Page 300 - Materials Chemistry, Second Edition

Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants 281

• Global warming potential (GWP) is estimated as GWP100. In this sense,
electricity consumption is considered as the main factor in the impact on
GHG.
• The potential thermal impact of the effluent is not included in this work.

3
The functional unit selected in this work is 1 m of wastewater, as it is usual in LCAs
on AD of municipal wastewater (Ioannou-Ttofa et al., 2016).
Life cycle inventory (LCI) analysis is generated by using energy, raw materials,
and air, land and water emissions (Ioannou-Ttofa et al., 2016).
The methodology chosen to analyze environmental impacts is CML 2 baseline
2000. The studied factors relevant to doing an environmental evaluation of an urban
wastewater treatment process are the energy consumption, the obtained energy asso-
ciated with biogas production, the composition of the final effluent, and the sludge
disposal.
After LCA analysis, SAnMBR for the treatment of medium-strength wastewater
shows the highest GWP compared with other processes. Thus, LCA analysis shows
that there is a problem with energy-related emissions. This energy consumption
decreases when the system operates at ambient temperature, because the associ-
ated heat energy needed to operate with the SAnMBR system at 33°C is very high.
However, temperature decrease alone is not the key to reducing energy emissions.
The solution consists in recovering the nutrient and the dissolved methane from
the effluent. AnMBR effluent is a suitable stream to be used in potable applications,
as it contains a low level of suspended solids and colloidal material, which can be
used for reverse osmosis treatment. However, in that case, the potential advantages
of nutrient removal could be lost (Smith et al., 2014).
On the other hand, methane losses (dissolved methane that escaped in the perme-
ate) drastically increase the carbon footprint of the process (the loss of methane in
municipal wastewater treatment can be around 30%–40% of the generated methane).
For this reason, the main approach for anaerobic treatment processes is to recover
this dissolved fraction (the effluent-dissolved methane). This methane can be used
for energy generation by employing an additional technology (Smith et al., 2014;
Krzeminski et al., 2017; Martinez-Sosa et al., 2011).
Thus, Krzeminski et al., (2017) report LCA results comparing anaerobic mem-
branes with traditional membranes. As can be seen, fresh-water and marine eutrophi-
cation show high impact values, whereas MBR systems imply higher energy-related
emissions. In submerged AnMBR systems, the procedures previously described to
reduce energy-related emissions are included. For that reason, the emissions from
energy consumption in MBR technologies may have higher values.

13.2.2 lca of anaerobic DigesTion of inDusTrial WasTeWaTer TreaTMenT
Wastewater streams generated in the industrial sector are usually characterized
by high and concentrated organic content and the presence of complex and slowly
biodegradable organic compounds, which are not easy to treat, depending on the
factory under study (Appels et al., 2008). The design of WWTPs, particularly for
industrial effluents, needs to consider those factors related to the characteristics of