The dual membrane system was considered and calculated because it was used to up-concentrate influent intermittently. During direct UF, the cleaning requirements and membrane deterioration were supposed to be higher than the values reported for UF of effluent in tertiary treatment because of more serious membrane fouling [37]. Nevertheless, no consumption for biological aeration and coarse bubble aeration occurred, which comprised a relevant fraction of the total (about 50% in submerged systems) [27]. Therefore, direct UF energy consumption has a lower energy demand, (i.e., 0.55 kWh/m3) compared with conventional MBR, (i.e. 0.98 kWh/m3). In addition, considering the implications of the treatment of real wastewater, higher COD concentration of feed could aggravate membrane fouling of RO membranes leading to increases in membrane cleaning and energy input. As shown in Table 2, energy consumption of 0.60 kWh/m3 for RO was 20% higher compared with that for RO of conventional tertiary treatment. However, the combined system (including UF-RO) demonstrated obvious advantages in energy consumption compared with conventional treatment. Throughout the operation of UF-RO system, 75% influent was converted into high-quality reclaimed water, which was the main recoverable resource from wastewater. In addition, nutrients were well preserved in the concentrate of the dual membrane system. During the minor flow operation of AnMBR, concentrated organic matter in the dual membrane system was converted into biogas, resulting in a high energy output (0.83 and 0.21 kWh/m3 according to the concentrate of 25% flow rate) via Eq. (3). By contrast, a large quantity of organic matter was vainly oxidized during the aerobic process in conventional MBRs, and no more than 0.08 kWh/m3 of power could be recovered during AD of sludge. Notably, one key sustainable benefit of AnMBR is that produced sludge is stabilized and no further digestion is required for disposal [28]. Compared with AnMBR, the energy consumption of CANON MBR was lower even though biological aeration (0.11 kWh/m3) was needed for ammonia oxidation. This phenomenon was mainly attributed to the serious membrane fouling and frequent membrane cleanings in AnMBR. The most important item contributing to energy consumption was biogas scouring for membrane, representing about two-thirds of the membrane combined system energy requirements. The next in importance was the rest membrane, which represented about 30–40% of the total energy requirements. Therefore, the main terms of energy consumption of combined system were related to filtration (representing about 80–90%). This energy consumption corresponded well with the values of coarse bubble aeration in conventional MBR, often exceeding 50% share of total energy consumption [27]. Hence, biogas scouring for continuous membrane cleaning in combined system is the main target for energy saving actions. Optimization of filtration in any operating range is strongly needed to improve the feasibility of MBR technology [28]. Furthermore, energy demand from the combined system evaluated in this study did not consider the energy consumption for nutrient recovery, which was considered in the recovery system as the references mentioned [38].
Clearly, from an energy perspective, the AnMBR and CANON MBR combined system was proven to be a promising sustainable system compared with other existing municipal wastewater treatment technologies under ambient temperatures. However, several issues still need to be addressed and improved prior to its future application. Membrane fouling is a most urgent problem that can be relieved by operating membrane modules at critical filtration conditions or installing a vibrated membrane filtration system [39]. Furthermore, for the AnMBR, an individual membrane room can be added to reduce cake layer on the surface of membrane caused by the low sludge concentration [40]. A significant part of the produced methane in AD is dissolved in the liquid phase and lost with the effluent, whereas only 40–45% of the organic carbon energy content is recovered in practice [4]. As a consequence, a post-treatment process should be required to relieve dissolved methane released to the atmosphere as a powerful greenhouse gas, as well as to improve the recovery efficiency of methane used as energy resource.