• Direct discharge in a river. In this case it was considered that N/P-CENT ¼ 0%
and that all N and P resulted in surface water pollution.
• Irrigation fields. In this case it was considered that N/P-CENT ¼ 100% since all N
and P was spread on fields. Not all N and P was actually taken up by crops, but the
agronomic efficiency of plant uptake is not taken into account in the calculation of
circularity.
• WWTPs with sludge spreading on fields. N/P-CENT is calculated by the proportion of N and P contained in sludge. The remaining P is released to a river as
pollution. The remaining N is either released to a river as pollution or denitrified
to the atmosphere in the non-reactive form N 2 . Despite their importance in terms
of the greenhouse gas effect and their high level of emission in Paris WWTPs
[17, 18], N 2 O emissions were not specifically estimated in this study since the
different forms of reactive N are not distinguished.
• WWTPs with sludge incineration. In this case, N/P-CENT ¼ 0%. Depending on
the treatment efficiency, a proportion of N and P is released to a river as pollution;
the rest of N is considered to go to the atmosphere in the non-reactive form N 2 and
P not released to the river is contained in ashes and usually used as construction
material with a loss of the fertilisation potential of P.
For households not connected to a sewer, we based our calculations on the fact
that 90% of N and 65% of P is excreted via urine and the remaining via faeces
[7]. N-DEC and P-DEC were calculated as follows:
• The N excretion of people was taken from N diet data [19] to which a 20%
decrease was applied to take into account non-ingested food. The multiplication
by the total population gave the total amount of N contained in excretions.
N-DEC was calculated on the basis of total quantity and N content of collected
night soil and final products obtained from cesspool management for the 1780s
[20] and the years 1817, 1852, 1869, 1911 and 1926 [6, 9, 21, 22]. Data were
linearly extrapolated for the missing decades. N-DEC was estimated at 0% for the
period since the 1980s.
• The P content of products obtained from cesspool management was not available
in previous studies. The P recycling rate is expected to be higher than the N
recycling rate since N does not precipitate and is usually lost in the form of NH 3
emissions in cesspool treatment processes, whereas P remains in the solid and
liquid phases [23]. Therefore, the best possible estimate for P-DEC was chosen as
the mean value between N-DEC and the recycling rate of night soil calculated in
volumes.
The combination of the five main parameters, N/P-DEC, N/P-CENT and
SEWER, allowed us to estimate the circularity of human excreta management as
the total amount of N and P returned to agricultural land via night soil processing,
field irrigation or sewage sludge spread onto agricultural lands and the total
amount of N and P pollution to soil, underground and surface water or air.
Uncertainty on most data is difficult to quantify. It is considered high for data
concerning decentralised management and concerning wastewater management in
122
F. Esculier and S. Barles
and that all N and P resulted in surface water pollution.
• Irrigation fields. In this case it was considered that N/P-CENT ¼ 100% since all N
and P was spread on fields. Not all N and P was actually taken up by crops, but the
agronomic efficiency of plant uptake is not taken into account in the calculation of
circularity.
• WWTPs with sludge spreading on fields. N/P-CENT is calculated by the proportion of N and P contained in sludge. The remaining P is released to a river as
pollution. The remaining N is either released to a river as pollution or denitrified
to the atmosphere in the non-reactive form N 2 . Despite their importance in terms
of the greenhouse gas effect and their high level of emission in Paris WWTPs
[17, 18], N 2 O emissions were not specifically estimated in this study since the
different forms of reactive N are not distinguished.
• WWTPs with sludge incineration. In this case, N/P-CENT ¼ 0%. Depending on
the treatment efficiency, a proportion of N and P is released to a river as pollution;
the rest of N is considered to go to the atmosphere in the non-reactive form N 2 and
P not released to the river is contained in ashes and usually used as construction
material with a loss of the fertilisation potential of P.
For households not connected to a sewer, we based our calculations on the fact
that 90% of N and 65% of P is excreted via urine and the remaining via faeces
[7]. N-DEC and P-DEC were calculated as follows:
• The N excretion of people was taken from N diet data [19] to which a 20%
decrease was applied to take into account non-ingested food. The multiplication
by the total population gave the total amount of N contained in excretions.
N-DEC was calculated on the basis of total quantity and N content of collected
night soil and final products obtained from cesspool management for the 1780s
[20] and the years 1817, 1852, 1869, 1911 and 1926 [6, 9, 21, 22]. Data were
linearly extrapolated for the missing decades. N-DEC was estimated at 0% for the
period since the 1980s.
• The P content of products obtained from cesspool management was not available
in previous studies. The P recycling rate is expected to be higher than the N
recycling rate since N does not precipitate and is usually lost in the form of NH 3
emissions in cesspool treatment processes, whereas P remains in the solid and
liquid phases [23]. Therefore, the best possible estimate for P-DEC was chosen as
the mean value between N-DEC and the recycling rate of night soil calculated in
volumes.
The combination of the five main parameters, N/P-DEC, N/P-CENT and
SEWER, allowed us to estimate the circularity of human excreta management as
the total amount of N and P returned to agricultural land via night soil processing,
field irrigation or sewage sludge spread onto agricultural lands and the total
amount of N and P pollution to soil, underground and surface water or air.
Uncertainty on most data is difficult to quantify. It is considered high for data
concerning decentralised management and concerning wastewater management in
122
F. Esculier and S. Barles