1 Â 10
17 for the WWTPs and 8 Â 10
17 for CSOs were estimated
for the very recent years. Even if these data are approximate values, they clearly
demonstrate that CSOs are nowadays the major source of microbiological contamination of the Seine River in the Paris area.
6 The EU Bathing Water Directive
In the 1970s, the World Health Organization (WHO) stated the existence of a link
between the microbiological quality of waters and the health of swimmers. Specific
European regulations for the management of the water quality of bathing areas
appeared in 1975 (76/160/EU). Based on WHO recommendations, the bathing
directive provided guidelines for grading the water quality of bathing sites using
FIB count data. Its main purpose was to safeguard public health against microbial
pollution.
The directive was modified in 2006 (2006/7/EC) to consider recent advances in
risk evaluation for bathers. In addition to previously existing epidemiological data
[41, 42], a more recent epidemiological study was available for fresh water in
Germany [43]. These epidemiological studies explore the most likely illnesses that
336
J.-M. Mouchel et al.