## 4 Concluding Remarks and Future Perspectives
Recently explored baffled osmotic membrane bioreactors can achieve simultaneous wastewater treatment, nutrient and OMP removal. Thus, this reactor design enables both aerobic and anoxic processes in an attempt to reduce the process footprint and energy costs associated with continuous aeration. Different redox conditions with extended anoxic cycle time can be linked with possible development of different microbial consortia responsible for diverse enzymes secretion responsible for efficient OMP removal [110; 103]. Moreover, research efforts are in progress to make outer selective hollow fiber FO membrane modules. This FO modules are capable of producing around 14 LMH initial water flux during wastewater treatment. Compared to inner selective hollow fiber FO membranes outer selective hollow fiber membranes are more promising in terms of higher sustained flux and less fouling propensity. However, both of those types of FO hollow fiber membrane reports are scarce and scope exists to scale -up and commercialize them [114]. Although anaerobic HRMBR was scarcely studied in the literature, given its potential for simultaneous wastewater treatment, biogas production and nutrient recovery and OMP removal potential this process is likely to be prioritized to be implemented at pilot and full-scale level [95]. Other novel concepts such as the coupling of electrically enhanced OMBRs with microbial fuel cell (MFC) for improvement in self-electricity generation should be tested in the future [52]. MFC alone leads to low-efficiency treatment and poor effluent quality due to limited biomass retention. Combination of MBR-MFC system known as an electrochemical membrane bioreactor (EMBR) offers a convincing option for wastewater treatment and energy recovery [82]. However, OMP removal employing EMBR studies are scarce [115] and scope exists to explore this process for micropollutant removal in cost-effective way. Today, OMBR is still an emerging technology limited to lab-scale examinations. To accomplish better OMP removal with OMBR longer SRT is recommended. However, this leads to salinity build-up in the reactor. MF membranes must be incorporated in OMBRs for inorganic salt discharge. MF effluent may be potentially used for toilet flushing, gardening or green-wall irrigation agricultural irrigation where the presence of phosphorous and nitrogen are beneficial [101]. Actually, concentration factor (CF), which is defined as the concentration increase of inorganic salts in the bioreactor and it is a ratio of SRT to HRT during HRMBR operation is an important parameter for controlling and optimizing salinity buildup in HRMBRs [106; 116]. Thus, an optimum CF value should be identified for balancing water recovery targets and salt accumulation in the bioreactor. Cost-effective treatment of membrane concentrates should be investigated. Further, microbial acclimation and the inoculation of halophilic microorganisms were put forward as feasible strategies to ensure a biologic treatment in the high saline environment. More important, easily biodegradable organically based draw solutes were tested and according to recent studies they do not contribute toward salinity build-up in the bioreactor [117]. Membrane fouling and energy consumption (aeration) are interconnected and considered as a major drawback in the application of MBR [82]. Further studies are necessary to ascertain the effects of the sludge cake layer on the rejection of OMPs--particularly the hydrophobic compounds, in the FO process [89; 118]. The combinations of different complementary technologies have produced promising results. Nonetheless, there is a lack of a holistic understanding of the nature of pollutants, their interactions and predictable relationships between the best-available specific technologies [104]. More lab scale and pilot plant experiments should be performed to evaluate the performance of both hybrid MBRs and high retention MBRs for scaling-up and make them next-generation sustainable and techno-economically feasible technologies.