Page 90 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 71

algae cultivation in which centrate and supernatant were recycled to grow algae
caused major eutrophication impacts. In algae cultivation PBRs, although a por-
tion of nutrients (N and P) was absorbed, around 60 % of N and 45% of P could
not be assimilated by the algae and therefore, remained in the wastewater exiting
the system. This stream caused significant release of nutrients. The system also
raised environmental concerns about terrestrial ecotoxicity, as the total release
8
reached 9.3 × 10 kg TEG in soil, and the land application of digestate accounted
for 99.9% of ecotoxicity. Lastly, the carcinogens released from the system were
214 kg C H Cl eq. per day, which was also high enough to raise a significant
2
3
human health concern. Unlike the exotoxicity, algae cultivation contributed 87%
of the total carcinogen release, which was mainly derived from using electricity
for algae cultivation. The land application of digestate also raised serious concern
over carcinogen release (9662 kg C H Cl eq.), but as the electricity generated in
2
3
the digestion stage accounted for high credits (−9334 kg C H Cl eq.), the overall
3
2
carcinogens from the digestion stage only accounted for 13% of total carcinogen
release.
The study also compared the LCA of the integrated system with the single diges-
tion + land application scenario. The results, seen in Figure 4.7, showed that the
integrated system had better performance or lower impacts on fossil fuel use, GHG
emissions, eutrophication, and acidification, and could reduce those impacts by 40%,
2170%, 8.5%, and 593%, respectively, compared with the single digestion system.
The reduction of impacts by the integrated system was attributed mainly to the bio-
fuel and electricity generation credits and the centrate treatment credits. However,
the land ecotoxicity and carcinogens of the integrated system increased by 33%
and 31% as compared with the single digestion system, because more digestate was
applied to land as fertilizer and consequently increased the impacts on ecotoxicity
and carcinogens.

100%
80% Integrated system
60% Single digestion
40%
20%
0%
–20%
–40%
–60%
–80%
–100%
Fossil fuel GHG Land Eutrophication Carcinogens Land
ecotoxicity acidifiation

FIGURE 4.7 Comparison of the integrated waste reuse system with the conventional sludge
digestion followed by land application.