Forward Osmosis: Fundamentals and Applications

This introduction presents an overview of the book’s key concepts. The book’s main objective is to introduce the principles and concept of the forward osmosis (FO) process and the potential for various applications, such as desalination, fertigation, wastewater reuse, and mine water treatment. The book also covers other important aspects of the FO process such as membrane fabrication and modeling. The introduction briefly outlines each chapter, including various water treatment technologies available on the market. One of the most advanced and reliable water treatment technologies is the membrane process. Depending on the membrane pore size and its removal capacity, the membrane process is classified into four different types: microfiltration, ultrafiltration, nanofiltration. and reverse osmosis.

This chapter introduces the forward osmosis (FO) process model. The objective of this chapter is to provide systematic insight into FO mass transport, to understand the governing mechanisms over flux behavior on a quantitative basis, and finally to determine how to improve the efficiency of the FO process. The contents include the basic concepts of the FO process, mass transport in the FO process, flux modeling of single-skinned FO membranes and advanced double-skinned FO membranes, modeling of salt accumulation in the osmotic membrane bioreactor, and modeling of a related pressure-retarded osmosis (PRO) process. The latter has become increasingly popular in recent years, owing to the surge in FO study. In the PRO process, a feed solution and a draw solution are placed on opposite sides of a semipermeable membrane.

This chapter deals with the impact of spacers on forward osmosis (FO) separation numerically by considering the complexity of concentration polarization. In addition, the chapter suggests the concentration polarization index as a concentration polarization (CP) indicator pertaining to the degree of concentration polarization and permeate flux in FO processes. CP is an important factor for analyzing the flux performance in membrane processes. CP usually diminishes the driving force for water permeation during membrane processes. The chapter primarily focuses on concentrative external concentration polarization for reverse osmosis separation. A membrane with high water permeability and low solute resistivity might be the most efficient way to maximize FO performance, rather than optimizing operating conditions such as feed and draw solution velocities.

This chapter discusses the modifications of standard pressure filtration element designs to transform them into forward osmosis (FO) element designs. Primarily this entails redesigning the permeate channels to accept a pumped draw solution. This is critically important in situations where the feed solution is highly fouling, because one of the major selling points of FO is its ability to concentrate fouling fluids, and thus plate and frame designs are often attractive. FO spiral elements are somewhat more complex because instead of the permeate flowing through the permeate spacer and out through the perforated tube, a draw solution must be pumped across the permeate side of the membrane. Multielement housings are by far the most economical design for spiral systems, but in FO they present a difficulty not seen in pressure filtration applications.

This chapter provides insight into the selection of suitable draw solutions (DS) and reviews different DS characteristics affecting the performance of forward osmosis (FO) processes. Although some commercial applications of FO technology exist, the development of an effective large-scale process is currently limited due to the lack of both suitable DS and membrane. The success of most FO applications also relies on how the DS can be recovered from the produced water. Therefore, in commercial FO processes, such as FO followed by reverse osmosis seawater desalination, emergency drinks and osmotic dilution are used without a DS recovery system—a simple and energy-saving solution. Research is still needed to develop more suitable DS to achieve full-scale commercialization of the FO process.

This chapter describes a new kind of forward osmosis draw solute, dextran-coated Fe₃O₄ magnetic nanoparticles (DF-MNPs), and evaluates its performance as a draw solute for the forward osmosis (FO) process. DF-MNP combines the advantages of the high osmotic pressure of dextran and the strong magnetism of Fe₃O₄ MNP. Two key components in the FO process include the FO membrane and the draw solute. This chapter focuses on the draw solute. An ideal draw solute for the FO process should have these characteristics: (1) high osmotic pressure that may generate high water flux, (2) easy recovery with low energy consumption, and (3) minimal reverse diffusion through FO membranes. This study further promotes the FO process as an environmentally friendly process for clean water production. In future research, the properties of DF-MNPs should be further optimized with the aim to further improve osmotic pressure for possible commercial applications.

This chapter discusses thermosensitive polymer hydrogels as a new class of draw agent for forward osmosis (FO) desalination. The effect of hydrogel chemistry is reviewed, including the effect of incorporating ionic chemical moieties and light-absorbing carbon particles and reduced graphene oxide nanosheets on FO desalination performance. The presence of ionic groups in the hydrogel networks provides a unique capacity for hydrogels to attract more water. The pure ionic and composite ionic hydrogels show higher flux than nonionic hydrogels do. In any FO desalination process, the separation of permeated water from the draw agent is also very important. The dewatering process of hydrogel needs the deswelling of the swollen hydrogels to release the water. Solar energy is also used to release water from graphene-incorporated hydrogel. The chapter also focuses on further optimization of hydrogel properties and structures of membranes to enhance the performance of the hydrogel-driven FO process.

This chapter (1) summarizes the background and state-of-the-art technologies for forward osmosis (FO) membrane fabrication, (2) analyzes the mass transport phenomena in FO membranes, and (3) discusses strategies to properly design FO membranes. It also highlights basic characterization methods for FO membranes. The chapter focuses on asymmetric membranes made of both phase-inversion and composite membranes. Three major components are involved in the forward osmosis (FO) process: (1) a draw solution comprising a high osmotic pressure, (2) a feed solution possessing a lower osmotic pressure, and (3) a semipermeable membrane working as a barrier between the two solutions. Similar to conventional pressure-driven membrane processes such as RO and others, concentration polarization affects FO performance. Apart from cellulose esters, polybenzimidazole is also a popular choice of materials for FO membranes.

This chapter introduces a novel forward osmosis (FO) membrane, nanofiber FO membrane, which is made of an electrospun nanofiber support layer and a thin active layer. Conventional FO membranes consist of a dense active layer and an asymmetric porous support layer that supports phase-inversion processes. One challenge this asymmetric porous support layer inevitably faces is the internal concentration polarization phenomenon. A customer-designed electrospinning setup was used to fabricate the nanofiber support layer on a nonwoven fabric. The pore structures of FO membranes were observed by a Zeiss Evo 50 Scanning Electron Microscopy and a Joel JSM 7600F thermal FESEM. The chapter paves the way for the development of next-generation FO membranes with high water production rate. The active layers could be modified with mesoporous materials to present ion-selective and high flux performance.

As the forward osmosis (FO) process utilizes natural osmosis as the pressure source, the energy demand can be significantly reduced compared with other pressure-driven processes, such as RO. In a surface-modification study conducted by the authors, membrane surfaces were coated with zwitterionic polymer 3-(3,4-Dihydroxyphenyl)-L-alanine (L-DOPA) to increase the fouling resistance of the membrane. The relationship between the zwitterionic coating and the improved antifouling effect was investigated by observing filtration flux and quantifying the change of water contact angle on the membrane surfaces. L-DOPA has both positively and negatively charged zwitterionic heads, which could lead to antifouling properties; it is also exceptionally adhesive to various surfaces, making it an ideal antifouling coating. One factor that affects the fouling behavior of FO membrane is the internal concentration polarization (ICP).

Several filtration systems have been applied to advanced water treatment and even conventional water treatment processes. Although the membrane filtration systems are the most feasible alternative for water treatment, several problems still remain for efficient operation of membrane filtration. Among these problems, fouling is one of the major challenges to overcome in membrane technology. Reverse osmosis (RO) membrane is a polyamide thin-film composite membrane and forward osmosis (FO) membrane is a cellulose-based membrane, thus fouling could differ between RO and FO due to different membrane surface characteristics, which are discussed in this chapter. The chapter also discusses various strategies that mitigate fouling formation in the FO process in terms of spacers, cross-flow velocity, air scouring, cross-flow type, and osmotic backwashing. The osmotic backwashing is applied to an osmotic membrane bioreactor system that utilizes a submerged FO membrane in the bioreactor.

This chapter reviews the most important aspects of biofouling in the forward osmosis (FO) process. The biofouling phenomenon differs significantly between reverse osmosis and FO membranes. The chapter discusses biofouling by transparent exopolymer particles, which is an emerging algal organic matter. Membrane biofouling is regarded as a severe problem in the field of FO when compared with other types of fouling. Membrane biofouling in membrane bioreactors is attributed to the physicochemical interactions between the biofluid (soluble materials suspended solids) and membrane. The chapter summarizes various parameters affecting membrane fouling in osmotic membrane bioreactors, including the configuration, feed/draw solution, membrane (type and material), cross-flow velocity, solids retention time, hydraulic retention time, and average flux. The FO membrane process requires further research into the prevention and control of biofouling.

This chapter provides an overview of the fundamental terminologies, concepts, performance discussions, and concerns concerning forward osmosis (FO) hybrid systems for water reclamation and reuse. Water reclamation and reuse attempt to alleviate water stresses by posing as viable and reliable water sources to create high-quality water supplies. The chapter examines several FO hybrid systems such as the FO dilution-reverse osmosis (FODN-RO) and FO-nanofiltration hybrids. Membrane fouling is one critical aspect in the operation of any membrane-based system, and it is of special importance and great concern in the present FODN-RO hybrid system because it uses impaired water as the feed stream. The FO-membrane distillation hybrid system has been successfully applied to concentrate proteins and could be a promising system to concentrate proteins and pharmaceuticals because of the low temperature and pressure needs.

This chapter introduces the principle of forward osmosis processes for desalination (FOD) and historical evolution of desalination technologies from reverse osmosis (RO) to FO, followed by an in-depth presentation of different case studies on FOD processes. Several case studies discuss important achievements in and engineering challenges and commercialization of the FOD process. One case study is about wastewater reclamation with FO and Red Sea seawater. In this case, the first objective was to achieve a new design for efficient FO configuration; the second objective was to study feed waters and draw solutions to achieve an optimal coupling of FO and RO. To achieve the objectives, a custom-made plate and frame configuration for the membranes was designed, implemented, and studied. The chapter summarizes FO operating conditions, performance, and key issues related to design/management, future trends, and research directions of FO for desalination.

This chapter reviews state-of-the-art treatment of shale-gas-produced water with a focus on the treatment of shale-gas flow-back water (SGW) by forward osmosis (FO). It briefly introduces the origin and chemical/physical characteristics of SGW. Management of shale-gas wastewater depends on multidimensional criteria, e.g., local regulations, site conditions, water quality, and economic feasibility. Approaches used to treat high-salinity wastewater include deep well injection, transport and centralized treatment, treatment and disposal, and reuse. The chapter analyzes the advantages and limitations of potential treatments methods and summarizes the process parameters and selection of membrane and draw solutions. Studies were performed to examine the effectiveness of FO in treating high-salinity wastewater produced from shale-gas exploitation. The chapter discusses the potential for using the FO process to treat SGW on a large scale.

This chapter reviews the current state of knowledge on the rejection of emerging trace organic chemicals by forward osmosis (FO) processes, beginning with a brief introduction to the occurrence of emerging trace organic chemicals in municipal wastewater effluent. Due to the frequent detection of emerging organic chemicals in secondary treated effluent, a multibarrier concept is widely utilized in the design of wastewater reclamation processes to ensure the quality of product water. The chapter outlines three major aspects to delineate how they affect rejection of trace organic chemicals; process parameters include properties of membrane and draw solution and operating conditions, membrane fouling, and FO-based hybrid processes. Potential benefits of the FO-based hybrid systems extend beyond better product water quality and energy and cost savings. The chapter ends with several concluding remarks that strengthen the potential of FO in effectively dealing with emerging trace organic chemicals.

This chapter explains the concept of the fertilizer-drawn forward osmosis (FDFO) desalination process and evaluates the potential of fertilizer solutions as draw solutions (DS). Different types of fertilizers are used to grow crops, and hence understanding what types of fertilizers are more suitable for the FDFO desalination process and how to screen and assess suitable fertilizer candidates for use as DS in the FDFO desalination process is important. FDFO desalination adds value to irrigation water and provides more opportunities for improving the efficiencies of water and fertilizer use. The major limitations of the FDFO desalination process are the challenge of meeting the irrigation water quality standards in terms of nutrient concentrations, which limits the direct use of FDFO product water for fertigation. Several options are discussed that could be integrated with the FDFO desalination process to reduce the final nutrient concentration closer to an acceptable limit.

This chapter examines the integration of forward osmosis (FO) with other water treatment methods with a particular focus on integrating FO and reverse osmosis (RO) technologies. It discusses the limitations of RO in relation to the quality of the feed water, in particular for the treatment of mining wastewater. In dry conditions, water management must focus on minimizing dependence on high-quality water. Adapting coal handling and preparation plants and dust suppression to use highly saline water reduces the volume of raw water that needs treatment at mine sites. The chapter presents different modes of integration and examines the advantages and limitations of different hybrid configurations. The hybrid system could potentially eliminate the extensive pretreatment required for conventional RO processes and thereby the overall cost of desalination. The mine site application of hybrid FO-RO technology has yet to be investigated.

This chapter presents an overview of pressure-assisted osmosis (PAO) processes and recent approaches aiming at the enhancement of water flux and reduction of reverse solute flux. It highlights the potential of PAO for desalination and wastewater reclamation. The reverse salt flux (RSF) in PAO has been reported to be lower than that in FO, due to the effect of the hydraulic pressure. Accordingly, the application of hydraulic pressure represents an improvement of the FO process, not only by enhancing water permeation but also by limiting the negative impact of RSF, one of the major limitations of FO technology. PAO membranes should be redesigned to have enough mechanical strength under intermediate or high pressure. Consequently, additional research on the design and operation of PAO systems should be pursued in parallel with the development and application of new membranes and spacers for PAO.

Salinity gradient energy (or osmotic power) has huge potential as a sustainable blue energy source. This chapter focuses on membrane development for osmotic power generation via pressure-retarded osmosis (PRO) processes. It discusses the concept of osmotic power generation and the fundamentals of PRO in terms of osmotic processes, water and salt permeabilities, concentration polarization, and power density. Theoretically, forward osmosis and PRO membranes share similar osmotic mechanisms, and both require highly selective membranes with high water and low salt fluxes. Materials with enhanced water transport properties and membranes with superior semipermeable characteristics and robust mechanical properties become absolutely necessary to harvest osmotic power from salinity gradient systems. The chapter reviews preferential types of PRO membranes and some recent exciting developments. The impressive mechanical stability and attractive power density suggest the great practicability of the newly developed composite membranes for harvesting osmotic energy via PRO processes.