Forward osmosis (FO)

Forward osmosis (FO) is an osmotic process in which water is separated from dissolved solutes through diffusion across a semi-permeable membrane. This process is powered by the osmotic pressure gradient that arises because of the difference in solute concentrations on either side of the semi-permeable membrane. FO is a naturally occurring process that is critical to nearly all living things. In plants, FO makes it possible for water to be transported from the root system to the leaves. In most other organisms, FO is responsible for allowing water to be transported in and out of cells. FO can also be performed manually—a fact that makes the process useful for a variety of practical applications. The three main practical applications of FO are product concentration, waste concentration, and the production of clean water. As a result, the FO process is employed in many industrial settings, including those tied to water desalination, oil and gas production, and mining.

Background

To completely understand FO, one needs to first understand the concept of osmosis itself. The process of osmosis was first discovered and described by French priest and scientist Jean-Antoine Nollet in 1748. However, he did not actually refer to the process as osmosis. That term was later coined in the nineteenth century by another French scientist, René Joachim Henri Dutrochet.

Osmosis is a process in which the molecules of a solvent pass through a semipermeable membrane from a dilute solution to a concentrated solution. As this happens, the concentrated solution becomes increasingly dilute. Although it can involve any liquid or gas, the process of osmosis most frequently occurs with water as the solvent. Essentially, osmosis works by equalizing the concentration of solute particles in a solvent on both sides of a semipermeable membrane. Because the solute particles themselves are unable to cross the membrane, the solvent is forced to pass the less concentrated solution on one side of the membrane to the more concentrated solution on the other side of the membrane.

The process of osmosis is ultimately governed by a force known as osmotic pressure. Osmotic pressure is the pressure that gradually builds on the side of the membrane where volume increases as osmosis proceeds. When osmotic pressure is low, the solvent can pass through the membrane toward the more highly concentrated side. In other words, low osmotic pressure allows osmosis to begin. As more solvent crosses the membrane, osmotic pressure increases until it reaches the point at which additional solvent cannot enter the other side. Once this point is reached, osmosis stops. The osmotic pressure of a given solution is therefore defined as the amount of pressure required to prevent osmosis of that solution.

One of the best examples of osmosis is seen in what happens to red blood cells when they are placed in water. As is the case with many cells, the cell membrane of a red blood cell is semipermeable. When placed in water, the concentration of solute molecules is higher on the inside of the cell than on the outside. This allows water to enter the cell through osmosis. As water is absorbed, the cell expands until the pressure of the cell membrane acting on the contents of the cell becomes too great.

Overview

Forward osmosis (FO) is one of the two types of osmosis, along with reverse osmosis (RO). FO can be thought of as the more traditional form of osmosis, in which an osmotic pressure gradient allows the molecules of solvent to pass from the highly concentrated side of a semipermeable membrane to the less concentrated side of the membrane. In this scenario, the high-concentration solution is referred to as the draw, while the low-concentration solution is referred to as the feed. This means that in FO, the osmotic pressure gradient allows solvent from the feed to pass through the membrane into the draw, thus decreasing the solution of the draw and increasing that of the feed. In effect, RO is the opposite of FO. In RO, hydraulic pressure or some other similar force is used to overcome the natural osmotic pressure gradient and push the molecules of a solution backwards through the membrane from the draw into the feed. In this way, RO can be used to purify water by separating the water itself from any contaminant particles that cannot pass through the membrane.

FO and RO can both be used to do work. The most common practical application for FO and RO is water filtration. Whether they rely on FO or RO, membrane-based water filtration systems make it possible to produce clean water from impaired water sources. In filtration systems that rely on RO, hydraulic pressure is used to force water across the membrane against the osmotic pressure gradient from the feed to the draw. As a result, the water left in the draw is free of any contaminants that cannot pass through the membrane. In FO water filtration systems, the natural osmotic pressure gradient pulls impaired water with a low solute concentration across the membrane from the feed and into a draw that contains a specially engineered solution with a high solute concentration. This process ultimately yields both a draw stream that is diluted with purified water and a concentrated feed stream.

The production of clean water is one of the three main categories of the practical applications of FO. The other two are product concentration and waste concentration. While some applications of FO combine all three of these, most focus primarily on one specific application. In terms of product concentration, FO is often used in the production of concentrated juices or chemicals. As it relates to waste concentration, FO can be used as a part of wastewater treatment processes. FO can also be utilized for purposes like brine concentration and desalination. In reference to the latter, FO can be used to produce freshwater from saltwater by separating the water itself from the salts found within it. For these and other reasons, FO has been adapted for use in a variety of industries, including the water reuse and desalination, oil and gas, mining, energy, and food and beverage production industries.

Bibliography

Elder, Marti. “Converting Seawater to Fresh Water with Forward Osmosis.” TechLink, techlinkcenter.org/technologies/converting-seawater-to-fresh-water-with-forward-osmosis. Accessed 21 Nov. 2024.

“Guide to Forward Osmosis Membranes.” ForwardOsmosisTech, www.forwardosmosistech.com/forward-osmosis-membranes. Accessed 21 Nov. 2024.

Helmenstine, Anne Marie. “Osmosis Definition in Chemistry.” ThoughtCo., 3 May 2019, www.thoughtco.com/definition-of-osmosis-605890. Accessed 6 May 2019.

“Osmotic Process Introduction.” International Forward Osmosis Association, forwardosmosis.biz/education/what-is-forward-osmosis. Accessed 6 May 2019.

Perry, Mark. “The Principles of Forward Osmosis (FO).” ForwardOsmosisTech, 24 Dec. 2013, www.forwardosmosistech.com/the-principles-of-forward-osmosis. Accessed 6 May 2019.

Perry, Mark. “Water Filtration By Forward and Reverse Osmosis Explained in 4 Paragraphs.” ForwardOsmosisTech, 9 Nov. 2016, www.forwardosmosistech.com/water-filtration-by-forward-and-reverse-osmosis-explained-in-4-paragraphs. Accessed 6 May 2019.

Tian, Miao, et al. "Forward Osmosis Membranes: The Significant Roles of Selective Layer." Membranes, 11 Aug. 2022, doi:10.3390/membranes12100955. Accessed 19 Nov. 2024.

Westerling, Kevin. “Forward Osmosis: How It Works, And Why It's Important.” Water Online, 24 Mar. 2014, www.wateronline.com/doc/forward-osmosis-how-it-works-and-why-it-s-important-0001. Accessed 6 May 2019.