In scenario 1, called “Linearity”, one can imagine that, due to difficulties in
developing sewage sludge use because of micropollutant contamination and the
complexity of its organisation, sewage sludge incineration increases from 50 to
100%. N and P circularity decreases to 0%, and human excreta management
becomes totally linear. In this scenario, N and P pollution depends on the ability
of the Paris conurbation to maintain its sewer network as well as intensive and
efficient N and P treatment at the WWTP.
In scenario 2, called “End-of-pipe”, one can imagine that circularity becomes a
goal of human excreta management, but only end-of-pipe solutions are
implemented. On one hand, with sludge P recycling, P soars to more than 80%
circularity, exceeding the 1900s recycling peak. On the other hand, N recycling
remains limited to 30% circularity, below the 1900s recycling peak.
In scenario 3, called “Circularity”, a combination of source separation and end-ofpipe solutions is implemented. Urine diversion is compulsory in all new constructions (faecal matter can potentially be source-separated also, as long as it does not
impair N and P circularity). In existing buildings, urine diversion is also
implemented using toilet integrated treatment. In this scenario, we assume that the
deployment speed of urine diversion in the Paris conurbation follows the same curve
as flush toilet deployment in the city of Paris after the 1880s (Fig. 8). Urine diversion
thus reaches 67% coverage in the 2040s and 98% coverage in the 2090s. Starting at
70% efficiency, the urine collection rate gradually increases up to 85% efficiency in
the 2050s. End-of-pipe P recycling is implemented as in “End-of-pipe” scenario.
Concerning N, increasing urine diversion deeply modifies the C/N ratio of sewage
water. When complete separation of urine is achieved, 85% recovery of N at the
sewage water treatment plants is assumed through N recovery from sludge with
primary decantation and short retention time activated sludge [37]. We assume a
linear progression of N recovery at the treatment plants.
With these hypotheses, N recycling already reaches 60% in the 2030s, with 25%
recycling at the sewage treatment plant and 35% in decentralised facilities. In the
2060s, N and P recycling exceeds 90%, equally distributed between treatment plants
and decentralised systems for P, mainly through decentralised systems for
N. Consequently, N and P pollution is necessarily very low in all compartments of
the environment. Figure 15 represents the N and P circularity of the Paris conurbation from 1800 to 2010 (historical data) with the addition of the 2020–2100 period
with the circularity scenario.
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developing sewage sludge use because of micropollutant contamination and the
complexity of its organisation, sewage sludge incineration increases from 50 to
100%. N and P circularity decreases to 0%, and human excreta management
becomes totally linear. In this scenario, N and P pollution depends on the ability
of the Paris conurbation to maintain its sewer network as well as intensive and
efficient N and P treatment at the WWTP.
In scenario 2, called “End-of-pipe”, one can imagine that circularity becomes a
goal of human excreta management, but only end-of-pipe solutions are
implemented. On one hand, with sludge P recycling, P soars to more than 80%
circularity, exceeding the 1900s recycling peak. On the other hand, N recycling
remains limited to 30% circularity, below the 1900s recycling peak.
In scenario 3, called “Circularity”, a combination of source separation and end-ofpipe solutions is implemented. Urine diversion is compulsory in all new constructions (faecal matter can potentially be source-separated also, as long as it does not
impair N and P circularity). In existing buildings, urine diversion is also
implemented using toilet integrated treatment. In this scenario, we assume that the
deployment speed of urine diversion in the Paris conurbation follows the same curve
as flush toilet deployment in the city of Paris after the 1880s (Fig. 8). Urine diversion
thus reaches 67% coverage in the 2040s and 98% coverage in the 2090s. Starting at
70% efficiency, the urine collection rate gradually increases up to 85% efficiency in
the 2050s. End-of-pipe P recycling is implemented as in “End-of-pipe” scenario.
Concerning N, increasing urine diversion deeply modifies the C/N ratio of sewage
water. When complete separation of urine is achieved, 85% recovery of N at the
sewage water treatment plants is assumed through N recovery from sludge with
primary decantation and short retention time activated sludge [37]. We assume a
linear progression of N recovery at the treatment plants.
With these hypotheses, N recycling already reaches 60% in the 2030s, with 25%
recycling at the sewage treatment plant and 35% in decentralised facilities. In the
2060s, N and P recycling exceeds 90%, equally distributed between treatment plants
and decentralised systems for P, mainly through decentralised systems for
N. Consequently, N and P pollution is necessarily very low in all compartments of
the environment. Figure 15 represents the N and P circularity of the Paris conurbation from 1800 to 2010 (historical data) with the addition of the 2020–2100 period
with the circularity scenario.
136
F. Esculier and S. Barles