Page 89 - Materials Chemistry, Second Edition

70 Life Cycle Assessment of Wastewater Treatment

The impact characterizations were based on Impact 2,000+ requirements. In
calculating GHG emissions, the CO emissions from digestion and biogas combustion
2
were not counted, because they are biological CO . In contrast, CH and N O emis-
2
4
2
sions were counted.
4.5.3 lca resulTs
The environmental impacts of the integrated system are shown in Figure 4.6. The
fossil fuel use, GHG emissions, and acidification were all negative, indicating that
the system could remove these three impacts in related ecosystems and benefit the
6
environment. The fossil fuel use was −2.33 × 10 MJ of primary energy per day, in
which the sludge digestion system contributed 95% of the total impact. This implied
that the system could produce energy to replace fossil fuel–based energy and save
resources. As the neutral lipid content in wastewater-based algae cultivation was
not high, the biodiesel yield was not significant. In contrast, the digestion system
generated much more energy as electricity than as biodiesel. Therefore, the fossil
fuel saving was mainly derived from the electricity generation. The total GHG emis-
sions were −3.49 × 10 kg CO eq. per day, indicating that the system could seques-
4
2
ter GHGs from the atmosphere. The result showed that digestion and algal biofuel
production made almost the same contribution to reducing emissions by replacing
petroleum-based diesel and electricity while replacing wastewater treatment of cen-
trate. The land acidification was −879 kg SO eq. per day, and the algae cultiva-
2
tion contributed to 83% of this acidification benefit. The acidification benefits were
mainly from removal of pollutants in centrate.
The other three impacts, that is, eutrophication, territorial ecotoxicity, and
carcinogens, were positive, indicating that emissions and discharges could be
released to the environment from the integrated system. The aquatic eutrophica-
4
tion potential was extremely high, reaching 7.42 × 10 kg PO eq. per day, and the
4
100%
80% Sludge digestion
60% Algae to diesel
40%
20%
0%
–20%
–40%
–60%
–80%
–100%
Fossil fuel GHG Land Eutrophication Carcinogens Land
ecotoxicity acidifiation

FIGURE 4.6 Environmental impacts of an integrated system that reuses centrate and sludge
created in the MWTP.