282
Z. Liang et al.
18.4 Results and Discussions
18.4.1 GHG Emissions of Different Sewage-Sludge
Treatment Scenarios
GHG emissions from different sources for the nine sewage-sludge treatment
scenarios (S1–S9) are shown in Table 18.5.
The GHG emission ranges (with different sludge scenarios) of SBR, AAO, and
OD are 58–60 ktCO 2 e/a, 122–127 ktCO 2 e/a, and 113–125 ktCO 2 e/a, respectively.
The direct GHG emissions of SBR (33.87 ktCO 2 e/a) are much less than AAO (109.78
ktCO 2 e/a) and OD (89.86 ktCO 2 e/a), while the indirect GHG emissions are similar,
namely 24.86 ktCO 2 e/a (SBR), 14.94 ktCO 2 e/a (AAO), and 30.18 ktCO 2 e/a (OD).
The ratio of direct to total GHG emissions were calculated to be 88% (AAO), 75%
(OD), and 58% (SBR). The contribution of GHG emissions from wastewater treatment accounted for many of the total emissions, which were 94.8% for AAO, 85.8%
for OD, and 95.6% for SBR.
In the nine sewage-sludge treatment scenarios, SBR-Combustion (S9) scenario
had the least amount of GHG emissions, while the AAO-Composting (S2) scenario
had the most GHG emissions. The total GHG emissions, in descending order, were
S2, S1, S5, S4, S3, S6, S8, S7, and S9. The total emissions of SBR scenario were
less than AAO and OD, even under different sludge treatment scenarios.
The ratio of GHG emissions from sludge treatment and total emissions were
approximately 5.2% (AAO), 14.2% (OD), and 4.4% (SBR). GHG emissions from
different sludge treatment scenarios (same sewage treatment), in descending order,
are composting, landfill, and combustion. The reduction rate of GHG emissions under
combustion scenario, when compared to landfill were 2.8% for AAO, 7.6% for OD,
and 2.4% for SBR; when compared to composting were 3.6% for AAO, 9.6% for
OD, and 3.1% for SBR. Therefore, the effect of sludge treatment process selection in
reducing GHG emissions is positive, without changing the sewage treatment process.
18.4.2 GHG Emissions from Different Sources
The GHG emissions of sewage-sludge treatment system were divided into six emission sources among nine different scenarios. The six emission sources were CO 2
from sewage treatment, N 2 O from sewage treatment, chemicals consumption from
sewage treatment, electricity consumption from sewage treatment, direct GHG emissions from sludge treatment, and indirect GHG emissions from sludge treatment, as
shown in Fig. 18.2.
The contribution of each emission source (in descending order) in the AAO
scenario were 80.12-83.12% (N 2 O), 11.62–12.05% (electricity), 2.66–5.77% (direct
emission from sewage treatment), 1.62–1.67% (CO 2 ), 0.004–0.49% (indirect emission from sewage treatment), and less than 0.005% (chemicals).
Z. Liang et al.
18.4 Results and Discussions
18.4.1 GHG Emissions of Different Sewage-Sludge
Treatment Scenarios
GHG emissions from different sources for the nine sewage-sludge treatment
scenarios (S1–S9) are shown in Table 18.5.
The GHG emission ranges (with different sludge scenarios) of SBR, AAO, and
OD are 58–60 ktCO 2 e/a, 122–127 ktCO 2 e/a, and 113–125 ktCO 2 e/a, respectively.
The direct GHG emissions of SBR (33.87 ktCO 2 e/a) are much less than AAO (109.78
ktCO 2 e/a) and OD (89.86 ktCO 2 e/a), while the indirect GHG emissions are similar,
namely 24.86 ktCO 2 e/a (SBR), 14.94 ktCO 2 e/a (AAO), and 30.18 ktCO 2 e/a (OD).
The ratio of direct to total GHG emissions were calculated to be 88% (AAO), 75%
(OD), and 58% (SBR). The contribution of GHG emissions from wastewater treatment accounted for many of the total emissions, which were 94.8% for AAO, 85.8%
for OD, and 95.6% for SBR.
In the nine sewage-sludge treatment scenarios, SBR-Combustion (S9) scenario
had the least amount of GHG emissions, while the AAO-Composting (S2) scenario
had the most GHG emissions. The total GHG emissions, in descending order, were
S2, S1, S5, S4, S3, S6, S8, S7, and S9. The total emissions of SBR scenario were
less than AAO and OD, even under different sludge treatment scenarios.
The ratio of GHG emissions from sludge treatment and total emissions were
approximately 5.2% (AAO), 14.2% (OD), and 4.4% (SBR). GHG emissions from
different sludge treatment scenarios (same sewage treatment), in descending order,
are composting, landfill, and combustion. The reduction rate of GHG emissions under
combustion scenario, when compared to landfill were 2.8% for AAO, 7.6% for OD,
and 2.4% for SBR; when compared to composting were 3.6% for AAO, 9.6% for
OD, and 3.1% for SBR. Therefore, the effect of sludge treatment process selection in
reducing GHG emissions is positive, without changing the sewage treatment process.
18.4.2 GHG Emissions from Different Sources
The GHG emissions of sewage-sludge treatment system were divided into six emission sources among nine different scenarios. The six emission sources were CO 2
from sewage treatment, N 2 O from sewage treatment, chemicals consumption from
sewage treatment, electricity consumption from sewage treatment, direct GHG emissions from sludge treatment, and indirect GHG emissions from sludge treatment, as
shown in Fig. 18.2.
The contribution of each emission source (in descending order) in the AAO
scenario were 80.12-83.12% (N 2 O), 11.62–12.05% (electricity), 2.66–5.77% (direct
emission from sewage treatment), 1.62–1.67% (CO 2 ), 0.004–0.49% (indirect emission from sewage treatment), and less than 0.005% (chemicals).