The Ammonia-Carbon Dioxide Forward Osmosis Desalination Process

By Jeffrey L. McCutcheon, Robert L. McGinnis and Menachem Elimelech

Introduction gies are the large thermal energy inputs membrane based separation process, like There is a finite amount of freshwa- required to vaporize water, the brine dis- RO, which relies on the semipermeable ter readily available for human consump- charges and the relatively low water re- character of a membrane to remove salt. tion and use. This supply is already coveries, with electrical requirements However, unlike RO, the driving force strained due to competing utilization that often approach those of reverse os- for separation is osmotic pressure, not demands for farming, industrial, com- mosis (RO). hydraulic pressure. By using a concen- mercial and domestic uses. Increasing RO continues to gain popularity as trated solution of high osmotic pressure populations place further strain on al- a successful desalination technology; called the draw solution, water can be ready-dwindling or otherwise impaired nearly all desalination plants built today induced to flow from saline water across water sources. With the world popula- are RO plants. Its benefits include the the membrane, rejecting the salt. The tion expected to exceed ten billion by capability of achieving higher recoveries (now diluted) draw solution must be re- 2040, the draw on these resources will than those typical of its thermal counter- concentrated, yielding potable water become ever less sustainable. Growth in parts, while using less overall energy. and recycling the draw solute.1 Figure 1 population necessitates the cultivation of However, electricity use is still high and presents the general process diagram. alternative water sources. brine discharge problems remain. The production of brine is a significant draw- The ideal draw solution Desalination back and requires that the plant be lo- Since the recovery or utility of the One such alternative is the vast cated near an ocean where discarded draw solute is critical to the successful quantity of saline water in the oceans and brine has less (albeit still a considerable) large brackish groundwater reserves of- impact on the environment. In brackish ten found in arid regions. These sources water applications, the quantity of brine Figure 1. Forward osmosis represent over 97 percent of the world’s produced precludes the use of economic desalination process schematic water and if tapped economically they solar pond evaporation and ground well diagram. would yield a virtually unlimited sup- injection will increase source water sa- Sea or Membrane ply of water. linity over time. These technologies, brackish The cost associated with desalinat- therefore, cannot be used to desalinate water Draw solute ing these types of water, however, is high. inland brackish groundwater sources as recovery Currently, most of the word’s desalina- the brine cannot be disposed of in a sus- tion capacity is based on thermal tech- tainable manner. Draw nologies, primarily multi-stage flash solution (MSF) distillation and multi-effect distil- FO not RO lation (MED). MSF and MED are popu- With issues of energy use and water Product lar in parts of the world where thermal recovery at the forefront of the desalina- water energy is readily available and inexpen- tion debate, many are investigating an Brine sive. The drawbacks of these technolo- alternative. Forward osmosis (FO) is a

Water Conditioning & Purification O CTOBER 2006 implementation of the FO process, vari- mechanism and a low toxicity. High solu- the saline feed source, the subsequently ous draw solutes may be used, depend- bility coupled with low molecular weight diluted draw solution may be treated ing upon the intended use of the allows for the generation of large osmotic thermally to remove its ammonium salt desalinated water. One current version pressures which lead to high product solutes, producing fresh water as the pri- of FO uses an edible solute, such as con- water flux and recovery. An easy removal mary product of the FO process. This centrated glucose. In this case, the con- mechanism is critical to the economic vi- thermal separation of draw solutes is centrated solute is diluted by the ability of the FO process since the over- based on the useful characteristic of these permeate water yielding a nutritious whelming majority of the energy used in salts to decompose into ammonia and drink. This is an example of FO that uti- the FO process is used for draw solute carbon dioxide gases when the solution lizes the draw solute rather than recov- recovery. A non-toxic draw solute is criti- is heated. The temperature at which this ering it. Other suggested draw solutes are cal for public acceptance of a new desali- occurs is dependent on the pressure of those that can be chemically or thermally nation technology, as trace amounts of the solution. If a vacuum distillation col- precipitated from solution for removal. the draw solute may be present in the umn is used for this separation, the tem- Some propose the use of dissolved gases product water. that can be removed by thermal means, Figure 2. Osmotic pressure as a or the use of larger molecular weight sol- The novel ammonia-carbon function of ammonia-carbon utes that can be separated by physical dioxide draw solution dioxide draw solution concen- means. With these two latter options, the The FO process currently being in- tration (FO line). draw solute is concentrated and recycled vestigated uses a recyclable solute com- Also shown for comparison are the seawater osmotic in a closed loop, as Figure 1 illustrates. posed of ammonium salts. These salts (a pressure (black dashed line) and approximate All of these various draw solutions rely mixture of ammonium bicarbonate, am- operational pressure of reverse osmosis (RO). on specific chemical and physical prop- monium carbonate and ammonium car- 4,000

250 Osmotic pressure (psi) erties of the solute which make them bamate) are formed when ammonia and FO ideal for certain FO processes.1 carbon dioxide gases are mixed in an 200 3,000 A draw solute for use in a continu- aqueous solution.2,3 The salts are highly 150 ous FO desalination process, in which the rejected by the semipermeable membrane 2,000 draw solute is recovered, must have cer- used in FO and are highly soluble, lead- 100 tain characteristics to make the process ing to the reliable generation of high os- RO operation pressure 1,000 50 economically viable. For this FO process motic pressures for the FP process (see Osmotic pressure (atm) Seawater osmotic pressure configuration (see Figure 1) the draw Figure 2). 0 0 123456 solute must have a high solubility, a low Once the concentrated draw solution Draw solution concentration (m) molecular weight, an easy removal is used to effect separation of water from

O CTOBER 2006 Water Conditioning & Purification perature of heat required can be quite poses. Figure 3 also illustrates the poor Figure 4. Illustration of internal low, in the range of 35-40°C (95-104°F) flux performance of the RO membranes concentration polarization in given an ambient temperature of 15-20°C when used for FO. Similarly, NaCl rejec- the FO process.4 π and π indicate the bulk feed and draw solution (59-68°F). tion of the RO membranes was also low Feed Draw The use of this ammonia-carbon di- (less than 80 percent). These results beg osmotic pressure, respectively. Note the external concentration polarization on the feed side of the oxide draw solute thereby allows for ef- the question: Why do the RO membranes membrane which is a result of the dilute feed solute fective desalination of saline feedwater perform poorly in FO? being rejected from the membrane surfaces as water flux sources using little more than low-grade occurs. Also note the osmotic pressure gradients that heat (very little electricity is required for Internal concentration exist inside the membrane due to the dilution of the draw solute in the porous support layer (internal unpressurized process pumping). Fur- polarization concentration polarization). Both of these phenomena thermore, the high osmotic pressures that RO membranes are designed to have reduce the effective osmotic driving force, ∆π. solutions of this type may generate al- a thin, dense, separating layer called the Porous low for very high feedwater recoveries. active layer, which is supported by mul- support Active This has the benefit of reducing brine tiple porous layers. The active layer re- layer layer discharge volumes, electrical require- jects the salt while the supporting layers ments for feedwater pumping and pro- provide mechanical stability to the mem- cess capital costs. brane during pressure-driven water flow. π As salt rejection occurs, a region of in- draw FO performance creased salt concentration forms near the Performance of the FO desalination membrane surface. This phenomenon, process was measured in the lab by de- referred to as concentration polarization, termining the permeate water flux and is reduced if turbulence is induced near salt rejection of several commercially the membrane surface, facilitating the available polymeric membranes3. Three diffusion of the concentrated solute back membranes were tested for water flux into the bulk solution. This is often ac- and salt rejection performance using the complished by using crossflow (at rela- ammonia-carbon dioxide draw solution tively high velocities) and creating ∆π and a sodium chloride feed solution. Two turbulence with spacers within the flow of these membranes (TFC-1, TFC-2, thin channels. film composite) were designed for use in The same phenomenon will occur in πfeed RO. The other (CA, a membrane derived FO on the feed side of the membrane. Water from cellulose acetate) was designed for However, a similar but dilutive effect si- flux FO landfill leachate dewatering. All multaneously occurs on the permeate (or membranes used were commercially draw) side, reducing the effective driv- available at the time of testing. Figure 3 ing force of the draw solution. This phe- shows the flux performance of each mem- nomenon is intensified by the presence the product water and recycled within brane under a set of specified experimen- of the porous support, which provides a the system by a thermal separation pro- tal conditions.3 protected environment where crossflow cess. The most readily adaptable technol- Figure 3 shows that water flux per- cannot mitigate the polarized layer. ogy for this purpose is a distillation formance of the CA membrane was high, Termed ‘internal concentration polariza- column, or more specifically, a reboiled at around 15 gfd (gallons per square foot tion’, this phenomenon severely reduces stripper. The diluted draw solution is of membrane area per day). For compari- the osmotic driving force when asymmet- introduced to the column at its top and son, typical RO systems treating water ric membranes are used in FO. Internal heat is introduced at its bottom. The equi- of similar salinity run between nine and concentration polarization is illustrated librium state of a column of this type, 11 gfd. The NaCl rejection for the CA in Figure 4.4 given an appropriate height, results in a membrane in these conditions also ex- The TFC-1 and TFC-2 membranes, fresh water stream from the base of the ceeded 95 percent, which is reasonable comprise a polymer porous support layer column and a vapor stream at the top, considering the membrane is not de- cast upon a thick fabric backing layer, containing nearly all of the ammonia and signed specifically for desalination pur- which provides mechanical strength. The carbon dioxide present in the dilute draw FO membrane also contains a porous stream (less than one ppm ammonia re- polymer support layer, but it is relatively mains in the product water stream). Figure 3. Comparison of flux thin and the membrane lacks an addi- Along with the gaseous solutes, performance of two RO tional fabric layer. Mechanical support is some water vapor is also captured in the membranes. instead provided by a mesh imbedded in gas stream from the column. To the de- (TFC-1 and TFC-2) and one FO membrane (CA).3 the porous polymer matrix5. This allows gree that this occurs, the solute separa- 16 6m draw / 0.5m NaCl feed, 50˚C CA for a thinner support layer, which results tion process loses a similar degree of 14 in a reduced prevalence of internal con- thermal efficiency. For this reason, one 12 centration polarization. The reduced in- important area of future research will be 10 ternal concentration polarization results in achieving better separation of the draw 8 in a greater utilization of the osmotic driv- solutes from the dilute draw stream. Pos-

Flux (gfd) 6 ing force which in turn leads to either sible avenues for this include membrane 4 higher water fluxes or increased recovery. distillation or pervaporation (a mem- brane process in which a liquid is main- 2 TFC-1 TFC-2 0 Draw solute recovery tained at atmospheric pressure on the The draw solutes are removed from feed or upstream side of the membrane

Water Conditioning & Purification O CTOBER 2006 while permeate is removed as a vapor possible to reduce the negative environ- due to the low vapor pressure existing mental impact of desalination of all types, on the permeate or downstream side). but these also open up the possibility of Success in the use of an alternate solute effectively desalting inland saline water recovery system will lead to improved ef- sources. With high recovery FO, it may ficiency overall for the FO process. It is be possible to obtain fresh water eco- important to note, however, that the cur- nomically from brackish groundwater rently available technology is already without producing a liquid brine stream. more efficient than existing thermal de- This could be of great benefit to arid re- salination methods.6 gions with such resources, such as the southwestern US. The greatest benefit of Potential benefits of ammonia- FO, however, will be realized in its po- carbon dioxide FO tential to reduce the total water cost. If it The use of osmotic pressure to effect were economically preferable to obtain the separation of fresh water from saline water from the ocean, rather than to ship sources will allow for higher feedwater it over long distances or from environ- recoveries, lower brine discharge vol- mentally sensitive streams and fresh umes, lower (and less expensive) energy water habitats, it would have tremen- use and a lower total water cost. Using a dously positive impacts on our natural vacuum distillation column for solute environment, while enabling continued, recovery, it is possible to use very low- sustainable economic growth. grade heat as the primary energy source for FO. This creates the potential to drive References water desalination with used energy ex- 1. T.Y. Cath, Childress, A.E., Elimelech, M., hausted from power plants or industrial Forward Osmosis: Principles, applications and recent developments, Journal of Membrane facilities, at near to or zero energy cost. Science, 281 (2006), 70-87. One way to estimate the improvement this represents is to compare FO to de- 2. R.L. McGinnis, Osmotic Desalination Process, US Patent Pending PCT/US02/02740. salination technologies on the basis of the value and quantity of energy used. A 3. J.R. McCutcheon, R.L. McGinnis, M. Elimelech, A novel ammonia-carbon dioxide term often used for this is equivalent work, forward (direct) osmosis desalination process, which uses the quantity of electricity con- Desalination, 174 (2005), 1-11. sumed by the process plus the electricity 4. J.R. McCutcheon, Elimelech, M., Influence that any thermal energy consumed could of concentrative and dilutive internal concen- be otherwise used to make. The compari- tration polarization on flux behavior in for- son, shown in Figure 5, makes the advan- ward osmosis, Journal of Membrane Science, tages of low-grade heat use clear.6 July 2006. 5. J.R. McCutcheon, M. Elimelech, Desalina- Conclusion tion by ammonia-carbon dioxide forward os- The high recoveries and subsequent mosis: Influence of draw and feed solution concentrations on process performance, Jour- low brine discharge volumes make it nal of Membrane Science, 278 (2006), 114-123. 6. R.L. McGinnis, Elimelech, M., Energy re- Figure 5. Comparison of desali- quirements of ammonia-carbon dioxide for- nation technologies on the ward osmosis desalination, Desalination, basis of ‘equivalent work’, or Accepted (2006). the quantity and value of the energy used. About the authors
Equivalent work is a calculation giving the electricity that Jeffrey McCutcheon (jeffrey.mccutcheon@ a given quantity of heat could create if that heat, as yale.edu) is a graduate student in the Depart- steam, were expanded in a turbine. Added to this is the ment of Chemical Engineering at Yale Uni- electrical energy consumed directly by the process. MSF versity. His research focuses on membrane above refers to Multi-Stage Flash Distillation; MED-TVC for Multi-Effect Distillation with Thermal Vapor transport and concentration/polarization Compression and MED-LT for Multi-Effect Distillation, phenomena in forward osmosis. Robert Low Temperature. FO-LT refers to Forward Osmosis, McGinnis ([email protected]) is an Low Temperature (vacuum column).6 NSF graduate student fellow in the Environ- 6 mental Engineering Program at Yale Uni- MSF 5 versity. His research focuses on membrane MED-TVC systems for the sustainable production of

3 4 water and power. Corresponding author MED-LT RO 3 Menachem Elimelech (menachem.elimelech@ yale.edu) is the Roberto Goizueta Professor of kWh / m 2 Environmental and Chemical Engineering, 1 FO-LT Chair of the Department of Chemical Engi- neering and Director of the Environmental 0 Engineering Program at Yale University.

O CTOBER 2006 Water Conditioning & Purification