The last PAH flux related to the sewer system was the removal of highly
contaminated sewer deposits during pipe cleaning. In 2014, 7,170 tons of sediment
was removed from the sewer system. The contamination of sewer sediment has been
documented in four studies [31, 38–40] and by the Paris Sanitation Department
between 2000 and 2014. The corresponding database gathered PAH measurements
from 406 samples, thereby ensuring good spatial and temporal representativeness.
Consequently, the flux estimation was awarded a quality grade of 4. The PAH flux
related to sediment cleaning in the sewer system was F22i ¼ 160 Æ 10 kg year
À1
(Σ15). Since sewer sediment is not suitable for amending agricultural land, it was
stored as hazardous material.
3.1.6 Wastewater Treatment Plants
To quantify the total flux of PAHs entering WWTPs (F22e), the period investigated
was divided between dry weather periods (208 days in 2014, average inflow,
2.2 Mm
3 day
À1 ) and wet weather periods (157 days, inflow, 2.8 Mm
3 day
À1 ). The
PAH input was estimated as the daily collected water volumes from each sewer type
multiplied by corresponding PAH concentrations measured in separate or combined
sewers. For this estimation, care was taken to use data from samples collected at
downstream parts of the sewer system. It was assumed that these samples were
representative of the mix of waters originating from different sources including
infiltration water. Consequently, entirely distinct databases were used to compute
the F22a, E22h, F12e and F22e fluxes. The data of 121 samples from nine studies,
including wastewater from the four main WWTPs, were used. The annual load of
PAHs was F22e ¼ 1,030 Æ 50 kg year
À1 (Σ15). The order of magnitude was also
consistent with a previous study that reported an annual flux of about 476 kg year
À1
for six compounds (FLH, BaP, BbF, BkF, BghiP and IcdP) [36]. Consequently, this
flux estimation was awarded a quality grade of 4.
Once they have entered WWTPs, PAHs may be degraded during the treatment
process, removed from the effluent through sorption onto sludge or discharged
into the Seine River at the outlet of the WWTP. To quantify the amount of PAHs
removed by the sludge removal process, the daily mass of sludge was multiplied
by the random PAH contents measured in this matrix. The database comprised
data from a previous study [36] or was provided by the SIAAP. It gathered data
for 16 samples collected at three WWTPs treating 74% of the Paris conurbation
population. Thus, the annual PAH amount removed from the WWTP was
560 Æ 40 kg year
À1 (Σ13, ACE and ACY excluded). The largest part was reduced
by the sludge thermal process (F13a ¼ 270 Æ 20 kg year
À1 , Σ13), the rest being
either composted (F13b ¼ 130 Æ 10 kg year
À1 , Σ13) or spread over agricultural land
without further treatment (F13c ¼ 160 Æ 10 kg year
À1 , Σ13). Considering that PAH
content in sludge has been steady over the past few years [15], good spatial and
temporal representativeness was expected, and the estimations of sludge-related
fluxes were awarded a quality grade of 4. No data for PAHs in composted sludge
was available (E13d). Therefore, the PAH degradation rates during the composting
Mass Balance of PAHs at the Scale of the Seine River Basin
173
contaminated sewer deposits during pipe cleaning. In 2014, 7,170 tons of sediment
was removed from the sewer system. The contamination of sewer sediment has been
documented in four studies [31, 38–40] and by the Paris Sanitation Department
between 2000 and 2014. The corresponding database gathered PAH measurements
from 406 samples, thereby ensuring good spatial and temporal representativeness.
Consequently, the flux estimation was awarded a quality grade of 4. The PAH flux
related to sediment cleaning in the sewer system was F22i ¼ 160 Æ 10 kg year
À1
(Σ15). Since sewer sediment is not suitable for amending agricultural land, it was
stored as hazardous material.
3.1.6 Wastewater Treatment Plants
To quantify the total flux of PAHs entering WWTPs (F22e), the period investigated
was divided between dry weather periods (208 days in 2014, average inflow,
2.2 Mm
3 day
À1 ) and wet weather periods (157 days, inflow, 2.8 Mm
3 day
À1 ). The
PAH input was estimated as the daily collected water volumes from each sewer type
multiplied by corresponding PAH concentrations measured in separate or combined
sewers. For this estimation, care was taken to use data from samples collected at
downstream parts of the sewer system. It was assumed that these samples were
representative of the mix of waters originating from different sources including
infiltration water. Consequently, entirely distinct databases were used to compute
the F22a, E22h, F12e and F22e fluxes. The data of 121 samples from nine studies,
including wastewater from the four main WWTPs, were used. The annual load of
PAHs was F22e ¼ 1,030 Æ 50 kg year
À1 (Σ15). The order of magnitude was also
consistent with a previous study that reported an annual flux of about 476 kg year
À1
for six compounds (FLH, BaP, BbF, BkF, BghiP and IcdP) [36]. Consequently, this
flux estimation was awarded a quality grade of 4.
Once they have entered WWTPs, PAHs may be degraded during the treatment
process, removed from the effluent through sorption onto sludge or discharged
into the Seine River at the outlet of the WWTP. To quantify the amount of PAHs
removed by the sludge removal process, the daily mass of sludge was multiplied
by the random PAH contents measured in this matrix. The database comprised
data from a previous study [36] or was provided by the SIAAP. It gathered data
for 16 samples collected at three WWTPs treating 74% of the Paris conurbation
population. Thus, the annual PAH amount removed from the WWTP was
560 Æ 40 kg year
À1 (Σ13, ACE and ACY excluded). The largest part was reduced
by the sludge thermal process (F13a ¼ 270 Æ 20 kg year
À1 , Σ13), the rest being
either composted (F13b ¼ 130 Æ 10 kg year
À1 , Σ13) or spread over agricultural land
without further treatment (F13c ¼ 160 Æ 10 kg year
À1 , Σ13). Considering that PAH
content in sludge has been steady over the past few years [15], good spatial and
temporal representativeness was expected, and the estimations of sludge-related
fluxes were awarded a quality grade of 4. No data for PAHs in composted sludge
was available (E13d). Therefore, the PAH degradation rates during the composting
Mass Balance of PAHs at the Scale of the Seine River Basin
173