Page 56 - Materials Chemistry, Second Edition

Life Cycle Assessment of Municipal Wastewater and Sewage Sludge Treatment 37

Mandatory elements Optional elements

Normalisation
Classification (dimensionless Weighting
• Models: mid-point • Calculation of quantity)
or end-point • Asignment of LCI category
• Impact categories results indicator results
• The substances are • Reference units
sorted into impact
Grouping Data quality analysis
categories
Selection Characterisation

FIGURE 3.3 Elements of Life Cycle Impact Assessment. (Adapted from ISO 14040.
Environmental Management and Life Cycle Assessment—Requirements and Guidelines.
International Organization for Standardization, 2006a.)

3. Characterization, the quantification of the impact associated with each
emission, which provides a way to directly compare the LCI results within
each category. Characterization factors are considered as equivalency fac-
tors; for example, CO for climate change.
2
4. Normalization, the stage when the different impact potentials and con-
sumption of resources are expressed on a common scale.
5. Weighting, which aggregates all impact categories into one single score
using subjective criteria such as expert opinion, monetization, or policy
goals.

Among the range of available methods, the choice of impact assessment models
should be tailored according to their relevance to urban wastewater systems. It is
recommended to use the EU recommended midpoint impact categories as described
in the International Life Cycle Data (ILCD)/Product Environmental Footprint (PEF)
guide (European Commission, 2010). Furthermore, the ILCD approach should be
combined with more complete models such as ReCiPe (Goedkoop et al., 2013). Other
LCIA methods, such as CML (Guinée et al., 2002), EDIP20036 (Hauschild and
Potting, 2003), or Impact 2002+ (Jolliet et al., 2003), present a set of impact cat-
egories such as eutrophication, global warming, and ecotoxicity. Thus, most LCA
practitioners would only need to choose the methodology that fits best the objective
pursued and the impact categories that are relevant for each scenario.

3.2.4 interpretation of reSultS
The interpretation phase assesses the outcomes of the study in a context of alterna-
tive scenarios as well as through sensibility and uncertainty analysis (Bauman and
Tillman, 2004). This stage should identify the hotspots of the process as well as
communicating conclusions, limitations, and recommendations. One of the widely
used ways of presenting the results is to take a reference scenario for which the