Forward osmosis
Encyclopedia
Forward osmosis is an osmotic
process that, like reverse osmosis
, uses a semi-permeable membrane to effect separation
of water
from dissolved solutes. The driving force for this separation is an osmotic pressure
gradient, such that a "draw" solution
of high concentration
(relative to that of the feed solution), is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the reverse osmosis process uses hydraulic pressure
as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. The simplest equation describing the relationship between osmotic and hydraulic pressures and water flux is:
where
is water flux, A is the hydraulic permeability of the membrane, Δπ is the difference in osmotic pressures on the two sides of the membrane, and ΔP is the difference in hydrostatic pressure (negative values of 
indicating reverse osmotic flow). The modeling of these relationships is in practice more complex than this equation indicates, with flux depending on the membrane, feed, and draw solution characteristics, as well as the fluid dynamics
within the process itself.
An additional distinction between the reverse osmosis (RO) and forward osmosis (FO) processes is that the water permeating the RO process is in most cases fresh water ready for use. In the FO process, this is not the case. The membrane separation of the FO process in effect results in a "trade" between the solutes of the feed solution and the draw solution. Depending on the concentration of solutes in the feed (which dictates the necessary concentration of solutes in the draw) and the intended use of the product of the FO process, this step may be all that is required.
The forward osmosis process is also known as osmosis or in the case of a number of companies who have coined their own terminology 'engineered osmosis' and 'manipulated osmosis'.
s or toxins
that are readily rejected by the FO membrane. With sufficient contact time, such water will permeate the membrane bag into the draw solution, leaving the undesirable feed constituents behind. The diluted draw solution may then be ingested directly. Typically, the draw solutes are sugars such as glucose
or fructose
, which provide the additional benefit of nutrition to the user of the FO device. A point of additional interest with such bags is that they may be readily used to recycle urine
, greatly extending the ability of a backpacker or soldier to survive in arid environments. This process may also, in principle, be employed with highly concentrated saline feedwater sources such as seawater, as one of the first intended uses of FO with ingestible solutes was for survival in life rafts at sea.
leachate
. A FO membrane separation is used to draw water from the leachate feed into a saline (NaCl) brine. The diluted brine is then passed through a RO process to produce fresh water and a reusable brine concentrate. The advantage of this method is not a savings in energy, but rather in the fact that the FO process is more resistant to fouling
from the leachate feed than a RO process alone would be. A similar FO/RO hybrid has been used for the concentration
of food products, such as fruit juice.
s formed from the mixing of ammonia
and carbon dioxide
gases in water. These salts can reach high concentrations, particularly as the ratio of ammonia to carbon dioxide is increased. An especially convenient property of these salts is that they readily dissociate into ammonia and carbon dioxide gases again, if a solution containing them is heated (to approx. 60 °C, at 1 atm pressure). Once the concentrated draw solution is used to effect separation of water from the FO feed solution, the diluted draw solution is directed to a reboiled stripper (distillation
column) and the solutes are completely removed and recycled for reuse in the FO system. An FO system of this type thereby effects membrane separation of water from the FO feed, using heat as its primary energy source. The quality of heat
used by this process can be very low, at temperatures as low as 40 °C. If FO of this type is used in a cogeneration
environment (waste heat
from a power plant, for example), its energy cost can be greatly reduced compared to RO.
A second area of current research in FO also involves direct removal of draw solutes, in this case by means of a magnetic field. Small (nanoscale) magnetic particles are suspended in solution creating osmotic pressures sufficient for the separation of water from a dilute feed. Once the draw solution containing these particles has been diluted by the FO water flux, they may be separated from that solution by use of a magnet (either against the side of a hydration bag, or around a pipe in-line in a steady state process).
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
process that, like reverse osmosis
Reverse osmosis
Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
, uses a semi-permeable membrane to effect separation
Separation process
In chemistry and chemical engineering, a separation process, or simply a separation, is any mass transfer process used to convert a mixture of substances into two or more distinct product mixtures, at least one of which is enriched in one or more of the mixture's constituents. In some cases, a...
of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
from dissolved solutes. The driving force for this separation is an osmotic pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
gradient, such that a "draw" solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
of high concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
(relative to that of the feed solution), is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the reverse osmosis process uses hydraulic pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. The simplest equation describing the relationship between osmotic and hydraulic pressures and water flux is:
where
is water flux, A is the hydraulic permeability of the membrane, Δπ is the difference in osmotic pressures on the two sides of the membrane, and ΔP is the difference in hydrostatic pressure (negative values of indicating reverse osmotic flow). The modeling of these relationships is in practice more complex than this equation indicates, with flux depending on the membrane, feed, and draw solution characteristics, as well as the fluid dynamicsFluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
within the process itself.
An additional distinction between the reverse osmosis (RO) and forward osmosis (FO) processes is that the water permeating the RO process is in most cases fresh water ready for use. In the FO process, this is not the case. The membrane separation of the FO process in effect results in a "trade" between the solutes of the feed solution and the draw solution. Depending on the concentration of solutes in the feed (which dictates the necessary concentration of solutes in the draw) and the intended use of the product of the FO process, this step may be all that is required.
The forward osmosis process is also known as osmosis or in the case of a number of companies who have coined their own terminology 'engineered osmosis' and 'manipulated osmosis'.
Emergency drinks
One example of an application of this type may be found in "hydration bags", which use an ingestible draw solute and are intended for separation of water from dilute feeds. This allows, for example, the ingestion of water from surface waters (streams, ponds, puddles, etc.) that may be expected to contain pathogenPathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s or toxins
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...
that are readily rejected by the FO membrane. With sufficient contact time, such water will permeate the membrane bag into the draw solution, leaving the undesirable feed constituents behind. The diluted draw solution may then be ingested directly. Typically, the draw solutes are sugars such as glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
or fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
, which provide the additional benefit of nutrition to the user of the FO device. A point of additional interest with such bags is that they may be readily used to recycle urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
, greatly extending the ability of a backpacker or soldier to survive in arid environments. This process may also, in principle, be employed with highly concentrated saline feedwater sources such as seawater, as one of the first intended uses of FO with ingestible solutes was for survival in life rafts at sea.
Desalination
Desalinated water can be produced from the diluted draw / osmotic agent solution, using a second process. This may be by membrane separation, thermal method, physical separation or a combination of these processes. The process has the feature of inherinently low fouling because of the forward osmosis first step, unlike conventional reverse osmosis desalination plants where fouling is often a problem. Modern Water plc has deployed forward osmosis based desalination plants in Gibraltar and Oman.Evaporative cooling tower – make-up water
One other application developed, where only the forward osmosis step is used, is in evaporative cooling make-up water. In this case the cooling water is the draw solution and the water lost by evaporation is simply replaced using water produced by forward osmosis from a suitable source, such as seawater, brackish water, treated sewage effluent or industrial waste water. Thus in comparison with other ‘desalination’ processes that may be used for make-up water the energy consumption is a fraction of these with the added advantage of the low fouling propensity of a forward osmosis process.Landfill leachate treatment
In the case where the desired product is fresh water which does not contain draw solutes, a second separation step is required. The first separation step of FO, driven by an osmotic pressure gradient, does not require a significant energy input (only unpressurized stirring or pumping of the solutions involved). The second separation step, however does typically require energy input. One method used for the second separation step is to employ RO. This approach has been used, for instance, in the treatment of landfillLandfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
leachate
Leachate
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....
. A FO membrane separation is used to draw water from the leachate feed into a saline (NaCl) brine. The diluted brine is then passed through a RO process to produce fresh water and a reusable brine concentrate. The advantage of this method is not a savings in energy, but rather in the fact that the FO process is more resistant to fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
from the leachate feed than a RO process alone would be. A similar FO/RO hybrid has been used for the concentration
Concentrate
A concentrate is a form of substance which has had the majority of its base component removed. Typically this will be the removal of water from a solution or suspension such as the removal of water from fruit juice...
of food products, such as fruit juice.
Research
One area of current research in FO involves the direct removal of draw solutes by thermal means. This process is typically referred to as the "ammonia – carbon dioxide" FO process, as the draw solutes are saltSalt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s formed from the mixing of ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
gases in water. These salts can reach high concentrations, particularly as the ratio of ammonia to carbon dioxide is increased. An especially convenient property of these salts is that they readily dissociate into ammonia and carbon dioxide gases again, if a solution containing them is heated (to approx. 60 °C, at 1 atm pressure). Once the concentrated draw solution is used to effect separation of water from the FO feed solution, the diluted draw solution is directed to a reboiled stripper (distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
column) and the solutes are completely removed and recycled for reuse in the FO system. An FO system of this type thereby effects membrane separation of water from the FO feed, using heat as its primary energy source. The quality of heat
Energy quality
Energy quality is the contrast between different forms of energy, the different trophic levels in ecological systems and the propensity of energy to convert from one form to another. The concept refers to the empirical experience of the characteristics, or qualia, of different energy forms as they...
used by this process can be very low, at temperatures as low as 40 °C. If FO of this type is used in a cogeneration
Cogeneration
Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....
environment (waste heat
Waste heat
Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...
from a power plant, for example), its energy cost can be greatly reduced compared to RO.
A second area of current research in FO also involves direct removal of draw solutes, in this case by means of a magnetic field. Small (nanoscale) magnetic particles are suspended in solution creating osmotic pressures sufficient for the separation of water from a dilute feed. Once the draw solution containing these particles has been diluted by the FO water flux, they may be separated from that solution by use of a magnet (either against the side of a hydration bag, or around a pipe in-line in a steady state process).