. . . an osmotic process that, like reverse osmosis (RO), uses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force of 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 water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the RO process uses hydraulic pressure as the driving force of separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from permeate to the feed.
An additional distinction between RO and 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.
FO is a comparatively new technology that can be used to remove difficult toxic waste and pollutants from industrial waste water. The main advantages of using FO are that:
- it operates at low or no hydraulic pressures,
- it has high rejection of a wide range of contaminants, and
- it has a lower membrane fouling propensity than pressure-driven membrane process.
FO membranes reject organics, minerals and other solids–similar to RO–but resist typical fouling problems.
One straightforward application of FO is found in “hydration bags” which use an ingestible draw solution and intended for separation of water from dilute feeds. This allows, for example, the ingestion of surface water that may contain pathogens 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 undesirable feed constituent behind. The diluted draw solution may then be ingested directly. Typically, the draw solutes are sugars such as glucose or fructose, which provide some nutrition to the user of the FO device.
Hydration bags can be employed with highly concentrated saline feedwater sources such as seawater. One of the first uses of FO with ingestible solutes was for survival in life rafts at sea. According to Sherwin Gormly, Process R&D Engineer for HTI (Hydration Technology Innovations), you throw the bag in the water:
The sugar being stronger than the seawater in terms of its osmotic pull (or its potential to attract water) will actually draw enough water out of ocean to produce a nice sweet sports drink. . . providing both food and drinking water.
Personal hydration bags can also work well as an early entry water solution for disaster relief situations. Between 5 and 10,000 small bags can but packed on a pallet and the single-use bags can be used in a variety of water sources. The end product has a great taste along with the added nutritional benefits of electrolytes.
According to Gormly, a bag “produces about one liter in four hours. The bag has a little hole and comes with a straw, so it is user-friendly in a developing-world-type application.” There is some debate among experts whether hydration bags provide water treatment per se because the product is not pure water but a sweet drink that can only be used for specific applications.
The FO process is useful to NASA because it can be used to recycle wastewater–including urine–as part of a closed-loop water system. Long-term human missions in space require a continuous and self-sufficient supply of fresh water for consumption, hygene, and maintenance. Long-range/long-duration missions, like lunar, Mars, or asteroid missions, depend on a water treatment system that recovers potable water from wastewater generated on board a spacecraft or in the planetary habitat. The three sources of wastewater that can be reclaimed and reused are:
- hygiene wastewater
- humidity condensate
Forward osmosis systems are ideally suitted for some types of industrial applications. Instead of hydraulic pressure (as in RO), the process uses salt brine on one side of a membrane. When the waste stream is introduced on the other side of the membrane, the salt pulls water from the waste stream by osmosis. FO removes 75 – 90% of the water from the waste stream, and the membrane is tight enough to retain the nutrients. The diluted salt brine is then reconcentrated using standard RO technology, recovering the brine for re-use and generating clean water for use in the food plant or disposal.
It is worth noting that in most industrial and wastewater applications FO is not the ultimate process, but rather a high-level pretreatment setup for the ultimate desalination process.
Renewable energy can be extracted wherever two streams of different salinity or different chemical potential meet. Pressure-retarded osmosis (PRO), a closely related process to FO, has been tested and evaluated since the 1960s as a potential process for power generation. PRO uses the osmotic pressure difference between seawater, or concentrated brine, and fresh water to pressureize the saline stream, thereby converting the osmotic pressure of seawater into a hydrostatic pressure that can be used to produce electricity.