A comprehensive review of forward osmosis and niche applications

Lijo Francis , Oluwaseun Ogunbiyi , Jayaprakash Saththasivam , Jenny Lawler and Zhaoyang Liu *
Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, PO Box 34110, Doha, Qatar. E-mail: zliu@hbku.edu.qa

First published on 10th June 2020

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Lijo Francis

Dr Lijo Francis is currently working as a Scientist at Qatar Environment and Energy Research Institute (QEERI), Doha, Qatar. He received his PhD in Chemical Sciences and Technology from the University of Genova, Italy in 2010. Then he pursued his postdoctoral research at the National University of Singapore (NUS, Singapore) and at the King Abdullah University of Science and Technology (KAUST, Saudi Arabia). Dr Lijo Francis has received several renowned awards including Young Scientist Award (from Leed's University, UK in collaboration with Calicut University, India in 2009), Best Research Paper Award from European Desalination Society (EDS 2014, Cyprus), Young Academics Award from European Membrane Society (EMS 2015, Italy) and a couple of Seed Fund Awards for his outstanding research contributions to the Membrane Science and Renewable Energy Driven Water Reclamation Processes. Dr Francis is the author of 8 U.S/European patent applications/PCT and more than 80 international conference/journal research papers.

Oluwaseun Ogunbiyi

Oluwaseun Ogunbiyi is currently a research scientist at QEERI (Qatar Environment and Energy Research Institute), Hamad Bin Khalifa University (HBKU), a member of the Qatar Foundation. He completed his doctorate research in Chemical Engineering at the University of Nottingham in 2007. He has up to 13 years of core industrial experience as a senior process engineer at Wessex Water & GENECO and technical consultant at CH2M within the water and wastewater sector in the United Kingdom. He was actively involved as lead process consultant in the design, construction and operation of several sewage treatment works in the south west region of the UK. His research interests include membrane-based treatment technologies, domestic and industrial wastewater reuse applications and process improvement. He has publications related to his work in peer reviewed journals including Chem. Eng Research and Design and Journal of Membrane Science and Desalination.

Jayaprakash Saththasivam

Dr Jayaprakash Saththasivam is working as a Scientist at Water Center, Qatar Environment and Energy Research Institute. He received his PhD degree in Mechanical Engineering from the National University of Singapore. His research interest are mainly in developing efficient treatment for wastewater and reuse. He has vast experience in designing heat and thermal systems and automated performance monitoring and control systems as well as building pilot plants related to wastewater treatment.

Jenny Lawler

Dr Jenny Lawler is the Senior Research Director of the Water Center at QEERI. She received her PhD in Membrane Separations from Dublin City University, Ireland. Her research focuses on the development of novel technologies and membrane-based strategies to solve environmental problems. She is particularly interested in the targeting of contaminants of emerging concern in the environment, especially those that have impact on human health. QEERI is at the forefront of development of technologies for protection of public health including treatment of water and other environmental matrices.

Zhaoyang Liu

Dr Zhaoyang Liu is Senior Scientist in Qatar Energy and Environment Research Institute (QEERI), Doha, Qatar. He received his PhD degree in Environmental Engineering from Dalian University of Technology, China. His research interests involve wastewater treatment and water desalination through process optimization and material development. He currently is engaging in multiple research projects as a lead for industrial and municipal wastewater treatment, with the subjects of oil/gas produced water treatment, purification of treated sewage effluent, and recycling of building grey water. He also holds 6 US and PCT patents related to water treatment.

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Water impact This article critically reviews the recent improvements in FO technology, and highlights the niche applications where FO shows distinct promise over mature technologies in desalination, municipal/industrial wastewater treatment, fertilizer-based solutions for agricultural irrigation, etc.

1. Introduction

Water scarcity is a pressing global issue and has driven the rapid development and use of various technologies to tackle it. Membrane technology is one of the major technologies used for water desalination and wastewater treatment due to the high quality of its product water after membrane filtration. Forward osmosis (FO) is an evolving and promising membrane-based water treatment process, which has been studied intensively from both academia and industry perspectives in recent decades.¹ When compared with pressure-driven membrane processes, the driving force in the FO process is the osmotic pressure gradient between both sides of a semipermeable membrane, which allows water to permeate from a low osmotic pressure region to the high osmotic pressure region without any external hydraulic pressure. Solutions with low and high osmotic pressure are known as feed solutions (FS) and draw solutions (DS), respectively. Water permeates from the FS to the DS through the FO membrane during the osmotic process until the osmotic pressure of the FS and DS reaches equilibrium. Thus the DS gets diluted and the FS gets concentrated during the process. It has been reported that some of the advantages of FO include lower membrane fouling propensity and greater energy efficiency compared to the reverse osmosis (RO) process.^2–5 FO has been applied to various areas including desalination, wastewater treatment, food processing, district cooling and power generation amongst many others.^6–9

Despite the high potential that the FO process has, it has not yet been fully commercialized due to several challenges, such as low membrane flux, concentration polarization, reverse solute flux, and high energy cost for the DS regeneration process.¹⁰ The successful implementation of the FO process in industrial applications relies heavily on improvements in multiple aspects, including membrane properties (water flux, concentration polarization and fouling mitigation), draw solutes (osmotic pressure, reverse solute flux and regeneration), and hybridization of the FO process with other processes.^1,11 All of these aspects need to be improved collectively, and the improvement in one aspect might not necessarily be good enough for FO to become commercially competitive with other technologies.

In this review article, we extensively discuss the aspects of membrane properties, draw solute selection, fouling mitigation, and various niche applications of FO. This article highlights comprehensive information related to: (i) the development of different approaches for the fabrication of FO membranes; (ii) various draw solutions employed for FO process operation, i.e. draw solutions based on the categories of regeneration, direct use and disposal; (iii) an understanding of membrane fouling factors and their mitigation strategies; (iv) identification of niche applications of the FO process, which provide a comprehensive overview on the landscapes where FO could outperform other technologies. The niche applications include desalination, brine management, zero liquid discharge (ZLD), wastewater treatment and reuse, produced water treatment, food and dairy industry, chemicals and pharmaceuticals industry, electronics industry, district cooling and irrigation. Fig. 1 shows an illustration of the various components of the FO process. After the niche applications section, the article delves further into the recent commercialization approaches of the FO process at various locations. Finally, the article describes the life cycle cost analysis (LCCA) and techno-economic feasibility of FO and hybrid FO processes for different applications that have been investigated by various authors. This LCCA and the techno-economic feasibility study are very important exercises that are undertaken in order to advance this technology to the market.

2. Forward osmosis membranes

Many industries including desalination and wastewater treatment are adopting membrane-based technologies due to their advantageous features, such as low energy consumption, small footprint, easy scale-up, etc. FO is a membrane-based low energy water separation process. An ideal FO membrane is characterized by a highly water permissible (A) active layer, which could minimize the reverse solute permeation (B), and a support layer which is capable of enhanced mass transfer and minimized concentration polarization effects.^11,12 An efficient FO membrane must be mechanically stable, chemically resistant and less vulnerable to the fouling phenomenon. In water desalination FO processes, salt rejection is one of the important membrane criteria where an RO or nanofiltration (NF)-like active layer is required and explored in FO membranes to reject monovalent and multivalent ions or other large molecules, respectively.^3,13 Several polymeric materials have been used to fabricate FO membranes. In the initial phase, some of the more commonly explored FO membrane materials include cellulose acetate (CA), cellulose triacetate (CTA), polyethersulfone (PES), and polybenzimidazole (PBI).^14–23 At the later phase, FO membranes were fabricated using polyamide thin film composites (TFC) via interfacial polymerization (IP) on polysulfone-based substrates, sulfonated substrates, cellulose acetate propionate (CAP) substrates and non-woven electrospun nanofibrous substrates.^18,24–27 The most common methods used for the fabrication of FO membranes are the non-solvent induced wet-phase inversion (NIPS) method formulated by Loeb and Sourirajan²⁸ and thin-film composite (TFC) formation on porous substrates via interfacial polymerization introduced by Cadotte.²⁹ In TFC membranes, thin-films are developed on the porous substrate by the in situ IP reaction between two monomers: namely, aqueous polyfunctional amines and apolar acid chloride solutions. TFC membranes show better FO process performance compared to conventional cellulose-based membranes. Other FO membrane fabrication methods are the layer-by-layer (LbL) coating of very thin layers of cations and anions on a charged porous support layer,^30,31 and fabrication of biomimetic FO membranes by the incorporation of aquaporin.^32,33 The most commonly used FO membrane materials and FO membrane fabrication methods are shown in Fig. 2. The state-of-the-art of the design and fabrication of TFC-FO membranes with their essential characteristics is depicted in Fig. 3.

Breton, in his dissertation entitled “water and ion flow through imperfect osmotic membranes” published in 1957, mentioned the theoretical hypothesis and experimental evidence of the semipermeable behavior of a cellulose acetate membrane in saline water.³⁴ In 1963, Loeb and Sourirajan reported a phase inversion technique for the fabrication of osmotic membranes from a casting solution of cellulose acetate, acetone and aqueous magnesium perchlorate. The fabricated membrane was tested for the demineralization of seawater and it was found that the membrane was capable of producing a flux of 5 to 11 gallons of 0.05% NaCl water per sq. foot per day from a brine solution containing 5.25% NaCl, under a pressure of 1500 to 2000 psig.²⁸ In 1976, Kessler and Moody reported the extraction of potable water from seawater using a concentrated solution of nutrients and a semipermeable membrane. They used CA membranes for fresh water reclamation from low quality water into a concentrated glucose–fructose solution, and compared the results with the predicted values.³⁵

In 1965, Batchelder utilized a cellulose-based semipermeable membrane and dissolved volatile solutes such as sulfur dioxide in seawater or fresh water as the DS and seawater as the FS, in order to facilitate the FO process.³⁶ Hydration Technology Innovation (HTI), established in 1986, commercialized the CTA FO membrane with a net or nonwoven support. The initial focus was on separating liquids and concentrating beverages and food products, with later development of flat sheet, spiral wound, and hollow fiber membrane elements for a wide range of water reclamation applications. HTI membrane water filtration systems were successfully employed by global military forces and humanitarian disaster relief organizations especially for the reclamation of fresh water from impaired quality storm waters.³⁷

In 2005, McCutcheon et al. demonstrated a novel FO desalination process using an ammonium bicarbonate DS in order to collect fresh water from an impaired quality FS through a semipermeable polymeric FO membrane. A high osmotic gradient across the FO membrane was created by the highly soluble ammonium bicarbonate DS, which yielded high water fluxes. Ammonium bicarbonate decomposes into ammonia and carbon dioxide gases upon moderate heating which can be separated and recycled as draw solutes, leaving the fresh product water.³⁸ The FO process followed by a second step for the regeneration of the draw solute is known as indirect forward osmosis.

Most of the aforementioned investigations on the FO process were performed using cellulose acetate-based RO membranes and cellulose triacetate-based FO membranes from HTI. However, the resultant water flux was much lower than theoretical or predicted values due to several reasons such as internal and external concentration polarization effects (ICP and ECP). The FO process performance is characterized by intrinsic parameters such as the A, B and S values of the membrane. All of these values are primarily dependent on the membrane structure and solute type. Cath and coworkers have developed a common methodology in order to compare the membrane performance from the literature.^39,40 Recently, there has been a significant development of CA and CTA-based flat sheet FO or hollow fiber FO membranes.^{15,18,19,21,41–44}

The membrane structural parameter (S) is a key factor in determining FO membrane and process performance. Although S is an intrinsic characteristic of a membrane, it has been used to decide the internal concentration polarization (ICP) phenomenon within the support layer of the FO membrane. The S value is constant for a specific system and varies with respect to the FS and DS concentrations. Recently, many researchers have investigated the FO process in order to optimize the S value. The S value can be determined by using the following equation:

where A is the water permeability (L m⁻² h⁻¹ bar⁻¹), B is the salt permeability (kg m⁻² h⁻¹), D is the salt diffusion coefficient, J_w is the water permeation flux (L m⁻² h⁻¹⁾), and π_ds and π_fs are the bulk osmotic pressures of the DS and FS, respectively.

Water permeability, A, is given by the equation;

A = Jw/ΔP

where ΔP is the transmembrane pressure.

Jw = Q/AΔt;

where Q is the volume of water permeate collected over a period of time Δt, and A is the effective membrane area.

In order to enhance the FO process performance by tuning the membrane structural parameters, researchers have introduced much advancement via chemical/physical membrane modifications and employing novel membrane materials and draw solutions. Several research groups have reported the achievement of high water flux by using a specially engineered TFC-FO membrane.^24,45–48 Many researchers are interested in hollow fiber FO membranes especially because of the fact that they can be used for high volume applications due to their high packing density and large surface area to volume ratio which results in small module footprint.^49–59 Bui et al. and Song et al. have reported the fabrication of highly porous nanofiber supports for high performance TFC-FO membranes.^26,60 Toyobo introduced a large-sized FO hollow fiber membrane prototype in Singapore International Water Week in 2014.⁶¹ According to Bui et al., the nanofiber support structure has high porosity and highly interconnected pore structure which cause the reduction of ICP in an FO process. It was demonstrated that the nanofiber-TFC membranes yielded 2- to 5-fold higher water permeation than the commercial HTI CTA membranes. Zhao et al. discussed a novel TFC-FO membrane fabrication approach with an intermediate layer of graphene oxide and multiwall carbon nanotubes, which offers a limited CP phenomenon with expected separation performance.⁶² Song et al. correlated water permeation with the structural parameter (S) of the membrane. As per their findings, the S values of the HTI-FO membrane, polyimide (PI) TFC-FO membrane and two different nanofiber TFC-FO membranes were calculated to be 620 ± 80, 450 ± 50, 106 ± 8 and 80 ± 6 μm, respectively. At the same time, the FO water flux values (in L m⁻² h⁻¹) while using those membranes were observed to be 6.5, 13.4, 33.9 and 37.8, respectively, and as such they concluded that the S value has a significant role in determining the FO water flux. The sponge-like structure of the support layer of the PI-TFC-FO membrane possibly blocks salt diffusion and results in a lower net transmembrane osmotic pressure difference, whereas salt diffusion is highly favorable in the support layer of electrospun nanofibrous TFC-FO membranes due to their interconnected pore structure, which results in a higher net osmotic pressure gradient and enhanced flux.

Tang and co-workers fabricated high flux FO membranes using an LbL structure of poly (allyl amine hydrochloride) (PAH) and poly(sodium 4-styrene-sulfonate) (PSS) on a porous poly acrylonitrile (PAN) substrate.^30,31 They have reported a FO water permeation flux of ∼100 L m⁻² h⁻¹ with the active-layer-facing-DS side using 2 or 3 M magnesium chloride as the DS and fresh water as the FS. Thus, the LbL method is very promising for the production of high permeation FO membranes. Recently, researchers showed great interest in biomimetic aquaporin membranes used for desalination and reuse applications. Aquaporin is reported to be the protein responsible for transporting and purifying water in living cells, and to be super-efficient and selective.^63–65 High water permeability and high salt retention are very important features of aquaporin proteins incorporated in the FO membranes.^32,33 In 2018, Ren and McCutcheon evaluated a novel commercially available hollow fiber FO membrane made using aquaporin proteins and reported that the membrane was good enough for application-based research and benchmarking purposes, with improvements in speed and in energy efficiency.⁶⁶ Recently, the modification of conventional membranes by blending them with nanostructured materials has been a new trend in the fabrication of efficient and fouling resistant membranes.^67–70 Apart from HTI, Aquaporin, Porifera, Modern Water, Trevi, FTSH2O, Toyobo and Oasys are the different start-ups that have been introduced into the market or are under development in the arena of FO commercial membranes and systems. Aquaporin introduced biomimetic hollow fiber and spiral wound membrane configurations, Toyobo introduced very special hollow fiber FO membranes and all other mentioned suppliers have introduced a spiral wound FO membrane configuration that could be deployed in the FO system. Recent investigations are based on nanostructured and fouling resistant TFC-FO membranes with hollow fiber and spiral wound configurations. The niche applications using those systems will be discussed in a later section of this article.

3. Draw solutions for forward osmosis

Forward osmosis is an osmosis-driven membrane separation process, which relies on concentrated draw solutions to draw fresh water across a semi-permeable membrane from feed solutions. Unlike pressure-driven membrane separation processes, the product of an FO process is not pure water, but a diluted draw solution. Therefore, a second process is often needed to regenerate draw solutions and simultaneously produce pure water. But there are some exceptions where the second process of draw solution regeneration becomes unnecessary, when diluted draws solutions can be directly used, or discharged if the FO process is purely used to concentrate feed solutions rather than produce pure water, as shown in Fig. 4. Since the regeneration process of draw solutions is normally considered as a cost-intensive process, the selection of draw solutions critically affects the practical applications of the FO process. An ideal draw solution is expected to have the following merits: 1) high osmotic pressure; 2) low reverse solute flux; 3) high diffusion coefficient to reduce ICP; 4) low cost for draw solute materials; 5) low cost for the regeneration process; 6) low toxicity. However, there may be some contradictions between these merits. For instance, draw solutes of small size, such as NaCl, tend to have high osmotic pressure and reduced ICP, but their sizes also cause the issue of high reverse solute flux. From the perspective of practical applications, a comprehensive overview of various draw solutions that have been reported in the literature will be provided. The draw solutions will be summarized based on the categories of regeneration, direct use and disposal, as shown in Table 1.

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3.1 Draw solutions with the need for regeneration

Draw solutions based on different regeneration methods were reviewed, including thermal separation, membrane separation (RO, NF, ultrafiltration (UF), thermal/membrane hybrid (membrane distillation (MD) separation), chemical precipitation and stimuli-responsive (e.g., heat, magnetic field) separation.

3.2 Draw solutions without the need for regeneration

4. Fouling and its mitigation in forward osmosis

Membrane fouling is an undesired complex phenomenon that is caused by the deposition of various types of organic and inorganic molecules/particles leading to the loss of membrane performance and efficiency.⁹² Adsorption of solutes/colloids and deposition of floc on the membrane surfaces and pores as well as formation of cake layers are among the fouling mechanisms that take place in a membrane.⁹³ Fouling adversely affects the membrane selectivity, contributes to the loss of permeate flux and quality, and decreases the life span of membranes.⁹⁴ It also leads to higher operating and maintenance cost as fouled membranes require frequent cleaning cycles. As shown in Fig. 5, the common governing factors of membrane fouling are (i) membrane properties, (ii) operating conditions, and (iii) feed water quality. There are many instances where fouling propensity is exacerbated by the combination of these factors.

Fouling in membranes can be broadly categorized into three different groups as shown in Fig. 6. They are: – (i) inorganic fouling, (ii) organic fouling and (iii) biological fouling. Inorganic fouling, which is commonly referred to as scaling, is caused by the precipitation of inorganic salts and metal hydroxides.⁹⁶ Scaling exacerbated by concentration polarization is a critical issue in NF and RO membranes, which usually deals with the removal of divalent and monovalent ions. As for the FO membrane, silica-scaling resulting from the polymerization of silica is one of the common inorganic foulants that hamper its performance. Organic fouling, on the other hand, is caused by the presence of natural organic matter such as polysaccharides, proteins, and fatty and humic acids in the feed water. This directly implies that organic fouling is likely to be the most dominant foulant in wastewater treatment systems. The adsorption of these organic compounds on the membrane led to the formation of a gel-like layer that increases the flux resistance.⁹⁷ The presence of organic fouling and biological fouling can be directly interlinked and correlated in the majority of fouling cases as organic substances act as precursors of biofouling.⁹⁴ Biofouling, which is defined as deposition, growth, and metabolism of bacteria cells or flocs on the membranes, is usually difficult to mitigate due to the protective extracellular polymeric substance layer.

In general, it is well-known that the impact of fouling on the performance of a forward osmosis membrane is less severe than on a reverse osmosis membrane due to its osmotic driven process. The low operating pressure of FO membranes plays a key role in lowering fouling tendencies as it helps to minimize the deposition of foulants and reduces cake layer compression. Yu et al. reported that the reduction in flux of the RO membrane was nearly twice that of the FO membrane.⁹⁸ Biofouling layers formed in FO membranes are generally looser than layers formed in RO membranes.⁹² Due to the loose attachment, biofouling can be sheared off from the surface of membranes. Despite their lower fouling tendencies resulting from minimal operating pressure, FO membranes are still susceptible to fouling governed by membrane properties and poor feed water quality. A comparison between CTA and TFC polyamide FO membranes by Wang et al. showed that the CTA membranes have a higher biovolume and thinner biofouling layer. They concluded that CTA FO membranes are more prone to biofouling, while TFC-based FO membranes are more susceptible to inorganic fouling. As FO membranes have higher fouling tendencies due to organic and biological fouling, the subsequent section outlines the common mitigation strategies used to address these two issues.

4.1 Organic fouling and biofouling and mitigation strategies

The mitigation strategies deployed in minimizing organic and biofouling in FO membranes are shown in Fig. 7. The common methods are as follows: (i) physical cleaning and process modifications, (ii) pretreatment, (iii) chemical cleaning and (iv) development of novel materials to minimize biofouling.

5. Recent developments and niche applications

There is increased global focus on water sustainability and focus on recycling and reusing produced water is gradually coming to the forefront of the agenda of the oil and gas industry. Water scarcity and water security issues are intrinsically related to the adverse environmental impacts of injecting produced water into disposal wells. These recent trends are a reflection of the need for produced water management including a fundamental improvement and subsequent implementation of these treatment technologies.¹²⁰ Independent research across the world in recent years has covered the novel application of forward osmosis technology to minimize the volumes of produced water generated for injection into disposal wells. This research has an impact on a global scale and is important in areas where there is increased oil and gas activity.

Forward osmosis-based technology applications cover a wide range of fields in industry that include bench-scale laboratory experiments and full-scale pilot demonstrations. Some of the practical uses of FO that are covered within this section range from the food and beverage industry, pharmaceutical and juice concentration, wastewater reclamation, brackish water/seawater desalination, fertigation in agriculture, zero liquid discharge (ZLD) concept and finally industrial scale-up and other industrial processes.^121,122 Depending on the application, implementing FO may have a detrimental effect on operational power requirements. In recent years, improvements in FO systems, including draw solution testing and membrane material development, have enabled the production of more flux whilst reducing the occurrence of the bane of membrane-based technology, internal concentration polarization, thus improving energy requirements and fouling effects.

There is huge potential for FO to be used as both an independent and a hybrid system, in conjunction with other existing technologies. Some of the applications for utilizing the benefits of forward osmosis include the food and beverage industry, chemical industry, coal processing, textile industry, pulp and paper industry, pharmaceutical industry, electronic industry, micro algae cultivation and car manufacturing, amongst many others. There have also been a number of peer reviewed FO publications about heavy metal elimination and cooling water treatment that have been identified and reviewed. The up-scaling to pilot or full-scale will be the essential next step for many of these processes and their reliability will depend on a number of factors including membrane performance, new methods for draw solution and feed solution regeneration, etc. Long-term fouling behavior, membrane cleaning methods, and operation procedures are also very important parameters that will need further assessment studies.

5.1 Desalination & wastewater treatment

The increasing frequency of water shortages around the world coupled with strict environmental policies in many countries and the movement towards sustainable development and the increasing demand for available freshwater resources make the ZLD technology a valuable wastewater treatment option. It is an important strategy for sustainable management of wastewater. There has been substantial investment in ZLD technology. Forward osmosis has increasingly become an attractive salt concentrating technique.¹²³

The forward osmosis system has been shown over time in various research studies to work effectively for the rejection of different pollutants and recovery of nutrients from various types of wastewater. The results from Chen et al. confirmed that using concentrated sewage could replace the anaerobic digestion feed and produce a high quality final effluent. However, there are still problems that need to be addressed to make the FO process practical for deployment in this particular area.¹²⁴

In recent years, a number of hybrid FO systems have been incorporated into existing or combined processes for various applications including seawater/brackish water desalination and wastewater treatment to replace conventional pre-treatment technologies or as post-treatment to reduce the extra volumes of industrial waste. The combination of FO and RO in a hybrid process, where FO uses a wastewater stream to produce high-quality water to dilute seawater prior to an RO step, is very feasible. In another study as documented by Pan et al., a forward osmosis–membrane distillation hybrid process was utilized to remove tetracycline from wastewater, with a rejection of 99.9% and a water recovery of 15–22%.¹²⁵ In addition to water recovery, FO can be used in wastewater treatment for nutrient and energy recovery.⁸

Some of the very useful applications of forward osmosis are the added value of integrating seawater desalination and municipal wastewater treatment to produce a much cleaner water product. This technology has shown relatively high rejection rates for many variables being monitored at treatment plants including COD, phosphate, trace metals and an average rejection rate for ammonia and total nitrogen. Forward osmosis has also proven to be a very reliable barrier in the rejection of a host of contaminants and salts found in wastewater that can be utilized as the feed solution or in seawater as a draw solution. The combination of municipal wastewater treatment and seawater desalination creates the added benefit for a considerable and sustainable energy-saving approach to a more sustainable urban water management and water reuse strategy. One of the main drawbacks in desalination technology is related to separation and recovery of the draw solution from the desalinated water. The successful implementation of forward osmosis related desalination in subsequent years will be reliant on the effectiveness of the draw solute recovery from desalinated water. Some of the industrial applications are detailed in Fig. 9.

Research has also been carried out using FO membranes with secondary wastewater effluent utilized as feed water and seawater as a draw solution, with results showing huge strides and indicating that the technology is encouraging. Indirect desalination using a hybrid system comprising forward osmosis/low-pressure reverse osmosis (FO/LPRO) has also been implemented, which requires only half of the energy used for normal high pressure RO desalination technology. The results show that very high quality product water can be realized from the feed.¹²⁶ One of the most commonly utilized desalination technologies currently employed, especially in the arid gulf region, is seawater reverse osmosis (SWRO). Scaling up of a FO/LPRO system for several industrial applications, and in particular seawater desalination, has been investigated as a cost effective solution for producing fresh water and minimizing fouling problems. The low energy requirements that are needed to remove water from the feed solution make forward osmosis a very attractive technology solution.¹²⁷

A working experimental procedure was developed in the laboratory for a novel “single-step” forward osmosis process concept where reject brine from thermal desalination plants was used as the draw solution to separate clean water from produced water, hence reducing the total volume of water generated. It runs in an osmotic dilution method of operation which guarantees low energy consumption.¹²⁸ It is also important to note that the process water realized is a mixture of offshore produced water and process water from onshore gas liquefaction operations. With regard to this application, one of the key advantages of the FO process over other conventional processes is the lower CAPEX and OPEX. The need for high pressure pumps is eliminated and this reduces electrical energy consumption.^1,129 FO desalination processes have a unique collection of potential advantages if they are installed on a large scale and transferred to a commercial scale. The low hydraulic pressure that is commonly applied whilst running an osmotically-driven FO process is responsible for the added advantages. It is also synonymous with low energy consumption which drives down associated costs if the draw solution recovery step is economically and technically sustainable.¹³ FO membrane contactors can also be used to remove natural steroid hormones from wastewater.¹³⁰ The basic concept for the desalination process involves two steps, the first is FO and the second is a recovery and separation step using RO. Researchers have laid claim to the fact that this process has lower energy consumption for desalination applications than a reverse osmosis plant coupled with advanced pre-treatment including ultra-filtration, when used to treat high salinity water. The RO step in the FO/RO combination operates at much higher recovery ratios than in a single RO step.

In this particular study, a novel module design to integrate forward osmosis (FO) and membrane distillation (MD) was proposed and investigated with a view to using a FO–MD novel integrated system for domestic wastewater treatment applications. The FO–MD integrated module does not require additional pumps, pipes or an intermediate tank and other ancillary equipment to ensure the flow circulation in both modules.¹³¹ The integrated FO–MD system captures the complementary advantages provided by FO and MD in a single step. However, this hybrid system has a serious potential problem associated with the configuration having the FO DS and the MD FS flowing in the same channel simultaneously, that is, the same stream in contact with the MD and FO membranes. The newly proposed integrated module has two separate flow channels within the same module, separating the FO DS and MD feed streams using an isolation barrier inserted inside the module with an opening on the top, allowing the MD brine (MD outlet) to flow in the FO DS inlet side. This feature makes the integrated module osmotically and thermally isolated, which ultimately increases the efficiency of both processes. However, the FO–MD hybrid can compensate for the individual weakness of FO and MD membranes and hence enhances the rejection of all contaminants.¹³²

The higher performance of the proposed integrated module will reduce the required membrane surface area compared to the typical integrated module. In order to evaluate the novel FO–MD integrated module, a number of iterations and investigations were carried out based on mass balance equations varying four different parameters: FO and MD recovery rates, initial DS flow rate, and initial DS concentration. The results showed that a high DS flow rate was a prerequisite for a high recovery rate in the FO–MD integrated module. The results and findings from the simulations show that the novel FO–MD integrated module can have very beneficial use for small-scale or mobile water and wastewater treatment plants. A very good example of the application of the integrated module is the treatment of hypersaline wastewater from shale gas fracking due to easier membrane fouling control and high rejection of volatile compounds and other toxic metals.

5.2 Water reuse

FO membranes across various applications were tested and investigated to determine their ability to reject a wide array of organic micro pollutants. The rejection rates across these investigations yielded values within the range of 40% and 95% for hydrophilic neutral compounds and hydrophobic neutral contaminants but were significantly higher, up to 99%, for hydrophilic ionic micropollutants. A hybrid system of FO and LPRO has also shown high efficiency in rejecting low molecular weight hydrophilic neutral micropollutants, with greater than 89% rejection. A hybrid FO/LPRO system also acts as an effective obstacle against micropollutants, including pharmaceuticals, hormones and other micro organic pollutants.¹²⁷ Forward osmosis has been adapted as an osmotic dilution process using seawater as the draw solution for water purification within a hybrid FO/RO process.¹³³ There are other configurations of FO/RO desalination systems that have been proposed to generate both potable water¹³⁴ and the osmotic power of brine from RO. Within these dynamic configurations, FO gives high quality drinking water as a result of the double layer protection, reduced RO fouling via pretreatment, recovery of osmotic energy of RO brine, reduced energy consumption and no pre-treatment requirements.

5.3 Produced water treatment & enhanced oil recovery

Collaborations between two companies, Bear Creek Services and Hydration Technology Innovations, LLC, with respect to treating wastewater being generated from oil & gas operations to assess the reliability of forward osmosis as a viable technology to treat drilling fluids from shale gas operations showed a high recovery value of around 90%.¹³⁵ Oasys Water Inc. was another company that carried out a feasibility study and assessed the use of forward osmosis as a technology option to treat flow back process water from shale gas operations using ammonium carbonate as a draw solution. Some of the results showed an average of over 60% reduction in volume of water.⁷⁴ Bench scale investigations using forward osmosis for treating produced water was carried out on drilling fluid in shale gas operations where the reduction in the volume of water was up to 80%, with TOC rejection greater than 99%.¹³⁶ The hybridization of forward osmosis and membrane distillation involved using an integrated FO–MD system for increased generation of process water from oily water mixtures. The combination of these technologies showed very effective treatment of oil and gas produced wastewater to produce potable water. This hybrid system had certain advantages that made it feasible to overcome many of the adverse effects of each process on its own function.¹³⁷ In an RO–MD hybrid system, clean water is recovered from oily effluents using RO brine as the feed constituent. The diluted brine is regenerated so that it can then be reused as a draw solution and also to produce up to 90% water recovery with little or no organics in the effluent.¹³⁷ Forward osmosis has been proved over the years via independent research by scientists and engineers as a viable option to treat high-salinity produced water. Forward osmosis is one of the few approaches considered to be a viable option to treat the high salinity of produced water solutions.¹³⁸ However, there needs to be some form of pretreatment as a step before using forward osmosis as a remediation technique to reduce the tendency for fouling. It was concluded that forward osmosis could be used to extract high quality water from high-salinity oil field brines and produced water.¹³⁹

5.4 District cooling

Forward osmosis is a technology that provides a low TDS make-up water supply to a recirculating cooling water system acting as an osmotic agent. The economics and robustness of the process are value added points to its implementation and practice; however, the relative emergence shows that it may take some time for the technology to be readily accepted by industrial stakeholders in this sector. The evaporative cooling process requires vast amounts of very high quality water to replace the water lost by evaporation, drift and consequently, blowdown. Desalination processes and the more recent utilization of treated sewage effluent, especially in Qatar and in other gulf countries, are the source of cooling and make up water. The recirculating feed water is processed into a draw solution and subsequently, the make-up water is pulled across a suitable forward osmosis membrane. There is contamination of the draw solution from ions, which are transferred across the forward osmosis membranes. The source of these ions is most likely the contaminants in the atmosphere. In order to retain the draw solution but filter out the target contaminants, a blow down recovery system is required and this has been developed and demonstrated.¹⁴⁰

5.5 Agricultural irrigation/fertigation

Fertigation is an agricultural irrigation process that is employed on a wide scale where water soluble fertilizers are added to the irrigation water in order to improve and enhance crop yield. The concept of fertilizer drawn forward osmosis (FDFO) for fertigation has been offered as a viable option for reducing the volumes of potable and desalinated water used in the process. The most interesting finding related to this study was that the diluted draw solution could be directly utilized and applied to irrigation without the added step of separation.⁸⁴ This process is an example of osmotic dilution and there have been investigations that have been carried out to evaluate the performance of fertilizer draw solutions using low energy fertilizer-driven forward osmosis desalination. Some of the issues that were realized in this application were the over concentration of the fertilizer after it has been diluted and the eutrophication of the receiving water body due to the presence of amounts of nitrogen and phosphorus. FDFO contributes to preserving fresh water supplies by reducing the volumes of desalinated water and fresh water used to dilute liquid fertilizer concentrates in the fertigation process. However, it is faced with quite a number of challenges which may hinder its large scale deployment as a viable technology.¹⁴¹

5.6 Dairy industry

The dairy industry uses raw milk to produce many food items including long-life milk, cheese, and yogurt, amongst others. Huge amounts of wastewater are generated from manufacturing and production as well as in situ and ex situ cleaning procedures and dewatering as investigated and documented.¹⁴² The recycling of wastewater within the industry is of huge concern and there have been a number of reviews on FO lab-scale experiments conducted using artificial whey solution generated using deionized water and whey powder. The economic implications of utilizing forward osmosis were also studied and assessed and conclusions were drawn on the basis that FO required a lot less energy and less space translating into lower overall costs.¹⁴³ A hybrid FO/MD configuration was developed and assessed from a feasibility study using real dairy wastewater as the feed solution and sodium chloride as the draw solution with different FO membranes. The results were conclusive and proved that FO could concentrate real dairy wastewater under normal operating conditions and MD could obtain the desalted water at the end of the process.¹⁴³ The optimum recovery of the draw solution for applications in the dairy industry is much easier than in the instances where products show up in the draw solution, with desalination as an example. In the production of fruit juices, milk and skim milk concentration, it is necessary to concentrate the feed stream and water goes into the draw solution but water is not important as a product. This is because the final product is actually the concentrated feed stream. This ultimately makes draw solution regeneration in dairy processing more practical and feasible than in the desalination process.

5.7 Chemical industry

FO technology shows potential for niche application in the chemical industry. Organic acids such as lactic acid¹⁴⁴ or succinic acid produced by fermentation could be concentrated by FO treatment. Glucose has been examined as a draw solution, with a permeate flux of 12 L m⁻² h⁻¹, which corresponded to an 84% water recovery value. However, there was only 56% rejection of lactic acid. Several other researchers have primarily focused on using FO for the treatment of acids. In one of the studies, different carboxylic acids were concentrated by FO.¹⁴⁵ These acids are utilized in many chemical processes. For this reason, they are likely to be contained in the wastewater from chemical production processing, and could be treated using FO.

5.8 Food processing

The use of forward osmosis technology in the manufacture of sugar drinks from a seawater and brackish water mixture has been explored and implemented and is amongst very few commercial applications developed for deployment in the US military.¹⁴⁶ A sugar solution (dextrose and fructose) within a bag served as a semipermeable FO membrane where water gradually flowed through the membrane after dipping it into an aqueous suspension. It could then be utilized as an electrolyte energy drink. The use of much larger systems has been practiced in certain countries for disaster relief and emergency scenarios and conditions.¹⁴⁷ The use of FO for the treatment of olive mill wastewater was also investigated using real wastewater rich in biophenolic compounds as the feed solution and magnesium chloride as the draw solution. These experiments were run continuously over a sustained period of time and the feed solution and draw solution were replenished on a daily basis. There was fouling as a result of the time taken for the experiments but pure water permeability could be restored to 95% via rinsing and osmotic backwashing. Various pre-treatment methods were tested with this configuration of FO treatment and it was found that microfiltration was the most effective method and subsequently increased the permeate flux of FO. Particle retention by microfiltration increased the FO permeate flux.¹⁴⁸

5.9 Pharmaceutical industry

Within the pharmaceutical industry, the application and utilization of forward osmosis includes osmotic drug delivery and enrichment of pharmaceutical products as outlined in various publications.^149–151 The osmotic drug delivery systems are based on the principle of osmosis and the second area relates to the enrichment of pharmaceutical products. These pharmaceutical products are heat sensitive and possess large molecules; this means that forward osmosis technology can bring some certain advantages that conventional methods will have difficulty with. FO was also applied to the concentration of anthocyanin and the results indicated that FO possessed many advantages over the alternative thermal concentration in terms of higher stability and a lower browning index.¹⁵² The enrichment of protein solutions using an integrated FO–MD system has also been assessed and investigated.¹⁵³ It is worth noting that in the fields of food and pharmaceutical product concentration, the concentrates of FO are the target products, which is quite different from desalination and wastewater treatment. FO has also been demonstrated to be very effective for dialysis fluid regeneration.¹⁵⁴

5.10 Electronics industry

The wastewater streams generated from the electronics industry usually contain valuable and precious heavy metals that may be toxic and pose significant risks. One example that was reported by Gwak et al. was the use of forward osmosis to treat wastewater from a printed circuit board manufacturer. Palladium containing wastewater was utilized as the feed solution and was concentrated up to 90%. It was also mentioned that the presence of inorganic fouling occurred on the feed side and hence, more investigations need to be had in order to make more valid claims regarding the effectiveness of this technology in this area.¹⁵⁵

5.11 Challenges, limitations and drawbacks

Some key technical issues, which are related to the successful operation and implementation of FO technology in niche applications, still need to be addressed. Some of them are the ideal membrane material, a suitable draw solute and its recovery approach, the mechanism of the target wastewater, membrane-fouling mechanisms, cleaning strategies and protocols, which still exist as hindrances and barriers and need the focus of future research patterns. The following aspects of FO that need attention in order to fully optimize its application are shown below:¹⁵⁶

a. The realization of high water flux

b. Low reverse salt flux (RSF)

c. High mechanical strength of the membrane

d. Low ICP film material modification and preparation

e. Special modification and preparation of properties or functional membrane materials

f. Development of new composite DS

g. The membrane fouling principle and fouling cake layer formation mechanism

h. The principle and solving methods of the accumulation of pollutants and salinity in the DS

i. Membrane fouling mitigation and membrane cleaning methods and strategies

j. The preparation uniformity of membranes

k. Preparation of the draw solutes applied to the appropriate field.

One of the biggest challenges that has been identified in the thermal desalination process is the issue of scaling, where the precipitation of salts like calcium sulfate and other sulfates and carbonates will lead to process issues and reduction in process efficiency in due course. Calcium sulfate, for example, precipitates above 115 °C, which limits the top brine temperature and has an adverse effect on the overall efficiency of the distillation process.¹⁵⁷ A good example of a strategy to overcome this issue is combining membrane distillation (MD) and forward osmosis (FO) to desalinate water because it is difficult for the standalone MD process. When utilizing a hybrid FO–MD system, FO is used as a pre-treatment step to reduce organic fouling and/or inorganic scaling which usually has detrimental effects on the MD process over time.

There are a few limitations in the developmental phase of large-scale applications of forward osmosis technology. One of the main ones is the concentration polarization phenomenon, which reduces the osmotic pressure driving force and results in a much lower water flux value. More work also needs to be done to optimize the thermal desalination process and improve the overall process efficiency.

Internal concentration polarization (ICP) causes a drop in the concentration gradient created across the porous support layer, which lowers the osmotic driving force and effectively reduces the water permeability through the membrane.¹⁵⁸ This effect depends primarily on the characteristics of the draw solution (DS), membranes and operating conditions employed during the FO process.¹⁵⁹ The concentration polarization phenomenon due to the accumulation of solute on the surface of the membrane or in the pores is fouling. This happens and effectively reduces the overall membrane performance. It causes either the formation of a cake layer on the membrane surface or clogging up of the pores. This results in a very steep decline in water permeation, a reduction in separation efficiency of the membrane and a diminished hydraulic resistance.¹⁶⁰

The financial and economic aspects of the need for an additional energy-consuming process, which is a post-treatment step to recover the draw solute, are considered as another drawback of FO for use as a standalone process.¹³ In terms of membrane development for use in FO systems, some of the drawbacks that were mentioned above must be overcome to achieve the successful installation of FO applications in industry. The issues of process problems associated with the development of the membrane should contribute considerably to the improved efficiency of the FO system.

The standard operation protocol for these modules are still hindered by membrane fouling and associated increased operational costs. After testing the membranes and carrying out performance tests, they became fouled and required cleaning and regeneration. However, there were no guarantees that the membrane would return to its original state. In the desalination process, high quality water is needed as the final product. Water goes into the draw solution and the final product needs to be extracted from the draw solution, but there are various challenges that need to be addressed in draw solution recovery. The energy consumption of forward osmosis will not be economically feasible because the draw solution is more highly concentrated than standard seawater.

The development of reliable and accurate predictive models to calculate energy expenditure is still one of the areas of focus that researchers can improve on. It is important to optimize the energy consumption of the well-established FO standalone system in comparison with the hybrid system to convert the process into a more feasible industrial application for water and wastewater treatment.

The impact of ICP on the FO water flux can be very severe, in some cases even leading to a water flux decrease of over 80%. It seems clear that there is still considerable uncertainty about the optimal support morphology for FO applications. Hydrophilicity is one of the parameters whose influence on FO performance has been extensively investigated. Not only does the hydrophilicity of the membrane support assist in wetting and water continuity within the membrane, it also mitigates ICP.

Developing a membrane with a low ICP is what is needed to improve the performance of FO technology. Modifying the membrane support layer is the correct option to explore since ICP cannot be controlled by fine-tuning the operational conditions of FO.¹⁶¹ As ICP is determined by support layer structure and hydrophilicity, a lot more research and development efforts should be carried out to improve those parameters.¹⁶²

There are very specific research areas in forward osmosis that have some challenges and limitations related to wastewater treatment and they include the choice and nature of the draw solution, economic cost, the degree of efficiency of the regeneration process and the configuration and manufacturing process of the membrane. These are some of the factors that will need to be overcome to make the process more attractive on a large scale.

5.12 Benefits

In the applications that involve challenging feed water with high fouling potential, high salinity, or containing specific micro-organic contaminants, FO has been proven to be an effective technology as a pre-treatment step to improve the overall efficiency of conventional desalination processes. Due to the many advantages of forward osmosis including the low hydraulic pressure and low energy consumption, this economically feasible and technically sound technology has been widely investigated in the application of seawater desalination. It should also be highlighted that the FO stand-alone process can treat wastewater to produce either a concentrated wastewater stream or a diluted wastewater stream, depending on the resulting application of the effluent stream. Using the FO process for the removal of organic matter has proven to be very effective and can yield a sustainable, optimized and relatively inexpensive process.⁸

The enhanced water flux of the PAFO (pressure-assisted forward osmosis) process gives a lot of room for the greater dilution of seawater than the standalone FO process, which results in greater energy savings in the RO step. PAFO also uses a lower surface area of the membrane in its operations and performance, thereby giving an overall cost saving. PAFO has been shown in many instances to enhance water productivity, which is attributed to higher water flux due to the application of hydraulic pressure. This improvement resulted in reducing the membrane area required compared to FO.¹⁶³

The advantages of FO as a standalone technology can be also applied to the treatment of wastewater with a high pollutant concentration. The water is drawn from the wastewater leading to the reduction of wastewater volume. Meanwhile, the pollutants are effectively rejected by the FO membrane. FO also offers up several advantages compared to other pressure-driven membrane processes, including the low pressure requirement and the ability to treat a high solid content solution.¹⁶⁴ The low fouling tendency of FO is due to the low hydraulic pressure and low permeating flux resulting in non-compacted and reversible fouling. Among the alternative processes, FO offers several attractive features allowing addressing the drawbacks of the thermal energy-based processes. In addition to its ability to produce a final product with a high solid concentration, FO can retain the bioactivity of nutraceuticals due to its mild operating conditions.

5.13 Future trends and direction

Currently, forward osmosis is very well suited for niche applications where water needs to be removed using very gentle and precise methods. Some of these include food processing and concentration of valuable chemicals such as flavors & fragrances and pharmaceuticals. This technology will be particularly applicable in the future when smart draw solution systems become industrially available that will leverage on utilizing waste heat or solar thermal energy for draw regeneration (e.g. the membrane distillation hybrid process). In order to commercialize the FO process, it is important to recognize industrial applications where the most value generated for FO and the technical challenges faced by companies wishing to commercialize FO processes can be tackled and addressed accordingly. In this regard, the most pressing concern is where there is a high guarantee of implementation and where efficiency and return on investment are considered with a cost benefit analysis and economic assessment.

The growing trend in wastewater treatment on a global scale related to the removal of organic materials and pollutants of emerging concern (especially organic materials) could be expected to continue in the next few years, with environmental compliance being the priority in many countries. This validates the area of fouling resistant membranes, which can be of great interest to researchers in the future.

The recovery system for the draw solution is related to the overall FO system which is crucial for the effective implementation of FO. The recovery rate efficiency when treating actual contaminated water is also important for the feasibility of the full-scale FO system. The regeneration of the draw solution has seen a noticeable research activity over the last 10 years with an increasing number of publications in the area.¹⁶⁵

The practicality of the FO process has been investigated and studied to enhance its efficiency by applying it to alternative hybrid processes such as FO–RO and FO–PRO. These hybrid processes have been shown to overcome some of the limitations from the standalone processes formerly employed. PAFO (pressure-assisted forward osmosis) has been proposed as an advanced concept for improving the performance of FO in applications related to agriculture and fertigation. The working concept of PAFO involves a hydraulic pressure that is directly applied on the feed side, where combined actions of osmotic and hydraulic pressures are used simultaneously as the driving force for water transport. This makes PAFO a well-equipped and enhanced technology to resolve one of the major problems associated with FO, which is low permeate water flux.

The additional hydraulic pressure within the PAFO process enhances the permeate water flux over concentration polarization and reduces reverse salt diffusion. PAFO can also be utilized in the FO–RO hybrid processes as the PAFO–RO configuration, since the low water flux of FO is still regarded as economically inappropriate for FO–RO processes due to its high investment and operational costs. In the FO–RO configuration, FO is used as a pretreatment step before RO, utilizing both secondary wastewater effluent as the feed water and seawater as the draw solution. This is used to dilute the seawater and save energy-related costs in the RO process.

One of the drawbacks of DS regeneration is the high energy consumption of pumping and high overall cost of the process which depends on the type of the used draw solution. However, the breakthrough is essential in the draw solution and regeneration system as these should be studied according to the overall system approach. A better understanding of the FO fouling mechanisms and fouling cleaning strategies will be beneficial for further application of FO. The main driving force of FO due to its membrane permeation process is osmotic pressure, different from UF, NF, RO, and other membrane filtration processes which require higher external pressure as the driving force, and membrane fouling is relatively light and easy to clean. However, membrane fouling remains a major drawback that cannot be ignored in FO systems. The FO membrane has shown promising prospects in both drinking water purification and wastewater treatment technology, especially its excellent high rejection rate performance and relatively low membrane fouling characteristics. Hence, it is likely to become a very important membrane technology in the future.

In the last 10 years, a large number of research papers focusing on FO have been published and the number of papers in various issues has increased year by year, especially in the issues of the draw solution and the membrane material.^160,166 The engineering of the forward osmosis process application and technologies is relatively scarce as a lot of the FO research is currently on the laboratory scale, with some progress in practical applications. However, there is still room for pilot scale trials and proof of concept studies. The advanced strategy of chemical cleaning remains to be further researched and developed.

FO membrane geometry should be chosen wisely in accordance with the intended application. Particularly in treating highly viscous solutions or solutions with a high solute loading such as in the food industry or in wastewater treatment, a hollow fiber geometry is preferred over flat sheet-based (spiral wound) membranes. It is well established that the hollow fiber geometry has a low fouling tendency, suitable for these applications. Widjojo et al. showed that the support's hydrophilicity had a greater impact on FO performance than morphology. They managed to fabricate supports with different morphologies and degrees of hydrophilicity by changing the amount of sulfonated material in the membrane support. Interestingly, the sponge-like more hydrophilic structure produced the best results in FO.⁴⁷ The ICP can be determined intrinsically and empirically, and each of these approaches has its shortcomings. More work is needed to find an accurate method and universal testing conditions to estimate the ICP. Only this will allow a proper development of the next generation of supports for FO membranes.¹⁶⁷

5.14 FO, agriculture and food security

Agriculture is by far the largest consumer of freshwater, accounting for almost 80% of water consumption worldwide. Energy-efficient desalination techniques could be a promising way for providing water for irrigation. Pressure-assisted forward osmosis (PAFO) has been recently introduced in a recent FDFO study. PAFO could potentially eliminate the need for further post treatment. In fact, the additional water flux generated in the process can enhance the final fertilizer DS dilution beyond osmotic equilibrium making the final product water suitable for direct fertigation.^7,168

FDFO desalination is not only energy efficient, but it also improves the efficiency of water and fertilizer applications. However, the major limitation of the FDFO process is its environmental compliance with the irrigation water quality standards in terms of nutrient concentrations, which limit the use of the product water for direct fertigation. This is an area where policy makers across the world have to use science-based research to drive the guidelines for proper use of the technology.

In the food industry, a very critical process step is to dewater liquid foods to improve product stability as well as to minimize packaging, storage, and transportation costs. The most common technique used in liquid food concentration is vacuum evaporation, but the high temperature applied in this technique results in the degradation of temperature-sensitive components such as vitamins, flavor molecules and phytochemicals.^169,170

FO's low fouling tendency is also a big benefit in the food industry in which major fouling of the membranes can be expected.¹⁷¹ Inherently, a dewatering step aims at a high overall recovery, or a high concentration factor. This means that, similar to wastewater applications, hollow fiber or tubular membrane geometries are preferred. FO showed almost 7.5 times higher rejection of urea as compared to the RO rejections. The operations in FO mode typically showed a better performance in rejecting small solutes such as urea and micropollutants than operation in RO mode. This makes FO also interesting for applications where small solutes need to be removed on a large scale.

A concentrated solution of the intended fertilizer is used as the draw solution for the desalination of either brackish groundwater or seawater. The advantage of this process is that the diluted draw solution would not require post-treatment to re-concentrate the draw solution.¹⁷² Another way to evaluate the potential of fertigation is to evaluate the relative amount of brackish groundwater that FO can contribute relative to other sources of irrigation water. The more brackish groundwater that can be utilized via FO treatment decreases the water usage from other sources. Ultimately, the potential utilization of brackish groundwater depends on the feed water osmotic pressure and draw solute, which dictates the required dilution necessary prior to application.

The promising aspect of fertigation is that it presents an FO application where concentration and recovery of the draw solute is not a primary concern. The disadvantage, however, is that the draw solute concentrations needed for practical fluxes generate a diluted draw solution that is still too concentrated for direct application to crops. The dependence of extraction capacity (and diluted draw solution concentration) on feed solution osmotic pressure limits the potential feed sources where this technology may be viable. From a practical standpoint, the feed solution likely has to have an osmotic pressure less than 5 atm and be pre-treated with an NF process, if the FS salts have divalent cations, for any substantial gains from implementing an FO process to be realized.

The rationales behind FO treatment of each application vary. The direct use of the diluted draw solution is promising for fertigation FO, but significant dilution prior to agricultural application likely limits the practicality of implementing FO without a low salinity water source for dilution or another treatment process. FO is a potential treatment alternative for highly saline brines for minimizing volume and recovering water. This technology may have a niche, because pressure-driven processes are not feasible as salinity increases. Identifying uses for the diluted draw solution, however, remains imperative. Ideally, brine waste minimization would be highly favorable if there was a direct use for the diluted draw solution or draw solute recovery did not include membrane processes.

If the original brine is hazardous, concentrating the hazardous material prior to shipping or disposal may yield large cost benefits despite treatment needs of the diluted DS. For desalination and water reuse applications, a benefit from FO is water transferred from a feed solution with high scaling or fouling potential to a solution with low potential so that membrane processes can operate at higher recoveries.

For FO to move from bench scale to full-scale applications, more focus on identifying specific niches where FO makes both economic and practical sense compared to more conventional treatment processes is required.

5.15 Forward osmosis – commercial applications

Over the last four years, the emergence and growth of the FO market and landscape have changed. Back in 2015, BlueTech¹⁷³ published a report on FO, where there was a lot of research and development, with an emphasis on commercialization around the technology, but there were some difficulties associated with achieving full commercialization. The FO market in its current stage and status now covers a varied spectrum from the ZLD market which was worth approximately $300 million in 2017 to a record of around $6 billion taking into account product concentration, ZLD and water reuse, amongst others. The market is dominated by FTSH2O, Porifera, Oasys and Modern Water. However, the FO market is small. None of the companies have more than a few commercial installations. FO as a technology is gaining market traction in ZLD/brine concentration, production of valuable products by dewatering, minimizing waste hauling costs, concentrating difficult to treat wastes, landfill leachate treatment and specialized desalination opportunities. Within the next five years, FO will likely be picked up by larger firms as a product offering within a suite of water treatment, brine management and water extraction solutions. The evaporator market will be more challenged by FO, but the FO addressable market will be more realistic than today's most optimistic projections. Table 2 shows the commercial entities that provide FO technologies for various applications.

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The niche applications of the FO process can be divided into two categories: a) application of FO with product water recovery and b) application of FO without product water recovery (i.e. osmotic concentration/dilution). Due to the various challenges with product water recovery, many FO applications are being classified as not cost effective. On the other hand, many of the FO applications that are shown to be promising & cost effective are based on the osmotic concentration/dilution concept. A part of this concept has been already discussed in section 3.2. The majority of the FO systems implemented in the market come with the application of FO with product water recovery. Recently, Koch Membrane Systems (KMS) has announced the launch of its TIDAL Forward Osmosis (FO) technology tailored to the food, beverage and life science sectors. The TIDAL FO technology enables product concentration without heat. This is another example of FO application without product water recovery. Aromatec Pte Ltd was established in 2018 and they offer a range of membrane-based solutions for “cold” concentration, such as concentration of flavours and fragrances, dairy production, and fruit juice and vegetable production. Thus Aromatec technology is also an interesting example for FO application with direct use of the concentrated FS. Bergh of Membrane Technology GmbH is another company which has demonstrated their visibility in the deployment of the FO process with and without product water recovery.

6. Life cycle cost analysis (LCCA) of the FO process

LCCA is a method to analyze the total cost as well as a wide range of cost elements of a system, including design, development and production costs, investment costs, operation and maintenance costs, and disposal costs throughout its life cycle, to determine the cost-effectiveness of the options among different configurations, systems, technologies, processes, services, feedstocks, and production alternatives.¹⁸⁹ In other words, LCCA of a system is the total cost of acquiring and utilizing a system over its entire life span. LCCA includes all costs incurred from the point at which the decision is made to acquire a system, through operational life, to eventual disposal of the system.¹⁹⁰ According to David and Lisnianski, the different steps for performing the LCCA process are depicted in Fig. 10.

Recently, several researchers have performed and reported the LCCA and techno-economic feasibility of the FO process and hybrid FO processes for different applications. Bryan et al.¹⁹¹ reported the economic assessment and LCCA of engineered osmosis and osmotic dilution processes for water treatment of Haynesville shale pit water. As per their investigations, FO is a more sustainable process for oil and gas (O&G) exploration wastewater treatment. Through their LCCA, they evaluated that FO, as an engineered osmosis and osmotic dilution (ODN) process, would reduce the economic and environmental impact for O&G exploration and associated pit water recovery. Low energy requirements of the FO process without any upstream pretreatment would significantly reduce the environmental impact. It was calculated that the pit water management cost could potentially be reduced by about 60% when the FO process was operated at 75% water recovery in comparison to cost requirements for deep well injection and pit water transportation cost, which can be reduced to 63%.

Hancock et al.¹⁹² carried out a comparative LCA methodology in order to distinguish between an innovative combined process consisting of conventional seawater reverse osmosis and FO–ODN mode and two more processes: (a) an independent SWRO system and (b) a hybrid SWRO and dual barrier water separation system with NF followed by RO. It was observed that the bottlenecks for the commercialization of the FO–SWRO process are optimized design of the FO membrane module and FO membrane fouling. System optimization analysis also revealed that the environmental impact of the combined FO–SWRO process compared to the SWRO process could be reduced by more than 25%, by tripling FO water flux (membrane efficiency), doubling membrane packing density and optimizing the FO process. The innovative combined NF–SWRO treatment of wastewater and seawater could also yield similar ranges of environmental impact.

In 2016, Linares et al.¹⁹³ carried out an interesting study on the detailed economic analysis on capital expenses (CAPEX) and operational expenses (OPEX) for: i) a conventional SWRO process, ii) a hybrid process of FO–LPRO, and iii) a hybrid of MBR–RO–AOP (membrane bioreactor–reverse osmosis–advanced oxidation process) for wastewater treatment and reuse. In this study, the most governing parameters influencing the economic viability of a hybrid FO–LPRO system have been derived by means of a detailed sensitive analysis. They have considered a FO–LPRO system with a production capacity of 100000 m³ d⁻¹ potable water for the LCCA. The important components involved in the analysis are costs for the FO and RO modules, required energy, characteristics of the feed and product water, materials, chemical operation and maintenance. The analysis reveals that the CAPEX for the FO–LPRO system is 21% higher than that for the conventional SWRO system. At the same time, the OPEX for the FO–LPRO system is 56% lower than that for the SWRO system because of the less fouling issue and less energy requirement. They calculated a 16% reduction in the total cost for the FO–LPRO system for the production of potable water compared to the conventional SWRO system.

The CAPEX and OPEX of FO–LPRO systems in comparison to MBR–RO–AOP systems have been calculated to be 7% lower and 9% higher, respectively. Therefore, there is no prominent reduction in the unit water production for both systems. With the development of efficient FO membranes and modules with optimized packing density and water permeation flux, the cost for overall water production could be further lowered and this would be advantageous to hybrid FO–LPRO systems over conventional water treatment processes.

In 2017, Kim et al.¹⁹⁴ carried out a study on the environmental and economic impacts of the fertilizer drawn forward osmosis (FDFO) and NF hybrid system and compared them with conventional SWRO hybrid systems using pre-treatment processes such as MF or UF. According to their findings, the FDFO–NF hybrid system using a TFC-FO membrane had less environmental impact than conventional RO hybrid systems due to lower consumption of energy and cleaning chemicals. The energy requirement for the treatment of mine impaired water by the FDFO–NF hybrid system was calculated to be 1.08 kW h m⁻³, which is 13.6% less than that of MF–RO and 21% less than that of UF–RO under similar initial feed solutions. The results of the above study clearly show that there is some positive potential of FDFO–NF for practical application with the development of a more efficient TFC-FO membrane and cost effective membrane module for the FO process. However, the improvement of efficient FO membranes towards the commercial scale is still open and thus detailed study of the fouling behavior and effective cleaning strategies is further needed for precise LCCA of the FO process.

In 2012, Mauter and Dzombak¹⁹⁵ investigated the techno-economic feasibility of the FO process using ammonia–carbon dioxide as the DS (NH₄HCO₃ FO) – which is a two-step waste heat driven process in which the DS draws fresh water across the membrane by means of an osmotic pressure gradient. The second step is the regeneration of the draw solution by means of waste heat using a distillation column. In this work they have considered the waste heat generated at U.S. coal, nuclear, and natural gas plants and they have estimated the quantity, quality, and spatio-temporal availability. The potential of NH₄HCO₃ FO in order to treat flue gas desulfurization (FGD) wastewater, gasification wastewater, and boiler feed-water has been assessed. The costs of NH₄HCO₃ FO using waste heat for FGD and gasification wastewater treatment to ZLD and boiler feed-water treatment are $2.20–$2.40 per m³ and $0.35–$0.65 per m³, respectively. In comparison to mechanical vapor compression and crystallization for ZLD wastewater treatment and reverse osmosis for boiler feed-water treatment technologies, waste heat driven NH₄HCO₃ FO is cost competitive for ZLD wastewater treatment, but not for boiler feed-water treatment. They have estimated that the NH₄HCO₃ FO process would reduce the cost of ZLD by $0.30– $1.20 per m³.

Most recently, Hamid et al.¹⁹⁶ reported the results of economic, energy and carbon footprint assessment of integrated FO–MBR for the treatment of urban wastewater using a FO membrane with 15 L m⁻² h⁻¹ flux, 80% water recovery and membrane cost of 50$/m² – expensive in comparison to a microfiltration (MF) membrane. They considered three different scenarios as illustrated in Fig. 11: (A) FO–aerobic MBR (FOAeMBR); (B) an integrated RO–FOAeMBR system; (C) a hybrid of FO–anaerobic MBR (FOAnMBR) with a partial nitrification/anammox (PN/AMOX) process. In this study, the overall costs in scenarios A and B were higher with FO than MF by $0.16 per m³ and $0.75 per m³, respectively. There is no significant difference in the cost of wastewater treatment using scenario C ($1.1 per m³), compared to the MF process. In addition to that, the GHG emission in scenario C is as low as 0.93 kg CO₂ equivalent per cubic meter and it was calculated to be 1.5 and 4.1 times lower than scenarios A and B, respectively. With the production of more efficient membranes with low cost and high flux, more optimized modules and processes could further reduce the overall cost and carbon footprint.

7. Conclusion

There is a wide range of industrial applications where osmotically driven membrane processes may be applied. There are also relatively few commercial applications and only a few successful companies that have focused on utilizing forward osmosis technology, but this is currently expanding. There are a number of industrial applications that have great potential for FO to be used as both an independent and a hybrid system, in conjunction with other technologies available. They have been covered in this review and they include the oil and gas industry, wastewater, desalination and pharmaceutical industries, amongst others. Unfortunately, it is still challenging to directly compare the efficiency of various application bench scale experiments and pilot scale studies because of the discrepancies and differences in the set ups and the operational variables and conditions involved. The focus on more research in the area of FO will further increase the chances of establishing FO as a viable treatment technology in the future, and give rise to even more industrial applications. The limitations and challenges as well as the benefits of forward osmosis as a viable technology have also been highlighted within this paper. These have to be taken into account and addressed by researchers in the future to ensure that it can be applied to many applications.

Despite operating at lower pressure, fouling is one of the major challenges in FO membranes as it can significantly affect the permeate flux and quality. Physical and chemical cleaning methods as well as pretreatment units are commonly used to address fouling issues in membranes. Physical cleaning that includes cross-flow shearing and bubbly flow is capable of minimizing flux decline over long-term operation, while chemical cleaning is much more effective in controlling biofouling. Pretreatment using coagulation, oxidation and adsorption processes can also be effectively used to mitigate fouling. Recent fouling related studies have focused on the development of novel materials for membrane surface modifications to alleviate fouling issues.

An effective draw solute must be low-cost, capable of generating high osmotic pressure, of low reverse solute flux, non-toxic, and economic to regenerate. Small ionic salts are capable of generating high osmotic pressures, have lower concentration polarization and can be regenerated with established RO, but on the other hand have relatively higher reverse solute diffusion. Other innovative draw solutes have been explored, including polyelectrolytes, hydrogels, stimuli (thermo, pH)-responsive polymers and nanoparticles coated with hydrophilic groups. Though these polymers and nanoparticles of larger sizes have lower reverse salt diffusion and easy regeneration approaches, they could not compete with smaller molecules for the high osmotic pressures. Osmotic concentration or dilution is the process to leverage the salinity difference between two natural solutions (such as seawater and brine water) to drive water permeation flux through FO membranes. Osmotic concentration or dilution offers the potential for energy-efficient FO operation without the need for a draw solution regeneration process. Moreover, environmental benefits could be achieved by reducing the salinity of brine water, or reducing the volume of the impaired water, discharged to the aquatic ecosystem.

As a membrane-based water treatment technology, efficient FO membranes are one of the inevitable components in an FO process. As a matured membrane technology, the RO membrane in an RO process was the benchmark of FO membranes at the earlier stages of FO process development. However, researchers have since developed various types of cost effective, fouling resistant, highly water permeable membranes with the ability to retain solutes and reject contaminants. Hollow fiber membranes have the advantage of a large surface area to volume ratio and thereby improved packing density with small footprint compared to flat sheet membranes. Several start-up companies have been already introduced into the market in order to commercialize FO membranes. The latest trends in FO membrane fabrication are based on the incorporation of nanostructured materials, fouling resistant materials and biomimetic molecules in order to enhance the membrane efficiency and process performance. Still, it is clear that there are lots of areas to be improved in the fabrication of FO membranes in order to make the FO process more efficient and economically feasible. LCCA and techno-economic feasibility study are also very important aspects of any technology in order to move forward to the market. Most of the studies reveal that FO can be a competitive water treatment technology with the help of more efficient membranes, ideal draw solutions, optimal process designs and identification of niche applications.

Conflicts of interest

There are no conflicts to declare.

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