On the other side of purity: what can and what can not reverse osmosis membrane
Reverse osmosis water is in every sense an illustration of the H2O dichotomy / Impurities and Useful / Harmful . We at AKVAFOR are accustomed to the world being divided into:
Both assumptions are matters of personal faith. We set the task to talk about how things are with the osmotic water in the real world and add pastel colors to the existing black and white picture.
Let's talk about the principle of operation of the membrane, about the difference between osmotic water from distillate and electrolyte, and also about whether to look for pores in it and boil it in acid.
')
Not really. The acquaintance of man with semipermeable membranes began with the history of the attentive French abbot Nola in the middle of the 18th century. He poured the wine into the pork bladder and left it in storage in the barrel with water. The wine became similar to juice, the bubble increased, and the phenomenon received from Nola the name osmosis (from the Greek “pressure”). The Abbot described the properties of a semi-permeable membrane and its main ability - to pass only water.
Later, natural scientists, botanists and physiologists who were interested in the natural manifestations of osmosis, in particular, the nutrition of plants and human cells, joined the research. Physicians and chemists, who were concerned about the task of “repeating the process on an industrial scale” for the desalination of fresh water and desalination of the sea, were a separate branch of the interested subjects.
Today, the reverse osmosis membrane is a thin polymer film deposited on an inert substrate that is completely permeable to water. The most important property of the membrane is the ability to swell - that is, to react with molecules and bind to them. This process is called hydration. Other substances dissolved in water cannot react with the membrane material, and when water pressure in the water supply system is applied to the swollen membrane, water molecules begin to leak (squeeze out) through the membrane.
When the osmotic pressure is compared with external, the transition of water through the membrane stops. At this point, the discharge of the concentrate into the drainage does not allow osmotic pressure to rise and the water molecules continue their way through the membrane into the storage tank.
Over the past decades, membrane materials have been modified, of the most common ones, we note:
- Polyacetate
Cellulose. The old generation of semipermeable membranes, which passed up to 50% of nitrates. The presence of prefiltration in this case does not help, because it also does not "see" nitrates. The cellulose base of polyacetate membranes provoked an active proliferation of bacteria.
- Polyamide
In the last decade, this type of membrane has gained widespread distribution due to its resistance to biogrowth and selectivity of 92–99%. In its reverse osmosis systems, AKVAFOR uses Polyamide 66, which is essentially nylon.
It is necessary to distinguish between household thin-film membranes and membranes that are used to desalinate seawater. The principle of operation of these membranes is the same, but technically the desalination membrane is arranged differently. In order to “squeeze out” H2O from seawater it is necessary to overcome its higher osmotic pressure, the thin-film membrane will break under such conditions. To work with high loads during desalination, a different technical design is required: the membrane is made of other materials and has a more dense substrate (for example, ceramic).
The opinion that the membrane works due to the presence of “very small pores” in it does not correspond to reality. The reverse osmosis membrane has no pores. The separation of water into permeate (purified water) and retentate (a concentrate of impurities leaving into drainage) occurs due to a process similar to the transfer of electric current through a metal semiconductor.
The ability to “conduct” water is a property of a certain class of polymeric materials, similar to the ability of metals to conduct electrical current. At the same time there are materials that do not conduct either one or the other.
Let's compare the filtration abilities of sorption and reverse osmosis filters by types of pollution:
Not all impurities are subject to 100% removal, even by a reverse osmosis membrane. Recall that the membranes were originally created for the desalination of water (in areas where the drinking water is noticeably salty, but not yet sea water). Therefore, standard tests for the removal of salts by the membrane were carried out on a solution of sodium chloride - sodium chloride. And indeed, osmosis can provide salt removal by 99%. However, when the water is very hard, the efficiency can be reduced to 93-95%, due to an increase in the “leakage”.
For household osmosis, membranes with selectivity from 97 to 99% are most often used. They are rationed for sodium chlorine, but this does not mean that it will be the same for other substances. Different pollutants "overshoot" may be different, it depends on their nature. For example, some boron compounds pass through the membrane quite successfully, while others, for example, large organic molecules, on the contrary, are removed almost 100%.
Proskok occurs for three reasons:
A small percentage of leakage can occur with any membrane, which is why measurements of salt content (and in fact, conductivity) with the help of a TDS meter show very low, but not zero, results. The effectiveness of the TDS-meter, by the way, is devoted to the previous post .
Simultaneously with the process of transferring water models through the membrane, a diffusion process takes place. Both contaminants and ions can diffuse through the membrane. The ions most actively diffuse when there are a lot of them on one side and not so much on the other. Selectivity can be different for different ions. The organic molecule is large, in a small concentration it will not actively “attack” the membrane and diffuse. And, for example, sodium, which is present in excess in hard water, will cause more active diffusion.
Compared with the main transfer of water molecules through the membrane, the volume of diffusion of ions is negligible. There are few of them and this is a very long process, however, we cannot fail to mention it when we speak about the origin of the selectivity of the membrane “99%”.
Diffusion is usually noticeable in the first portion of water, which is obtained after a long stagnation - idle filter. During this time, the concentration of salts on both sides of the membrane has time to level and the filter can release salt water in the first glass.
There are substances that easily fool the membrane. Among them - boron / borates. At neutral pH, boron is in solution in the form of an H3BO3 molecule and, by some properties, is very similar to water in a membrane. This allows boron to pass through the membrane for the company. If the pH is changed to alkaline, then boron will be in solution in the form of a charged ion — an anion of boric acid or tetraborate. In the form of an anion, boron is already perfectly cut off by a membrane.
Some highly volatile organic compounds are characterized by high diffusion activity. For example, chloroform is able to penetrate the membrane, but is easily removed with a carbon prefilter. The membrane is not designed to remove gases, in particular, hydrogen sulfide. Residents of megalopolises should not worry, water with hydrogen sulfide will not be put into the water supply network, and boron is not toxic in all forms. Boric acid, for example, is buried in children’s ears.
The reasons for which the membrane fails:
Paradoxically, the ability to almost completely purify water from impurities can be considered by many as a disadvantage. The very principle of filtration with the use of drainage water under the auspices of sitting on the “needle of operating costs” is also recorded in the minuses. We have compiled a small FAQ on these and similar issues.
What the fans of the term “dead” water with respect to reverse osmosis filters mean is not completely clear to us. From the point of view of official science there is neither living, nor dead water. Each H2O molecule on the planet was once processed by the body and isolated as waste products. New molecules are not formed, but there is a cycle and an excellent option is to thoroughly clean the aqueous solution of all the superficial with a membrane. To maintain the body processes it requires exactly H2O, the rest is divided into two groups:
The absence of impurities, and with them the so-called “useful minerals”, does not stand up to nutritional criticism. Calcium is almost not absorbed by our body from water, because it is there in the form of inorganic salts. Even if this calcium was absorbed, it would be difficult to drink 17 liters of Moscow medium hard water to satisfy the daily need for this element - about 1000 mg. Magnesium can only digest well from water, but we talked about this separately .
Typical example
I had to make a serious calculation and find out that 300 rubles per cubic meter of clean water is about 30 kopecks per liter. We offer to compare with the cost of a liter of drinking water in the store, because its quality in plastic is similar, if not worse . Depending on the pathos of the outlet, the price of a liter of the same osmotic water will be from 15 rubles per liter.
Water is not a way for us to get energy, not food, not a way to get “bricks” for building the body. This is the environment in which the chemical and physical processes of the body take place. And she herself is rather inert and almost never participates in these processes. We do not split it into hydrogen and oxygen, the body does not undergo the process of electrolysis.
Understanding this role of water, you can drink distilled water, in which there are no “useful minerals”. Having a balanced diet, you will not get any problems.
We share the find:
An electrolyte is any liquid that conducts electrical current. For example, soup or compote. That is all liquid that conducts electrical current due to the movement of ions.
Washing the membrane really does. However, this applies to industrial membranes and special formulations - alkaline or acidic detergents. This is a whole arsenal of surfactants, depending on which particles “stuck” onto the membrane. In the case of industrial membranes, everything is known about these particles and the composition is selected individually.
Watching videos in youtube about how membranes are boiled in citric acid is a little sad, because before our eyes people waste time in vain - the membrane loses its properties from heat.
Whether or not to use a reverse osmosis membrane is a matter of motivation and desire to minimize the impact of negative environmental factors. There is no useful water, as well as useful air. Useful air is inhalation, but it is no longer air. And the formula of pure water from our laboratory is:
- those who believe that the osmotic filter cleans everything except karma and conscience
- and those who pour osmotic water into the tea of ​​the enemy, considering it dead.
Both assumptions are matters of personal faith. We set the task to talk about how things are with the osmotic water in the real world and add pastel colors to the existing black and white picture.
Let's talk about the principle of operation of the membrane, about the difference between osmotic water from distillate and electrolyte, and also about whether to look for pores in it and boil it in acid.
')
Made by military scientists for submarines?
Not really. The acquaintance of man with semipermeable membranes began with the history of the attentive French abbot Nola in the middle of the 18th century. He poured the wine into the pork bladder and left it in storage in the barrel with water. The wine became similar to juice, the bubble increased, and the phenomenon received from Nola the name osmosis (from the Greek “pressure”). The Abbot described the properties of a semi-permeable membrane and its main ability - to pass only water.
Later, natural scientists, botanists and physiologists who were interested in the natural manifestations of osmosis, in particular, the nutrition of plants and human cells, joined the research. Physicians and chemists, who were concerned about the task of “repeating the process on an industrial scale” for the desalination of fresh water and desalination of the sea, were a separate branch of the interested subjects.
The principle of operation of the household reverse osmosis membrane
Today, the reverse osmosis membrane is a thin polymer film deposited on an inert substrate that is completely permeable to water. The most important property of the membrane is the ability to swell - that is, to react with molecules and bind to them. This process is called hydration. Other substances dissolved in water cannot react with the membrane material, and when water pressure in the water supply system is applied to the swollen membrane, water molecules begin to leak (squeeze out) through the membrane.
When the osmotic pressure is compared with external, the transition of water through the membrane stops. At this point, the discharge of the concentrate into the drainage does not allow osmotic pressure to rise and the water molecules continue their way through the membrane into the storage tank.
What are modern membranes made of?
Over the past decades, membrane materials have been modified, of the most common ones, we note:
- Polyacetate
Cellulose. The old generation of semipermeable membranes, which passed up to 50% of nitrates. The presence of prefiltration in this case does not help, because it also does not "see" nitrates. The cellulose base of polyacetate membranes provoked an active proliferation of bacteria.
- Polyamide
In the last decade, this type of membrane has gained widespread distribution due to its resistance to biogrowth and selectivity of 92–99%. In its reverse osmosis systems, AKVAFOR uses Polyamide 66, which is essentially nylon.
It is necessary to distinguish between household thin-film membranes and membranes that are used to desalinate seawater. The principle of operation of these membranes is the same, but technically the desalination membrane is arranged differently. In order to “squeeze out” H2O from seawater it is necessary to overcome its higher osmotic pressure, the thin-film membrane will break under such conditions. To work with high loads during desalination, a different technical design is required: the membrane is made of other materials and has a more dense substrate (for example, ceramic).
Osmosis is not a sieve!
The opinion that the membrane works due to the presence of “very small pores” in it does not correspond to reality. The reverse osmosis membrane has no pores. The separation of water into permeate (purified water) and retentate (a concentrate of impurities leaving into drainage) occurs due to a process similar to the transfer of electric current through a metal semiconductor.
The ability to “conduct” water is a property of a certain class of polymeric materials, similar to the ability of metals to conduct electrical current. At the same time there are materials that do not conduct either one or the other.
The mechanism of transfer of water molecules through a membrane is similar to the process of transferring current through a metallic conductor. In it, as well as in the membrane there are no holes, nevertheless, the current in the form of electrons flows through the material from the place where there are many of them in the direction of lower “concentration”.
Why membrane selectivity is not always 100%?
Let's compare the filtration abilities of sorption and reverse osmosis filters by types of pollution:
Not all impurities are subject to 100% removal, even by a reverse osmosis membrane. Recall that the membranes were originally created for the desalination of water (in areas where the drinking water is noticeably salty, but not yet sea water). Therefore, standard tests for the removal of salts by the membrane were carried out on a solution of sodium chloride - sodium chloride. And indeed, osmosis can provide salt removal by 99%. However, when the water is very hard, the efficiency can be reduced to 93-95%, due to an increase in the “leakage”.
For household osmosis, membranes with selectivity from 97 to 99% are most often used. They are rationed for sodium chlorine, but this does not mean that it will be the same for other substances. Different pollutants "overshoot" may be different, it depends on their nature. For example, some boron compounds pass through the membrane quite successfully, while others, for example, large organic molecules, on the contrary, are removed almost 100%.
Proskok occurs for three reasons:
- "Mimicry". If there is something in the water that, in its chemical behavior, resembles a water molecule, it can form bonds with the material of the membrane and “pass for the company”.
- Diffusion, we will tell about it in more detail later.
- Damage or poor membrane quality.
About mimicry. Imagine a line of workers passing the chain of bricks. If several bricks are replaced with something very similar, that is, “heavy and square”, it is unlikely that someone in the chain will notice the substitution.
A small percentage of leakage can occur with any membrane, which is why measurements of salt content (and in fact, conductivity) with the help of a TDS meter show very low, but not zero, results. The effectiveness of the TDS-meter, by the way, is devoted to the previous post .
Diffusion - a parallel process
Simultaneously with the process of transferring water models through the membrane, a diffusion process takes place. Both contaminants and ions can diffuse through the membrane. The ions most actively diffuse when there are a lot of them on one side and not so much on the other. Selectivity can be different for different ions. The organic molecule is large, in a small concentration it will not actively “attack” the membrane and diffuse. And, for example, sodium, which is present in excess in hard water, will cause more active diffusion.
Compared with the main transfer of water molecules through the membrane, the volume of diffusion of ions is negligible. There are few of them and this is a very long process, however, we cannot fail to mention it when we speak about the origin of the selectivity of the membrane “99%”.
Diffusion is usually noticeable in the first portion of water, which is obtained after a long stagnation - idle filter. During this time, the concentration of salts on both sides of the membrane has time to level and the filter can release salt water in the first glass.
Any material is subject to diffusion. Do you think polyethylene is sealed? Gases pass through it with a whistle, albeit quiet. Helium diffuses much faster from a balloon.
What does not even clean the reverse osmosis filter?
There are substances that easily fool the membrane. Among them - boron / borates. At neutral pH, boron is in solution in the form of an H3BO3 molecule and, by some properties, is very similar to water in a membrane. This allows boron to pass through the membrane for the company. If the pH is changed to alkaline, then boron will be in solution in the form of a charged ion — an anion of boric acid or tetraborate. In the form of an anion, boron is already perfectly cut off by a membrane.
Some highly volatile organic compounds are characterized by high diffusion activity. For example, chloroform is able to penetrate the membrane, but is easily removed with a carbon prefilter. The membrane is not designed to remove gases, in particular, hydrogen sulfide. Residents of megalopolises should not worry, water with hydrogen sulfide will not be put into the water supply network, and boron is not toxic in all forms. Boric acid, for example, is buried in children’s ears.
Membrane Health Factors
The reasons for which the membrane fails:
- Physical damage
- Loss of ability to hydration due to exposure to chlorinated water or other oxidizing agents (ozonation) or due to membrane drying. Drying can become irreversible, therefore, drying of an already running membrane should not be allowed. And in order to prevent membrane damage due to chlorine, the reverse osmosis filter necessarily includes carbon blocks of preliminary cleaning.
- Deposition on the surface of insoluble salts and mechanical impurities present in the water (poor / insufficient prefiltration)
- Insufficient water flow to drainage or “saving” drainage water.
Best the enemy of the good?
Paradoxically, the ability to almost completely purify water from impurities can be considered by many as a disadvantage. The very principle of filtration with the use of drainage water under the auspices of sitting on the “needle of operating costs” is also recorded in the minuses. We have compiled a small FAQ on these and similar issues.
1. Is “dead” water? (our favorite)
What the fans of the term “dead” water with respect to reverse osmosis filters mean is not completely clear to us. From the point of view of official science there is neither living, nor dead water. Each H2O molecule on the planet was once processed by the body and isolated as waste products. New molecules are not formed, but there is a cycle and an excellent option is to thoroughly clean the aqueous solution of all the superficial with a membrane. To maintain the body processes it requires exactly H2O, the rest is divided into two groups:
- optional, as it comes from food;
- unhealthy in the short or long term.
The absence of impurities, and with them the so-called “useful minerals”, does not stand up to nutritional criticism. Calcium is almost not absorbed by our body from water, because it is there in the form of inorganic salts. Even if this calcium was absorbed, it would be difficult to drink 17 liters of Moscow medium hard water to satisfy the daily need for this element - about 1000 mg. Magnesium can only digest well from water, but we talked about this separately .
2. Is it expensive to buy and expensive to maintain?
Typical example
I had to make a serious calculation and find out that 300 rubles per cubic meter of clean water is about 30 kopecks per liter. We offer to compare with the cost of a liter of drinking water in the store, because its quality in plastic is similar, if not worse . Depending on the pathos of the outlet, the price of a liter of the same osmotic water will be from 15 rubles per liter.
3. Why is reverse osmosis water not a distillate?
Water is not a way for us to get energy, not food, not a way to get “bricks” for building the body. This is the environment in which the chemical and physical processes of the body take place. And she herself is rather inert and almost never participates in these processes. We do not split it into hydrogen and oxygen, the body does not undergo the process of electrolysis.
Understanding this role of water, you can drink distilled water, in which there are no “useful minerals”. Having a balanced diet, you will not get any problems.
The danger of distilled water is that it can just be “dirty.” Evaporation does not relieve water from the impurities of organic matter, the boiling point of which is below 10 ° C.There is a huge difference between the water after the reverse osmosis membrane and distilled water. Osmosis does not completely cut off the dissolved salts, and during distillation, salts are completely left in the still. Volatile substances, that is, almost all organic matter, are transported with steam to the distillate. In addition, before the distillers had rubber tubes, which added “tasteless” to the resulting water.
4. Why is reverse osmosis water not an electrolyte?
We share the find:
An electrolyte is any liquid that conducts electrical current. For example, soup or compote. That is all liquid that conducts electrical current due to the movement of ions.
Electrolytes are a delicious lunch.
5. Does the membrane need flushing?
Washing the membrane really does. However, this applies to industrial membranes and special formulations - alkaline or acidic detergents. This is a whole arsenal of surfactants, depending on which particles “stuck” onto the membrane. In the case of industrial membranes, everything is known about these particles and the composition is selected individually.
Watching videos in youtube about how membranes are boiled in citric acid is a little sad, because before our eyes people waste time in vain - the membrane loses its properties from heat.
What did we want to say?
- Historically, reverse osmosis membranes are good because they are able to perfectly desalinate water, for this they were created. This property removes suffering from those who have drinking water either very hard or just salty.
- Household membranes can filter the maximum of harmful impurities that enrich the water of the modern world. This ability of membranes exceeds the capabilities of assembly filters. Sorption filters can also reduce stiffness with ion-cartridge cartridges. However, there are a number of annoying limitations on efficiency and resource.
- The harm of water after an osmotic filter has not been proven. The cleaner the water, the easier it is for metabolic processes in the body.
- The cost of a liter of pure water using a membrane will be slightly higher than that of a sorption water purifier. But you need to compare the degree of purification with the bottles from the store, since the water in the plastic is purified by the same technology.
Whether or not to use a reverse osmosis membrane is a matter of motivation and desire to minimize the impact of negative environmental factors. There is no useful water, as well as useful air. Useful air is inhalation, but it is no longer air. And the formula of pure water from our laboratory is:
Water should be invisible in tea.
Source: https://habr.com/ru/post/441376/
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