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Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants  275


                resulting digestate is separated by press filtration, after which the N- and
                P-rich liquid phase is used for natural stable fertilizer production, and the
                solid phase is pyrolyzed to obtain biochar. Despite the novelty of this con-
                cept, there is a clear economic imbalance, as the physical separation pro-
                cesses are highly energy consuming, which could impose up to 92% of total
                plant costs.
              •  An improved version of the Verstraete concept was proposed by Batstone
                et al. (2015a). The first step of separation is performed by microbial organ-
                isms (partition step) in an enhanced membrane bioreactor. The microbial
                biomass is then submitted to anaerobic digestion, where biogas is produced,
                and the nutrients are released (release step). Finally, the digestate is sepa-
                rated into two phases, closely following the Verstraete concept (recovery
                step). Unlike the “mother” concept, the partition-release-recovery proposal
                has been proved to be energetically positive if the inlet wastewater concen-
                tration exceeds 600 mgCOD L .
                                         −1
           A recurring technological solution for novel wastewater concepts is the use of the
           anaerobic membrane bioreactor (AnMBR). Although the AnMBR concept is not
           new, with the first attempts made between the 1970s and the 1980s, novel develop-
           ment of efficient membrane operation procedures in aerobic MBRs has improved
           the operability and sustainability of their anaerobic counterparts (Liao et al., 2006).
           There are three AnMBR configurations: side-stream (external, cross-flow), sub-
           merged within the reactor, and submerged in a separate chamber. The membrane
           filtration is the economic limitation of the process; typical energy demand values
           are between 0.2 and 1 kW h m . However, this has not been an impediment for pro-
                                    −3
            cess applications to a wide range of industrial wastewater, such as slaughterhouse,
            molasses, landfill leachate, dairy manure, pharmaceutical, and diverse types of food
            industry wastewater. The main advantage over CSTR configurations is the separa-
            tion between SRT and HRT, leading to AD operation of short HRT (a few days) and
            long SRT (between 20 and 700 d) (Dvořák et al., 2015). In these reactors, biomass
            concentration can be increased up to 75 g L , which is like the biomass concentra-
                                               −1
            tion in granular UASB-type reactors.
              New anaerobic processes have been recently proposed to be part of the frame-
            work of wastewater treatment by AD, although these processes are not related to
            biogas production through methanogenesis (Batstone et al., 2015b). Instead, these
            are more oriented toward direct resource recovery and the decrease of the energetic
            demand of WWTPs (Puyol et al., 2017b), and therefore are dedicated to including
            wastewater treatment into a circular economy concept in water management:

              •  Fermentation targeting chain elongation. Particulate and soluble organic
                matter contained in wastewater can be transformed anaerobically into
                CO , volatile fatty acids (VFAs), and light alcohols by fermentative bac-
                   2
                teria. These compounds can be further chain-elongated to high value-
                added products following three different pathways: (1) homoacetogenesis
                of CO  to acetate, (2) succinate formation from glycerol, and (3) reverse
                     2
                β oxidation of VFAs and light alcohols to form n-butyrate and n-caproate
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