Osmosis is a natural process. In its simplest form, water flows through the membrane into a solution. It is important for plant and animal life. Nutrients must enter cells and waste products leave. Trees must bring nutrients from the soil to the upper most leaves (See: Maple Tree Details at DrOsmosis.com).
Experimenters found that a large pressure must be applied to the solution to stop the uphill flow of water through the membrane into the solution. (This osmotic pressure is commonly labeled as PI (π).)
When a pressure larger than PI (π) is applied to the solution, pure water flows from the solution across the membrane. This is called reverse osmosis. It is commonly used to extract pure water through a membrane from a solution (more commonly sea water or wastewater). Recently, other methods have been used to extract pure water from dirty water. Forward osmosis and manipulated osmosis are two examples. Many of today’s explanation of the new processes still rely on reasoning from the 1900’s. Although osmotic pressure is a dominant and measurable variable in the osmosis experiment, it is not the driving force for water flow across a membrane.
We have proposed that vapor pressure is the driving force for osmotic flow through a membrane (Osmosis: The Molecular Theory, Larry Howlett, 2013). It is well known the water vapor pressure increases with applied pressure and temperature. Vapor pressure decreases when solute concentration is increased.
The following table shows various osmosis/reverse osmosis processes. In all cases, pure water flows from the higher vapor pressure volume through the membrane to the lower vapor pressure region.
| sugar | sugar | External | Vapor | |||||
| oz | Vol 1 | Vol 2 | oz | Pressure | Pressure | Flow | ||
| Osmosis | ||||||||
| water | water | none | Pv1=Pv2 | none | ||||
| water | sugar-water | 1 | none | Pv1>Pv2 | 1→2 | |||
| 1 | sugar-water | sugar-water | 1 | PI | Pv1=Pv2 | none | ||
| Reverse Osmosis | ||||||||
| water | sugar-water | 1 | >PI | Pv1<Pv2 | 1 ←2 | Reverse Osmosis | ||
| water | sugar-water | 1 | none | Pv1<Pv2 | 1 ←2 | sugar water heated | ||
| 2 | sugar-sugar-water | sugar-water | 1 | none | Pv1<Pv2 | 1 ←2 | forward osmosis | |
| Pv = vapor pressure of water | PI=Osmotic Pressure |
References
Water demineralization by membrane distillation, M Gryta, 2018, Polish Journal of Chemical Technology
New Processes in seawater Desalination, AE Al-Rawafeh & MA Zarooni, 2008, Bentham Science publishers Ltd.
A Review on PV-RO Process: MC Garg & H Joshi, 2015, Separation Science and Technology.
Forward Osmosis: Where are we now? DL Shaffer, JR Werber, et al, 2014, Elsevier.
Scaling and fouling in membrane distillation for desalination applications: A review, DM Warsinger, J Swaminathan, E Guillen-Burrieza, HA Arafat, JH Lienhard, 2014, Elsevier.
Osmotic Potential: An overview of draw solutes for forward osmosis, DJ Johnson, WA Suwaileh, AW Mohammed, N Hilal, 2017, Elsevier.
© Larry Howlett HTMD Engineering 2021
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