Page 309 - Materials Chemistry, Second Edition

290 Life Cycle Assessment of Wastewater Treatment

reason, the increase of AD efficiency, the reduction of chemicals in sludge treatment,
and the improvement of treatments related to waste liquids from sludge thickening
were identified as key aspects to improve the environmental feasibility of ADWWS.
Along these lines, Li et al. (2017) studied different options to increase the digestion
of organic matter present in the wastewater sludge. For that purpose, mesophilic
(conventional) and thermophilic AD were compared considering a system similar to
that depicted in Figure 13.3. The main conclusion was that thermophilic conditions
present environmental advantages, reducing the environmental impacts due to more
biogas being obtained, and therefore, the avoided impacts are higher because of the
replacement of grid electricity.
As well as the use of biogas, the end-use of organic digestate must be taken into
account to evaluate the environmental sustainability of ADWWS using the LCA
approach. The digested sludge is a source of nutrients and energy, thus allowing dif-
ferent end-uses. For this purpose, the use of the digested sludge as fertilizer (nutrient
recovery) or in a cement kiln (energy recovery) has been studied in the literature
(Murray et al., 2008; Peters and Rowley 2009). From both approaches, fertilizer is
the most common application in a typical ADWWS system, since the replacement of
fertilizers by digested solids avoids impacts (in terms of emissions and energy con-
sumption) to a larger extent than energy applications. Nevertheless, the agricultural
application of digested sludge must be undertaken with caution, as wastewater sludge
can contain pollutants, whose environmental impacts must be evaluated, as reported
by Hospido et al., (2010). This work studied the environmental consequences of
applying digested sludge in agriculture by considering the presence of not only
nutrients (nitrogen and phosphorus) but also a number of micropollutants (mainly
heavy metals, pharmaceuticals, and personal care products) that can be present in
the wastewater sludge. The authors identified nutrient-related emissions as the main
environmental impacts, heavy metal emissions to the soil being the most important
contributors to toxicity.
As described earlier, biogas has been considered as the main product for obtain-
ing energy. However, Mills et al. (2014) proposed the possibility of obtaining energy
from both the biogas and the digested sludge. For that purpose, the biogas obtained
by ADWWS is burnt in a combined heat and power (CHP) system, whereas the
digested sludge is dried and used as fuel, thus replacing coal. Although this scheme
is superior to the conventional approach, regulations concerning the combustion of
dry sludge could be an important drawback.

REFERENCES
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Appels, L., J. Baeyens, J. Degrève, and R. Dewil. 2008. Principles and potential of the anaero-
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Barrera, E. L., E. Rosa, H. Spanjers, O. Romero, S. De Meester, and J. Dewulf. 2015. Modeling
the anaerobic digestion of cane-molasses vinasse: Extension of the anaerobic digestion
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