Page 305 - Materials Chemistry, Second Edition

286 Life Cycle Assessment of Wastewater Treatment

reduction (adding or excluding functions, for instance, those related to co-products).
When studying wastewater sludge management systems by LCA, the most usual
objective is to compare different technologies, and therefore, the systems are sim-
plified by excluding functions related to water and/or part of sludge lines. On the
other hand, substitution is also applied in LCAs of sludge management systems.
This approach consists of giving an environmental credit to the process due to
avoided impacts of secondary functions with respect to the product/service replaced
(Heimersson et al., 2017). Finally, an allocation approach is also included in ISO
14044:2006 as a possible alternative way to solve multifunction systems. This
approach consists of allocating the environmental impacts between the different
products obtained in the process. This allocation can be performed in terms of prod-
uct quantity (mass or volume) or by using economic criteria.

13.3.2 applicaTion of lca To anaerobic DigesTion
of WasTeWaTer sluDge (aDWWs)

LCA has been widely used to evaluate the environmental and economic feasibility
of AD applied to sludge management. As reported by Yoshida et al. (2013), a large
number of works have focused on studying AD from an environmental perspective
using LCA.
ADWWS has been traditionally modelled as an input-output system, but this
concept has changed toward more complex systems, in which additional features
have been included, mainly due to the possibility of recovering valuable materials
from wastewater sludge. Although both approaches are different, the most usual way
to define the functional unit is in terms of quantity of dry sludge to be managed
(Yoshida et al., 2013).
In establishing the boundaries of the system, most LCAs consider ADWWS sepa-
rately from WWTP. This leads to considering only the processes related to anaerobic
digestion, thus not including upstream processes. Figure 13.3 shows the typical pro-
cesses considered in LCA studies on ADWWS, including all the operations (from
sludge pre-treatment to recovery of energy and nutrients). Most LCAs reported in
the literature studied ADWWS at mesophilic temperature (35°C) and treating waste-
water sludge with total solid content of 3%–6% (Li et al., 2017). The retention time
values usually vary within the range of 12–30 days (Mills et al., 2014).
As can be seen in Figure 13.3, the upstream processes (wastewater treatments) are
not included; thus, it is considered that the sludge enters AD free of environmental
impacts. This implies that the sludge is considered as waste, although energy and/or
nutrients are recovered, and actually, the sludge should be considered as waste-to-
product. This assumption, called the zero burden assumption as already explained,
is usually applied in LCAs on ADWWS regardless of the aim of the study (Pradel
et al., 2016).
The modeling of ADWWS depends on the consideration of sludge as waste
or as waste-to-product (if energy or materials can be recovered). In the first case,
ADWWS function would be to treat the waste and consequently, all the impacts
would be allocated to the wastewater treatment. However, as shown in Figure 13.3,
the LCA of ADWWS is not restricted to the sludge treatment but also includes the