Page 306 - Materials Chemistry, Second Edition

Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants 287

Inputs (materials, energy)

Recovered electricity and heat

Sludge Anaerobic Biogas Energy Electricity and heat
Waste sludge
conditioning digestion recovery
Digestate

Dewatering

Land
application
(nutrient recovery)

Outputs

FIGURE 13.3 Scheme process of waste sewage sludge management through anaerobic
digestion.
recovery of energy and nutrients. Thus, as reported by Heimersson et al. (2017), the
main function of the system would be the sludge treatment and the end-use, whereas
the secondary function would be production of biogas and digestate, which in addi-
tion can have further functions, such as energy and nutrient recovery. In these cases,
the most common approach to solve multifunctionality is by expansion/reduction
or by substitution. In the first case, some functions can be included (expansion) or
excluded (reduction), as already explained. These approaches are usually applied to
make different systems comparable, and consequently, they are most commonly used
to compare ADWWS with other technologies (Pradel et al., 2016). In the substitu-
tion approach, additional functions are considered (energy and/or nutrient recovery)
and the avoidance of impacts related to the replacement of using energy or resources
(Heimersson et al., 2017). This approach is the most recommended, according to ISO
14044:2006, when secondary functions (use of biogas and/or digestate) are studied
to quantify the avoided emissions related to the substitution of energy and nutrient
resources. Finally, the allocation approach is hardly used in LCAs of ADWWS, since
the functions of this system (waste treatment, biogas, and organic digestate) cannot
be related by a common factor. Regarding economic allocation, great uncertainty
exists in determining the profits and cost of the different functions (Heimersson
et al., 2017).
Quality of inventory data is essential to ensure the reliability of LCA results.
There is a wide variety of ways in which inventory data are obtained for LCAs on
ADWW. The most suitable option should take data from real systems (WWTPs) or
use experimental data obtained in the laboratory. This is the most common option