Forward osmosis is increasingly being used in a range of water treatment applications, from food and beverage production to heavy industry. Here are answers to a few common questions about the process.
What is forward osmosis?
Forward osmosis (FO) is a water separation process in which a semipermeable membrane is used to separate water from dissolved solutes. FO uses natural energy in the form of osmotic pressure to transport water through the membrane while retaining the dissolved solutes on the other side.
How does forward osmosis work?
In FO systems, a solution of lower solute concentration (known as the feed solution) flows on one side of the membrane, while a solution of higher solute concentration (the draw solution) flows on the other side. Osmosis induces water to flow from the feed solution through the membrane and into the draw solution. As the water moves through the membrane, the draw solution becomes diluted and the feed solution is concentrated.
The entire process can be run without additional hydraulic pressure. The draw solution can consist of a simple salt/water mix or a substance specifically tailored for the application.
What can forward osmosis be used for?
FO is generally used for three types of application:
- Product concentration: Water is extracted from the feed solution, leaving a valuable concentrate (for example, a coffee concentrate).
- Waste concentration: Water is extracted from the feed solution, leaving a concentrated waste product that is easy to dispose of.
- The production of clean water: In both valuable product concentration and waste concentration, water moves from the feed solution into the draw solution. This water can then be recovered to produce clean water.
What is the difference between forward osmosis and reverse osmosis?
Like FO, reverse osmosis (RO) is a membrane water treatment solution. The main difference between RO and FO is how water is driven through the membrane. In RO, the water is forced through the membrane using hydraulic pressure. FO uses natural osmotic pressure to induce the flow of water through the membrane.
What is a ‘forward osmosis solution’?
A forward osmosis solution employs a draw recovery system to separate the clean water from the used draw solution. The draw solution is then re-circulated for reuse.
In FO, the water molecules are drawn from the feed solution into the draw solution. In order to produce clean water and recover the draw solution for reuse, the two need to be separated using another type of water treatment system – a draw recovery system. The draw recovery system can be based on either evaporation, membrane-based (typically RO) or responsive thermal draw solutes.
How does forward osmosis contribute to zero liquid discharge (ZLD) or minimal liquid discharge (MLD)?
When it comes to ZLD or MLD, FO can be integrated into water treatment systems to improve recovery rates and reduce the energy required for evaporation.
For effluent treatment plants, for example, the ultrafiltration (UF)/nanofiltration (NF) stage can be fully or partly replaced by FO (option 1 and 2). This can simplify the treatment process, increase system robustness when dealing with challenging contaminants and increase the amount of water sent for reuse. FO can also be added after RO (option 3) to further reduce the volume sent to the evaporator and so reduce operating expenses.
These ZLD/MLD systems are becoming more efficient. For instance, Canadian company Forward Water has created a water treatment system that cleans flow back water and produced water from the oil & gas industry with very little energy required for evaporation. This is achieved using a unique draw solution that converts into gas with just a small amount of heat, such as secondary heat from an existing industrial process.
What about reverse flux / reverse transfer?
Like all semipermeable membranes, some solute from the draw solution can diffuse back through the membrane into the feed solution. This is known as reverse flux or reverse transfer and has to be taken into account when designing an FO solution. Reverse flux is influenced by the choice of draw solution and membrane. The best FO membranes keep the reverse flux to a minimum.