Reverse osmosis
The reverse osmosis or reverse osmosis is a physical process of membrane technology for the concentration of dissolved substances in liquids which, with the pressure of the natural osmosis is reversed process.
Working principle
The medium in which the concentration of a certain substance is to be reduced is separated from the medium in which the concentration is to be increased by a semi-permeable membrane. This is exposed to a pressure that has to be higher than the pressure created by the osmotic desire to balance the concentration. This allows the molecules of the solvent to migrate against their “natural” osmotic direction of propagation. The process pushes them into the compartment where dissolved substances are less concentrated.
Drinking water has an osmotic pressure of less than 2 bar , the pressure used for the reverse osmosis of drinking water is 3 to 30 bar, depending on the membrane used and the system configuration. A pressure of 60 to 80 bar is required for seawater desalination, since seawater at around 30 bar has a significantly higher osmotic pressure than drinking water. In the Dead Sea there is even an osmotic pressure of 350 bar. In some applications, e.g. B. for the concentration of landfill leachate, even higher pressures are used.
The osmotic membrane , which only allows the carrier liquid (solvent) through and holds back the dissolved substances ( solutes ), must be able to withstand these high pressures. If the pressure difference more than compensates for the osmotic gradient, the solvent molecules fit through the membrane like a filter , while the "impurity molecules " are retained. In contrast to a classic membrane filter, osmosis membranes do not have continuous pores. Rather, the ions and molecules migrate through the membrane by diffusing through the membrane material . The solution-diffusion model describes this process.
Scheme drawing of a reverse osmosis system (seawater desalination) with a pressure exchanger.
1: sea water inflow, 2: fresh
water flow (40%), 3: salt water concentrate (60%), 4: sea water inflow (60%),
5: drainage of the salt water concentrate,
A: inflow by high pressure pump (40%), B: circulation pump,
C: Osmosis unit with membrane, D: pressure exchanger
The osmotic pressure increases as the difference in concentration increases. If the osmotic pressure equals the applied pressure, the process comes to a standstill. There is then an osmotic equilibrium. A steady drainage of the concentrate can prevent this. At the concentrate outlet, the pressure is either controlled by a pressure regulator or used by a pressure exchanger to build up the pressure required in the system inlet. Pressure exchangers reduce the operating costs of a reverse osmosis system very effectively through energy recovery . The energy consumption per cubic meter of water is 4–9 kWh.
The solutes must be prevented from crystallizing ( precipitating ) in the membranes. This may (by the addition of anti-plaque agents English Antiscaling ) or acids can be achieved. Antifouling agents are polymeric compounds based on phosphate or maleic acid, which surround the crystallites that are formed and thus prevent crystalline precipitates from forming on the membrane. It may still be necessary to clean the membrane.
Filters can be connected upstream to prevent damage to the membrane. A fine filter can prevent mechanical damage , while an activated carbon filter can prevent chemical damage (e.g. from chlorine ).
It may also be necessary to free the system from biological pollution, especially when treating seawater. Biofilms that form discontinuously are removed using biocides (mostly based on bromine). Chlorine is mainly used for disinfection in southern countries. Because the membranes are sensitive to chlorine, it has to be removed again at great expense.
Applications
Military and space travel
Mobile systems based on the principle of reverse osmosis have been developed for military use, which can obtain drinking water from almost any water source. The systems are either housed in containers or trailers, but can also be installed in stand-alone vehicles. Although there are different mobile reverse osmosis systems for each area of application ( reverse osmosis water purification unit , ROWPU ), they all work on a similar principle. The water is pumped from the source to the facility, where it is treated with a polymer to start coagulation . The water then passes through a filter, which is responsible for the ion exchange. A downstream, spiral-shaped cotton filter cleans the pretreated water from impurities that are larger than 5 micrometers. It then passes through several vessels in which reverse osmosis takes place. Finally, the water is subjected to chlorination.
On behalf of NASA , procedures for applying reverse osmosis to one's own urine during stays in space were developed.
Drinking water treatment
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Reverse osmosis is used as an intermediate step in numerous treatment systems for drinking water in households. Depending on the water quality, such systems work with combinations of membranes and filters (different pore sizes, activated carbon filters) and possibly ultraviolet light to remove microbes that were not kept away by the filters and membranes. In some systems, the pre-stage of the activated carbon filter is replaced by a cellulose acetate membrane. This membrane is broken down unless chlorinated water is used. A downstream activated carbon filter removes the previously added chlorine.
Mobile treatment systems are sold for personal use. A line pressure of at least 280 kPa is required for the functioning of these systems . Such treatment plants are mainly used in rural regions without clean water that are not connected to a water treatment plant. Another possible application is the production of drinking water on the high seas or in countries where tap water is contaminated. In the production of mineral water in bottles, the water goes through a reverse osmosis system to rid it of impurities and microorganisms. Such systems work with germ barriers that are connected upstream of the osmosis membrane. However, such an approach is not allowed in the EU. Some of the microorganisms pass the filters in reverse osmosis systems due to small cracks or irregular pore sizes. Such systems therefore work with additional cleaning stages (ultraviolet light, ozone, sterile filters).
Domestic water treatment
In industry, boiler water is subjected to reverse osmosis in power plants in order to remove minerals from it. This process is intended to prevent the evaporated water from leaving calcifications. Pre-cleaned brackish water is also subjected to reverse osmosis and is also used in the production of demineralized water and aquarium water .
Other uses
Reverse osmosis can also be used to increase the concentration of dissolved substances, in which case the carrier substance is concentrated e.g. B. in the production of fruit juice concentrates or for the compression of must in wine production . Reverse osmosis is also used in the production of non-alcoholic beer , milk concentrates and protein powders .
See also
literature
- Matthias Kraume: Transport processes in process engineering. Springer-Verlag, 2004, pp. 262-263
Individual evidence
- ↑ KW Böddeker, H. Strathmann: The membrane filtration, chemistry in our time, 8th year 1974, p. 105.
- ↑ Melin, Thomas and Rautenbach, Robert: Membranverfahren , p. 264 f., Springer, 2007, ISBN 3-540-00071-2 .
- ↑ Reverse Osmosis Water Purification Unit (ROWPU)
- ↑ Fethi BenJemaa: Logistics for deploying mobile water desalination units (2009), State of California: Department of Water Resource. - Not available on April 6, 2016.
- ↑ Rotating Reverse Osmosis for Wastewater Reuse, article on www.nasa.gov (PDF file; 2.53 MB), page 4, last accessed on February 27, 2011
- ^ Robert Rautenbach: Membrane Process: Basics of Module and System Design (2013), Springer-Verlag
- ↑ Erik Voigt, Henry Jaeger, Dietrich Knorr: Securing Safe Water Supplies: Comparison of Applicable Technologies (2013) Academic Press
- ↑ Council Directive of 15 July 1980 on the approximation of the laws of the Member States relating to the exploitation and marketing of natural mineral waters (PDF)
- ^ Vishal Shah: Emerging Environmental Technologies, Volume 1 (2008), Springer Science & Business Media
- ^ Andrej Grabowski: Electromembrane Desalination Processes for Production of Low Conductivity Water (2010), Logos Verlag Berlin