A critical review of the history, development and future prospects for Forward Osmosis. Anthony Jacob

Abstract Osmosis is a physical phenomenon that has been extensively studied by scientists in various disciplines of science and engineering. Early researchers studied the mechanism of osmosis through natural materials, and from the 1960s, special attention has been given to osmosis through synthetic materials. Following the progress in membrane science in the last few decades, especially for reverse osmosis applications, the interests in engineered applications of osmosis has been spurred. Osmosis, or as it is currently referred to as forward osmosis, has new applications in separation processes for wastewater treatment, food processing, and seawater/brackish water desalination. Other unique areas of forward osmosis research include pressure-retarded osmosis for generation of electricity from saline and fresh water and implantable osmotic pumps for controlled drug release. This paper provides the state-of-the-art of the physical principles and applications of forward osmosis as well as their strengths and limitations. Key words: Osmosis; Forward osmosis; Direct osmosis; Desalination; Reverse osmosis; Pressure-retarded osmosis

1. Introduction

2. History

Prior to beginning, we have to specify this review is mainly based on the article: Forward osmosis: Principles, appplications, and recents developments [1] published in the journal of Membrane Science. For a few decades, membranes are more and more used in the industry for many applications. Reverse osmosis (RO) membranes are well-known and often used for desalination or food industry; reverse osmosis, as his name indicates it, is the opposite of the forward osmosis. Osmosis is a physical phenomenon which is defined as the transfer of solvent (water in most cases) through a selectively permeable membrane driven by a concentration gradient. A selectively permeable membran allows passage of water, but rejects solute molecules or ions [1,3]. Build theories to explain osmosis – or forward osmosis (FO), or direct osmosis (DO) – permits also to design RO membranes. But paradoxically, forward osmosis which is the natural phenomenon is much less studied and developped than reverse osmosis for applications (a search on http://www.sciencedirect.com/ gives us more than 2,500 results for reverse osmosis whereas for forward or direct osmosis less than 200 results) and FO membranes manufacturers are very few in comparison to RO membranes manufacturers. Nevertheless, forward osmosis has some advantages and the research makes advances to discover new applications for FO.

Osmosis is a physical phenomenon that has been exploited by human beings since early days of manking. Early cultures realized that salt could be used to desicate foods for long-term preservation. Effectively, in saline environments, most bacteria, fungi, and other potentitally pathogenic organisms become dehydrated and die or become temporarily inactivated because of osmosis [1]. In 1748, abbot Nollet noticed that while separating water and alcohol by an animal bladder, the water passes in the alcohol but not the alcohol in the water [2]. Working on aqueous solutions between 1827 and 1832, Ren´e Dutrochet proposes the terms “endosmosis” and “exosmosis” to name this phenomenon. K. Vicrordt was also interested in the phenomenon in 1848. In 1854, Thomas Graham worked on colloid substances and discovered that they cannot pass through an animal membrane. M. Traube invented the first artificial membrane made in copper ferrocyanide Cu2 F e(CN )6 in 1864. In 1877, W.F.P. Pfeffer made copper ferrocyanide precipitate in a porous material, that gives a membrane which has a good mechanical resistance. In 1884, de Vries worked on plasmolysis and animal cells turgescence. In 1886, van’t Hoff published an analogy between aqueous solutions and perfect gases and applied thermodynamics to osmosis. He established a law similar to the

Preprint submitted to Elsevier

28th November 2006

Gay-Lussac law and he proposed the adjective“semipermeable” to refer to membranes. He is awarded by the Nobel Prize of chemistry in 1901 for his work. In 1899, A. Crum Brown used three liquid phases - calcium nitrate aqueous solution saturated with phenol at the bottom, a pure phenol layer in the middle, and a water solution saturated with phenol at the top - and he noticed an osmosis phenomenon : water passes from th top phase to the bottom phase, the middle liquid phase acting as semipermeable membrane. Thus, he established the importance of the solubility of diffusing species in the membrane. From 1901 to 1923, H. N. Morse and J. C. W. Frazer undertook a systematic work on measuring permeability for different gelatinous precipitates : ferrocyanides and phosphates of uranyl, iron, zinc, cadmium, and manganese.

a1 P V1

solvent (water) activity which decreases when solute concentration increases, pressure upon the solution (Pa), solvent molar volume (m3 /mol).

At the equilibrium, the chemical potential of water in the diluted solution is the same than in the concentrated solution: µ1 = µ01 RT ln a1 + ΠV1 = 0 so, the expression of the osmotic pressure is:   RT ln a1 Π=− V1

(1)

This relationship was obtained in assuming the solvent is incompressible. The activity can be determined from partial pressures measures (Raoult’s law): p1 (2) a1 = ∗ p1

3. The principles of operation 3.1. Osmotic pressure

with p1 vapour pressure of the solution, p∗1 vapour pressure of the pure solvent, thus  ∗ RT p1 Π= ln (3) V1 p1 In assuming that the solution is diluted and using limited developments, we can deduce from the relationship (3):

Before explaining in details the principles of operation, we have to view the differences between each osmotic processes: FO, RO and PRO. Considering a system with two compartiements separated by a selectively permeable membrane and containing two solutions of different concentrations (see Figure 1). The osmosis phenomenon will give a water flow coming from diluted solution to concentrated solution. If we try to prevent this water flux in applying a pressure on the concentrated solution, the transfered water quantity by osmosis will decrease; that is called pressure-retarded osmosis. At a moment, the applied pressure will be such that the water flux will be equal to zero; that is called osmotic equilibrium. If, to simplify, we assume that the diluted solution is pure water, the equilibrium presssion is callled osmotic pression. An increase of the pression beyond the osmotic pressure will give a water flow in the opposite direction of the osmotic flow, i.e. from the concentrated solution to the diluted solution: that is the reverse osmosis phenomenon.

Π = CRT

(4)

where C is the molar concentration (mol/m3 ). The law is known as Van’t Hoff ’s law and can be compared to the perfect gas law. It is important to note that this law is only valid for the diluted solution, i.e. for low osmotic pressures. The reasoning realized until now has ignored the solute characteristics and, particularly, has assumed that there was only one species. In reality, if the solute is dissocied in i ions of different nature, the osmotic pressure will be i times higher: Π = iCRT Van0 t Hoff 0 s law

(5)

In fact, in some documents [5], we can find that osmotic pressure is given by the following relationship: X Π = 103 mi RT (6) P with mi sum of the molalities of each ion or molecule non-ionised (mol/kg). Actually, as in most cases the solvent is water, we have:

3.1.1. Van’t Hoff Law In the case where there is water transfer from diluted solution to concentrated solution, we have to admit that the water chemical potential is higher in the diluted solution than in the concentrated solution [1,3]. The water chemical potential µ1 in the concentrated solution is given by the relationship:

C = 103 × m = 103 ×

µ1 = µ01 + RT ln a1 + (P − 1)V1 with µ01 chemical potential of water in diluted solution (J/mol), R gas constant (8.314 J/mol · K), T thermodynamic temperature (K),

solute mole solvent mass

3.2. Membranes materials From 1960’s, special attention has been given to osmosis through synthetic materials, but currently few membranes 2

P =Π

M

M

M

P >Π P Π

P >Π

P ∆P . The flux directions of permeating water in FO, PRO, and RO are illustrated in Figure 1. The FO point, PRO zone, and RO zone, along with the flux reversal point are illustrated in Figure 2. 4.2. Model of phenomenologic type transfer or “black box” This model [3] established linear phenomenologic relationships between flows Ji and associated transfer gradients Fi , or not associated Fj , by intermediary of phenomenologic coefficients Lij (Onsager’s theory): X Ji = Lij Fi + Lij Fj (11)

This phenomenon is well understood and has been extensively modeled, and its effects can be largely mitigated by increasing the shear rate and turbulence by increasing flow accross the membrane.

i6=j

The flow Ji of one specie is not only connected to its associated force Fi but results of all implied forces in the global process: there are coupled transfers. In the case of membranes, there are only two flows (solvent and solute) and two transfer gradients (pressure and concentration). The previous relationship can be simplified in assuming that crossed coefficients are equal: Lij = Lji for i 6= j (Onsager’s reciprocity relationship)

4.3.2. Internal concentration polarization Internal concentration polarization (ICP) is a closelyrelated phenomenon, but is exclusive to FO. ICP occurs within the support layer of the membrane, and is characterized by differing solute concentrations at the transverse boundaries of that layer [6].

(12) 4.3.2.1. Modeling concentrative internal concentration polarization

Kedem and Katchalsky applied previous relationships to transfers through membranes and got the following results: J1 = Lp (∆P − σ∆Π) J2 = ω∆Π + (1 − σ) C¯2 J1

4.3.2.2. Influence of internal concentration polarization on water flux

(13) (14)

J1 and J2 (in m/s) are flows of solvent and solute through the membrane. Lp and σ are phenomenologic Lij of relationship (11), with: Lp solvent permeability (m/s · Pa), ω solute permeability (m/s), σ reflection coefficient (or Staverman’s). If σ = 1, there is no interaction between solvent flow and solute flow (case of very selective membrane) and KedemKatchalsky’s moldel is identical to solubilizationdiffusion model (4.1). 2

5. Limitations and known problems Compared to classical separation processes, the FO process seems to have a number of advantages. Low energy use, low operating temperatures and pressures, and high products concentrations are the main advantages. However, as each separation process, it has its limitations and some problems are well-known. Limitations are mainly due to the lack of optimized membranes and in some applications like food processing or seawater desalinisation for example, an effective recovery process for the draw solution misses to transform FO into a full-scale process.

In some cases, A can be found in m3 /m2 · s · Pa.

4

roduction

} }

(∆P = ∆Π)

Water flux

(∆P > ∆Π)

Flux reversal (∆P = ∆Π) point

∆P

(∆P=>∆Π) ∆Π) (∆P

∆Π

(∆P> > ∆Π) ∆Π) (∆P (∆P < < ∆Π) ∆Π) ∆P ∆P ∆Π ∆Π

(∆P = ∆Π) Pressure-retarded 00 (∆P > ∆Π) Osmosis

1. Introduction 1. Introduction

Prior to begin ing, we have to specify this revi Posmorsior: tPorbinecgipnleisn,ga,pweplhicavtei tno,spaencdifyretchinstredvei

Speed

Pressure

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Futhermore, an other limitation of membranes, in general, is the cost. Actually, membranes are still quite expensive, thus researchs haven’t probably explored all the fields where we could use membranes yet, and it is all the more true that FO has a high potential of applications due to its properties. 6. Potential industrial applications FO process is very less used for applications in industry, but for many years researchers have realized experiments which lead to potential industrial applications. In this paper, we won’t present all potential applications but just those for which we find the most interesting documents, i.e.: hydration bags, seawater desalinisation and power production. Of course, it exists much more applications in different fields like: food processing, pharmaceutical industry or wastewater treatment and water purification. To give just most famous examples in these fields, we can cite concentration of fruit juices in food processing; osmotic pumps in pharmaceutical industry; and in wastewater treatment and water purification, the following: – concentration of dilute industrial wastewater, – concentration of landfill leachate, – direct potable reuse for advanced life support systems, – concentration of digested sludge liquids, – forward osmosis for source water purification – hydration bags (seen in 6.1).

Figure 4. Drawing of HTI’s flagship water purification bag, X Pack [8].

but a sweet drink that can only be used for specific applications [1]. 6.2. Seawater desalinisation To get a water from saline water, we use a draw solution with a higher osmotic pressure to obtain a water flux from the saline water (feed solution) to the draw solution. An effective draw solution solute must have very specific characteristics. Of course, it must have a high osmotic efficiency, meaning that it has to be highly soluble in the water and have a low molecular weight in order to generate a high osmotic pressure. Higher osmotic pressure leads to higher water flux and feed water recovery. Most important, for processes involving the production of potable water without a comestible solute, the draw solute must be easily and economically be separated and recycled.

6.1. Hydration bags Osmotic filters are unlike other filters in that they produce a nutrient drink rather than simple water. This makes them inappropriate for producing cooking or hygiene water; however, the drinks produced are a superior source of water, calories and electrolytes for active people in areas where safe water is not available. Customized mixes can be created and individual flavor additives are available from the manufacturer so that users can formulate their own drink mix with the addition of a sweetner. Hydration Technologies (HTI) design several disposable or reusable hydration bags (see Figure 4) which are preloaded or loaded with nutrient powder and use membrane bag or filter bag inside a sealable pouch; that permits to get some drinking water liters in few hours from non-potable water without spending energy. HTI developed also stationary application filters and hiker’s backpack which use a nutrient syrup and spiral osmotic filter tubes [7]. Although slower than other water purification devices , FO hydration bags require no power and only foul minimally, even when used with muddy water. But, yet, there is debate among experts whether hydration bags provide water treatment per se because the product is not pure water

The novel forward osmosis desalinisation uses a draw solution of two highly solubles gases – amonia (NH3 ) and carbon dioxide (CO2 ) – which satisfied the presented ideal draw solution criteria. The concentrated draw solution is made by dissolving ammonium bicarbonate salt (NH4 HCO3 ) in water [9]. Separation of the fresh product water from the draw solution can be achieved with relative ease. Upon moderate heating (near 60 ◦ C), ammonium bicarbonate can be composed into ammonia and carbon dioxide gases. Thus, the saline feed water and the draw solution are fed to forward osmosis unit in each side of the membrane. Water from the seawater transport through membrane into the draw solution, by this saline water becomes brine. Diluted NH4 HCO3 is sent to a separation unit, comprising a distillation column or a membrane gas separation unit (i.e. pervaporation) (see Figure 5). This ammonia-carbon dioxide FO is a viable desalinisation method. Previous studies on desalination using FO were unsuccessful or economically unviable because of the 6

Saline feed water

FO membrane unit

Brine

Concentrated draw solution recycle

Draw solute separation

Potable water

Draw solute solution

Figure 5. Schematic diagram of the ammonia-carbon dioxide forward osmosis desalinisation process. Adapted from [9]

lack of an appropriate draw solution having solutes that both produce high osmotic pressure and can easily be separated from the product fresh water [9].

drinking very quickly. Hydration Technologies’s products are mainly designed for military applications or eventually for emergency, although for this last sector pure water is preferable. US army and NASA are the main clients of HTI, but as the US Department of Defense budget is superior to that of the first ten world powers, we can suppose that the sector has a great potential.

6.3. Power production This process is not exactly forward osmosis but pressureretarted osmosis and has been developed from 1970’s by Sidney Loeb [10]. Seawater, which has a mean NaCl concentration of 35 g/L, is said to have an osomotic pressure about 27 bar. For example, for a concentration in NaCl of 35 g/L and temperature of 15 ◦ C, we get (calculated with (5)) the following osmotic pressure:

7.2. Governments At moment where global warming and decrease in fossile fuels resources are more and more present in the consciences, renewable energy systems are much more developed than in the past. Thus, application whom interest the most the european governments is surely power generation (seen 6.3). Actually, an intensive study of PRO power generation is being funded by European Union and some other companies and universities. The main objective is to develop membrane for PRO power that have a production capacity equivalent to at least 4 W/m2 .

35 × 8.314 × 288 58 × 10−3 = 2.89 × 106 Pa ≈ 2.9 bar

Π=2×

Thus, if we use seawater and fresh water (for example from a river), we get a natural water flux from fresh water to saline water that we can exploit to produce electrical energy with a turbine in using PRO process [11] as the Figure 6. Power generated by PRO from seawater has certain features that make it very attractive. It is a large and unexploited resource; it is renewable; its use has minimal environmental impact; and compared to other potential source of energy from the ocean, its density (i.e., power capacity per physical size) is high [1].

8. Future growth areas

7. Market assessment

As pharmaceutical industry, energy and wasterwater treatment fields could be interested by quite recents applications for FO. For them, the interest is in the energy profit that FO process can bring, notably in processes where FO and RO processes are coupled.

7.1. Military

9. Concluding remarks

In my opinion, recreation market for hydration bags is not very big, because it ever exists several devices to get a

Forward osmosis is undoubtedly the most interesting membrane process for two major reasons. The first reason 7

Pressure exchanger Brackish water Membrane modules

Turbine Brackish water

Seawater Fresh water

Fresh water bleed

Figure 6. Energy production with PRO process

is that osmosis membranes are the most selective membranes and consequently can produce the best quality of a pure liquid (pure water interests a lot electronics industry); the second reason is that FO is a physical phenomenon of which solvant transport is natural and require no energy and consequently can help to reduce the energy consumption of processes. Unfortunately, as we discussed earlier, a lack of FO membranes slows its development of new applications because results obtained with RO membranes are often not as good as we could hope.

[1] Tzahi Y. CATH, Amy E. CHILDRESS, and Menachem ELIMELECH. Forward osmosis: Principles, applications, and recent developments. Journal of Membrane Science, May 2006. [2] Wikipedia. Osmose. Available from: http://fr.wikipedia.org/ wiki/Osmose [cited 22nd November 2006]. [3] Alain MAUREL. Techniques s´ eparatives ` a membranes consid´ erations techniques. Techniques de l’Ing´ enieur, June 1993. [4] Tzahi Y. CATH. Forward osmosis: Principles, applications, and recent developments. In Rocky Mountain Water: Reuse it or Lose it!, Golden, Colorado, August 2006. Colorado School of Mines. Available from: http://www.rmwea.org/tech_papers/ #Reuse. [5] Patrick DANIS. Dessalement de l’eau de mer. Techniques de l’Ing´ enieur, June 2003. [6] Gordon T. GRAY, Jeffrey R. McCUTCHEON, and Menachem ELIMELECH. Internal concentration polarization in forward osmosis : membrane orientation. Desalination, February 2006. [7] Robert J. SALTER. Forward osmosis. Water Conditioning & Purification, April 2006. [8] Hydration Technology. Available from: http://www.hydrationtech.com/ [cited 23rd November 2006]. [9] Jeffrey R. McCUTCHEON, Robert L McGINNIS, and Menachem ELIMELECH. A novel ammonia-carbon dioxide forward (direct) osmosis desalination process. Desalination, November 2004. [10] Sidney Loeb. Large-scale power production by pressure-retarded osmosis, using river water and sea water passing through spiral modules. Desalination, January 2002. [11] L` aszl` o PANYOR. Renewable energy from dilution of salt water with fresh water: pressure retarded osmosis. Desalination, March 2006. [12] Rocky Mountain Water Environment Association. Technical resources. Available from: http://www.rmwea.org/tech_ papers/ [cited 22nd November 2006]. [13] Menachem ELIMELECH. Journal publications with pdf. Available from: http://www.yale.edu/env/elimelech/publications.htm [cited 23rd November 2006]. [14] Wikipedia. Forward osmosis. Available from: http://en. wikipedia.org/wiki/Forward_osmosis [cited 22nd November 2006]. [15] Wikipedia. Osmosis. Available from: http://en.wikipedia. org/wiki/Osmosis [cited 22nd November 2006].

Nomenclature C¯i mean concentration in the membrane (kg/m3 ) ¯ Di diffusion coefficient of consituant i in the membrane (m2 /s) ∆Π osmotic pressure difference of each side of the membrane (Pa) ∆P pressure difference of each side of the membrance (Pa) γ polarization factor µ chemical potential (J/mol) Π osmotic pressure (Pa) σ reflection coefficient A water permeability coefficient (kg/m2 · s · Pa) a1 solvent activity B permeability of membrane coefficient (m/s) C molar concentration (mol/m3 ) H coefficient of solute distribution between solution and membrance J flux (kg/m2 · s) m molality (mol/kg) P pressure upon the solution (Pa) R gas constant (8.314 J/mol · K) V partial molar volume (m3 /mol) z effective thickness of membrane (m)

References 8