Page 82 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 63

is used, as each technology varies greatly in productivity, efficiency, and operating
conditions.
Many studies have conducted LCAs to assess the environmental performance
of wastewater-driven algal biofuels. The study by Clarens et al. (2010) compared
the ecological harm from biomass production of algae using municipal wastewa-
ter versus terrestrial biomass (corn, switchgrass, and canola). The results showed
a reduction of environmental impacts with algae biomass cultivation, because
conventional fertilizers are replaced by wastewater in these pathways. Another,
later study by Clarens et al. (2011) also compared wastewater-derived algal bio-
diesel and bioelectricity production via lipid extraction, combustion, and diges-
tion technologies. Their results showed that algae combustion outperformed the
other two technologies in energy use and GHG emissions, because it required less
upstream electricity and heat and fewer chemical inputs. A life cycle environmen-
tal impacts analysis for wastewater-derived biofuels was conducted by Mu et al. in
2014. The study found that using centrate as a feedstock produces environmental
benefits relative to conventional fuel. No matter what algae conversion technology
is chosen, centrate-derived bioenergy/biofuels have better life cycle performance
than petroleum-based fuels. Algae cultivation is the most energy-intensive stage
in all pathways compared, and it consumes over 50% of total life cycle fossil
fuels. The environmental benefits arise because of the replacement of wastewater
treatment, the absorption of CO , and the removal of nitrogen and phosphorus
2
from wastewater. When growing algae in wastewater, the PBR is better than using
an open pond.

4.4 LIFE CYCLE ASSESSMENT (LCA) OF WASTE REUSE
IN WASTEWATER TREATMENT FACILITIES
The LCA of waste reuse from the waste streams, like all other LCA projects, should
follow the ISO 14,000 standards, which include four standard steps: goal and scope
definition, LCI, life cycle impact analysis, and interpretation. This chapter only high-
lights considerations on LCA and modeling when focusing on the reuse of waste
streams.

1. Scope and system boundary: Figure 4.4 shows the potential technology
pathways of waste reuse. The system boundaries for LCA analysis start
with the various wastes collected from the treatment facility, through waste
conversion technologies, all the way to the final use and disposal in land/
vehicles/landfills. The reuse of waste streams could also be combined into
the LCA of a wastewater treatment plant. In this scenario, the reuse brings
environmental benefits to the plant. The LCA could just focus on the waste
to products stage, excluding the final use and disposal. This technique could
be useful in comparing different waste reuse technologies.
2. Functional unit: Functional units within the LCA of waste reuse could
be input functional units (wastes) or output functional unit (products).
The selection of the functional unit depends on the goal and scope of the