Page 304 - Materials Chemistry, Second Edition

Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants 285

phosphorus); on the other hand, it may contain toxic substances, pathogens, heavy
metals, and organic contaminants that can be harmful to the environment, and for
that reason, it requires specific treatment to recover profitable materials (Buonocore
et al., 2016). Traditionally, direct disposal in landfill has been the most frequently
used sludge management method, but this approach is in decline, because the con-
cept of sludge as waste has shifted toward the consideration of sludge as an end-
product from which energy and/or materials can be recovered (Pradel et al., 2016).
Energy recovery is a key aspect to improve the energy and environmental perfor-
mance of traditional WWTP (Li et al., 2017). For this purpose, there are different
technologies, such as incineration, gasification, pyrolysis, and AD (Cao et al., 2017).
ADWWS is extensively used in WWTPs, as it allows the volume of the sludge to be
stabilized and reduced (Gourdet et al., 2016). Moreover, and this is the main feature
of this treatment, ADWWS fulfills one of the key objectives of wastewater sludge
management: the integrated use of the sludge. Thus, as is well known, AD allows
the recovery of biogas (which can be used to produce electricity and/or heat) and a
digestate with a high content of nutrients (nitrogen and phosphorus), which can be
further applied as fertilizer (Mills et al., 2014; Heimersson et al., 2017).

13.3.1 lca MoDeling of WasTeWaTer sluDge ManageMenT sysTeMs
The modeling of wastewater sludge management requires a change of perspective
regarding the main objective of LCA application. Waste management systems are
focused on determining the environmental aspects related to the process of handling
the waste. Hence, the functional unit (FU) in these systems is defined in terms of
amount (mass or volume) of sludge to be treated (Yoshida et al., 2013).
Usually, LCA studies on sludge management are focused on comparing differ-
ent technologies, thus supposing that all the functions to generate the sludge are the
same in the studied scenarios. In these cases, the most usual approach is to assume
the concept of zero burden, also called the cut-off approach (Pradel et al., 2016;
Schrijvers et al., 2016). This simplification means that the LCA study is only focused
on the sludge treatment, excluding all the functions that generate the sludge. This
has implications for the allocation of environmental impacts, since it implies that
the sludge is free from environmental burden regarding its production. According
to Pradel et al. (2016), this approach could be valid if the sludge is considered as
a waste with no further treatment. However, if sludge is considered as waste-to-
product (energy and/or materials are recovered), the application of the zero burden
concept may be questionable.
Waste management systems can be modeled in LCAs by using different
approaches regarding the main functions considered. If only one function is con-
sidered (for example, to treat the waste), the system could be modeled as a mono-
functional system. However, this is not the approach commonly used, since sludge
management systems comprise additional functions (energy/material recovery), and
consequently, these must be taken into account, so that the system is modeled as a
multioutput process.
According to Pradel et al. (2016), multifunction wastewater sludge management
systems are usually solved by modifying the system boundaries by expansion or