Page 289 - Materials Chemistry, Second Edition

270 Life Cycle Assessment of Wastewater Treatment

in novel applications of AD (such as, for example, lignocellulosic biomass conver-
sion). However, new paradigms in the production system have opened up possibilities
to expand the concept of AD into the field of resource recovery from waste sources.
The engagement of AD with the circular economy concept was probably intro-
duced, but certainly supported with data, by Zeeman and Lettinga (1999). The cir-
cular economy concept for sustainable industrial production encompasses the life
cycle of a product “cradle-to-cradle” by partially reusing components or completely
recycling the basic components to build new raw materials (Gregson et al., 2015).
Sustainable water management adds water recycling and resource recovery into the
storyline. The introduction of “full” sustainability of water management necessitated
setting prices for water components and, most importantly, to the effort for sustain-
able water management (Verstraete and Vlaeminck, 2011). But the energetic consid-
erations are imperative for achieving sustainability in a short timeframe. According
to the waste-to-energy supply chain (WTE), solving the dilemma of energy demand,
waste management, and greenhouse gas (GHG) emission for communities globally
is not only an energetic opportunity (especially for fossil fuel–importing countries)
but ultimately, a human need (Pan et al., 2014). In this context, AD emerges as the
core part.
Increasing energy demands has driven the change from fossil fuels to renew-
able energetic sources as primary energy. Biomass is the only one to be currently
integrated into the production system at all levels. AD plays a key role in transform-
ing the biomass from different origins (domestic and industrial organic waste and
wastewater, sewage waste, cattery waste, and agroforestry waste) into a transportable
and fully valuable energy source (in the form of biogas or ultimately, biomethane)
(Weiland, 2010). Among the possible applications for biomass transformation into
energy, AD is considered the most sustainable from a GHG emissions perspective,
especially for closed reactors, with an average CO emission potential of 54.7 kg
2
–1
CO -eq. GJ (Fruergaard et al., 2009). This can save up to 196.2 kg of CO -eq
2
2
per ton of biomass treated compared with current processes for waste management
(Masullo, 2017). But more prominent is the energetic efficiency of AD processes,
especially in novel developments.
The energy balance of the AD process has been recursively calculated to be posi-
tive. This essentially means that the transformation of low-value waste into bioen-
ergy as biogas is always energetically and economically favorable. Methane has a
calorific value around 50–55 MJ kg , which is the second highest among the com-
−1
mon fuels, lower only than hydrogen. This turns AD into a highly attractive process
even from an economic perspective. By analogy, the application of AD for waste-
water treatment with organic contamination should be energetically positive as well.
As an example, a single-stage AD was used for treating beer factory wastewater,
achieving an energetic potential of 90 KJ L of wastewater treated with a methane
−1
−1
production of 2.5 L CH L and a mass balance of 86% of chemical oxygen demand
4
converted into methane (Nishio and Nakashimada, 2007). In this way, wastewater
(which has essentially no value as per the existing perspective) is fully treated to
discharge limits, and energy is produced during the process. The concept of positive
energy in wastewater treatment, considered as non-credible just a while ago, has
become fully possible by basing the treatment on AD (Batstone and Virdis, 2014).