In recent years, the research community has made constant efforts to develop new technologies for the recovery and valorization of water, nutrient and energy content of municipal wastewater. However, the recovery process is significantly limited due to the low-strength of sewage. Over the last 10 years, the Forward Osmosis (FO) process, has gained interest as a low-cost process with low membrane fouling propensity, which can convert municipal wastewater into a concentrated low-volume effluent, characterized by high organic and nutrient concentration. This chapter presents the main configurations that have been implemented for the concentration of municipal wastewater using FO, including their performance in terms of contaminant removal and water/reverse salt flux (Jw/Js). Furthermore, the draw solutions and respective concentrations that have been used in FO for the treatment of sewage are reported, while at the same time the positive and negative characteristics of each application are evaluated. Finally, in the last section of this chapter, the spontaneous FO followed by anaerobic process is integrated in a municipal wastewater treatment plant (WWTP) and compared with a conventional one. The comparison is done, in terms of the mass balance of the chemical oxygen demand (COD) and in terms of the energy efficiency.

The main bottleneck towards resource and energy valorization of municipal wastewater is the low strength of sewage; characterized by low organic matter and nutrient concentrations. In this study, a bench scale Forward Osmosis (FO) system with a cellulose triacetate membrane (CTA) was used as a pre-treatment system in order to concentrate synthetic municipal wastewater. Increasing concentrations of sodium chloride (NaCl) (0.6, 1.2, 1.7, 2.7 and 3 M) were tested as draw solution (DS), achieving a concentration factor ranging from 1.4 to 4.4. Afterwards, the different concentrated effluents of the FO were anaerobically treated. Based on the results, the most concentrated sewage with the higher water recovery rates yielded higher methane production. The concentration of 1.2 M NaCl (similar concentration to the brine produced in seawater desalination) proves to be the most attractive solution. Ηigh water flux (Jw) can be achieved by utilizing the by-product of the reverse osmosis (RO) system as draw solution and closing the loop on the management of two non-conventional water sources.

Mediterranean (MED) islands and coastal areas are under pressure due to water shortage. Water reserves are depleted, while tourism in the summer months burdens the limited water sources. A persistent issue arising from the above activity is the increased seasonal loads of sewage that wastewater treatment plants (WWTPs) receive and need to cope with. On the top of that, the circular economy concept is not implemented in the MED region and particularly in small WWTPs. Water, nutrients, energy and chemicals contained in sewage are not usually valorized. At the same time, in several MED islands, energy intensive seawater desalination is applied to produce potable water thus increasing the energy demand and producing large quantities of brine water, which is not further valorized. As a consequence, activities within the concept of circular economy are required, considering the water-energy-food nexus. The HYDROUSA Horizon2020 Innovation Action project will provide innovative, regenerative and circular solutions for (1) nature-based water management of Mediterranean coastal areas, closing water loops; (2) nutrient management, boosting the agricultural and energy profile; and (3) local economies, based on circular value chains. The services provided lead to a win-win-win situation for the economy, environment and community within the water-energy-food-employment nexus.

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