Page 298 - Materials Chemistry, Second Edition

Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants 279

selection of the evaluation method (Renou et al., 2008; Corominas et al., 2013;
Meneses-Jácome et al., 2016).
Different life cycle impact assessment (LCIA) methodologies are reported in
LCA studies on anaerobic wastewater treatments. Among them, CML and ReCiPe
impact assessment methods are usually applied.

13.2.1 lca of anaerobic DigesTion of urban WasTeWaTer TreaTMenT
Urban WWTPs (UWWTPs) are conceived to reduce the environmental impacts of
municipal wastewater, which is characterized by low organic strength (Pintilie et al.,
2016; Martinez-Sosa et al., 2011). As previously described, anaerobic processes show
several advantages over aerobic ones: they obtain biogas as an additional source of
energy, the sludge production is lower than in aerobic systems, and it is possible to
reuse the inorganic nutrients contained in the effluent stream (Martinez-Sosa et al.,
2011).
The main objective of anaerobic processes is to operate with a long SRT due to
the slow growth of microorganisms (related to the low organic strength of munici-
pal wastewater). In this sense, AnMBRs are a promising alternative, as they can
decouple HRT, reducing the reactor volume and maintaining sludge concentration
(Martinez-Sosa et al., 2011). As reported elsewhere, these systems can also reduce
the sludge production compared with an aerobic process, eliminate the aeration
energy consumption, and generate methane (Pretel et al., 2016).
From 2008 until 2016, the number of patents related to AnMBR systems has
grown constantly (Krzeminski et al., 2017), as they present some advantages over
other systems. Thus, biogas of excellent quality can be obtained by converting a
large amount of input COD (Skouteris et al., 2012). Using mesophilic AnMBR tech-
nology, the methane production is reported to be in the range of 110–320 ml/g COD
(Pretel et al., 2013), which can be increased by using novel technologies (Gu et al.,
2017; Wei et al., 2014).
In comparison with other anaerobic systems, AnMBRs’ anaerobic membranes
can also operate in mesophilic conditions at lower temperatures (15–30°C) or at
around 55°C in the thermophilic range. They do not show the limitation of operat-
ing at high temperatures to increase the microorganisms’ growth rate that is usually
registered in anaerobic UWW systems.
On the other hand, AnMBR technologies show some disadvantages, such as mem-
brane fouling and high cost of membranes. Regarding the former, energy is needed
to prevent membrane fouling, which could reduce membrane permeate fluxes, caus-
ing an increase in the operating costs (Skouteris et al., 2012). To limit this problem,
gas bubbling is used to increase the flux level. Additionally, AnMBRs require the
recovery of nutrients and methane from the effluent to improve the environmental
feasibility of this technology. Finally, AnMBRs show operating difficulties when
fluctuations in wastewater composition occur, or when the influent presents toxic
compounds (Skouteris et al., 2012; Wei et al., 2014; Smith et al., 2014).
As described previously, AnMBR systems can be installed in three configura-
tions, two of which correspond to immersed configuration. These systems are called