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Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants  273


           To fulfill these limitations and expand the niche of anaerobic technologies, novel
           concepts of anaerobic wastewater treatment have emerged recently.


           13.1.3  novel concepTs in anaerobic DigesTion
           The year 2014 was the 100th anniversary of the activated sludge (AS) process.
           During these years, biological wastewater treatment has been settling and consoli-
           dating, and nowadays, it is unusual to find a WWTP with no AS treatment. However,
           there is increasing concern about limitations of the AS process, mainly due to its
           high energy demand, high GHG emission potential, and limited recovery potential
           for C and other resources. Therefore, the fourth generation of WWTPs is becoming
           a reality, and four strong alternatives to the AS process have been recently proposed
           (Figure 13.1), all of them related to AD processes and the circular economy in some
           way (Puyol et al., 2017b):

              •  McCarty et al. (2011) proposed low-energy anaerobic wastewater treatment
                in the main line. The concept is based on substituting the AS process by an
                anaerobic secondary treatment, leading to a 100% higher biogas potential, a
                50% reduction of sludge to be disposed, and a net energy production within
                the whole plant. This concept is considered as “low cost” and has been suc-
                cessfully applied in India and Latin America, especially by using UASB-
                type reactors (Chernicharo et al., 2015). However, the concept does not
                include efficient management of nutrient recovery, and the effluent is merely
                proposed to be used for irrigation. An interesting option is to combine this
                concept  with the anaerobic ammonium oxidation (anammox) process  to
                remove excess N, but still the P recovery is inefficient (Winkler et al., 2012).
              •  van Loosdrecht and Brdjanovic (2014) introduced the concept of trans-
                forming a WWTP into a biofactory to reclaim the resources contained in
                the wastewater, particularly the organic contamination, and produce high
                value-added resources that can be re-introduced into the market in a cra-
                dle-to-cradle fashion. As direct transformation of the organic components
                into human-usable products is hardly possible due to pathogens and heavy
                metal contamination, some interesting options have appeared so far, such
                as the indirect combination of the C coming from AD of sewage waste
                with hydrogen from solar-driven water electrolysis to grow chemolithotro-
                phic biomass that can be further used to extract edible single-cell proteins
                or prebiotics (Matassa et al., 2015). This concept leads to full use of the
                residual N from wastewater.
              •  The first concept in considering all the wastewater components as valuable
                resources was proposed by Verstraete et al. (2009). The basis consists of
                a first step of physical separation of water from both soluble and particu-
                late components by sequenced ultra- and nanofiltration followed by reverse
                osmosis (main line, 90% of flow) and a second step of up-concentration of
                a minor flow (10% of flow), whereby all the wastewater components are
                concentrated. In this process, AD plays a fundamental role, as it is the first
                step where all the biodegradable organics are converted into biogas. The
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