Reverse Osmosis is the opposite of the naturally occurring osmosis process achieved by inducing high pressures. While this has been used in water treatment for some time now, forward osmosis is gaining momentum for the past few years. Today, it is actively used for treating wastewater and enabling zero or minimal liquid waste discharge for industries.
The FO Process:
In FO, the driving force is the draw solution which is more concentrated than the feed. The net flow of water molecules from the feed solution through the semi-permeable membrane is induced by a higher concentration of the draw solution. This semi-permeable membrane is very selective to water molecules.
In FO, solute diffusion is possible in both directions. The diffusion happens based on the composition of both the solutions. This means the draw solutes may diffuse to feed and vice versa. For any particular FO process therefore, the required permeate water and concentration of the feed plays a very important role in designing the concentrate of the draw solution.
Unlike in RO, the membrane between the two solutions simply results in the trade of solutes between them. Depending on the concentration of the feed and the intended purpose of the FO process, trade is all that may be required from it. This is also called engineered or manipulated osmosis due to the adjustment of concentrate in draw solution to obtain required permeate water.
Applications
Forward Osmosis Applications include but are not restricted to:
• Emergency drinks or hydration bags: FO is a helpful tool in emergency situations. With enough time for contact, the membrane readily rejects contaminants to allow passage of water to the draw solution. Once it is diluted enough, it can be ingested directly. The concentrates in the bag are mostly glucose, fructose, and other sugar compounds.
• Desalination: FO in a combination of membrane separation, physical or thermal, can produce a diluted draw solution. It has an advantage of low fouling which is a big problem with membranes in conventional RO desalination plants. National Geographic cited this process as one of the three promising technologies that can reduce the energy consumption of desalination plants in 2010.
• Evaporative Cooling Towers: In this process, the coolant is used as the draw solution and the evaporated water is replaced by brackish or seawater. The energy consumption is lower and fouling is comparatively very low in forward osmosis.
• As Pre-treatment Process: FO is an asset in pre-treatment of feed water before it goes through RO. Used as FO/RO, it removes major fouling components before they reach the RO membrane. This saves a lot of energy in RO.
• Retention Of Required Compounds: FO is the only process that helps retain essential compounds. The draw solution concentrate can be designed to retain aromas, flavors, nutrients, and colors. This is very helpful in the dairy, beer brewing, coffee brewing, and beverage industry.