On the other side of purity: what a reverse osmosis membrane can and cannot do

Reverse osmosis water is, in every sense, an illustration of the H2O / Impurity dichotomy.

The world is divided into:

1. those who believe that the osmotic filter cleans everything except karma and conscience

2. and those who pour osmotic water into the enemy's tea, believing her to be dead.

Both assumptions are matters of personal belief. We set ourselves the task of telling how things are with osmotic water in the real world and adding 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 it is worth looking for pores in it and boiling it in acid.

Man's acquaintance with semipermeable membranes began with the story of the attentive French abbot Nolet in the middle of the 18th century. He poured the wine into the pig's bladder and left it in a barrel of water for storage. The wine became like juice, the bubble increased, and the phenomenon received from Nole the name osmosis (from the Greek “pressure”). Abbot described the properties of a semi-permeable membrane and its main ability to pass only water. If the bubble of wine that Nole put in the water did not have the ability to stretch, the penetrating water would raise the pressure and the process would stop. The pressure that must be applied to keep water out of the wine is called osmotic pressure. It depends on the difference in concentrations of solutes on both sides of the membrane.

If the abbot had thought of squeezing a bubble of wine and “squeezing out” excess water from it, he could have invented a reverse osmosis filter at the same time.

Later, natural scientists, botanists and physiologists, who were interested in the natural manifestations of osmosis, in particular, the nutrition of plants and cells of the human body, joined the research. A separate branch of interest was made up of physicists and chemists, who were concerned about the task of “repeating the process on an industrial scale” for desalination of fresh water and desalination of sea water.

The principle of operation of a household reverse osmosis membrane

Today, a 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 is applied to the swollen membrane, only water molecules begin to seep (squeeze out) through the membrane.

When water passes through the membrane, the concentration of dissolved substances in front of the membrane increases, and the osmotic pressure increases accordingly.

If the osmotic pressure is equal to the pressure in the system, the passage of water through the membrane will stop. To prevent this from happening, the concentrate is constantly discharged into the drain.

What are modern membranes made of?

Over the past decades, membrane materials have been modified, of the most common, we note:

— Polyacetate

Cellulose. The old generation of semi-permeable membranes, which passed up to 50% of nitrates. The presence of carbon pre-filtration in this case does not help, because it also does not “see” nitrates. The cellulose base of polyacetate membranes provoked active reproduction of bacteria.

— Polyamide

In the last decade, this type of membrane has become widespread, specifically due to its resistance to biogermination and selectivity of 92–99%. In its reverse osmosis systems, AQUAPHOR uses Polyamide 66, which is essentially nylon.

A distinction should be made between household thin-film membranes and membranes used for seawater desalination. The principle of operation of these membranes is the same, but technically the membrane for desalination is arranged differently. In order to “squeeze out” H2O from sea water, 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 denser 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 is not true. The reverse osmosis membrane has no pores. The separation of water into permeate (purified water) and retentate (concentrate of impurities that go to the drain) occurs due to a process similar to the transmission 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 electric 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 the membrane is similar to the process of current transfer through a metal conductor. There are no holes in it, as well as in the membrane, nevertheless, the current in the form of electrons follows through the material from a place where there are many of them in the direction of lower “concentration”.

Why is the membrane selectivity not always 100%?

Let's compare the filtration capacity 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 initially the membranes were created for water desalination (in areas where drinking water is noticeably salty, but not yet sea). Therefore, standard tests for the removal of salts by a membrane were carried out on a solution of common salt (sodium chloride). Indeed, osmosis can achieve 99% salt removal. However, when the water is very hard, the efficiency can be reduced to 93-95% due to the increase in "breakthrough".

For domestic osmosis, membranes with selectivity from 97 to 99% are most often used. They are normalized for sodium chloride, but this does not mean that it will be the same for other substances. For different pollutants, the “breakthrough” may differ, it depends on their nature. For example, some boron compounds pass through the membrane quite successfully, while other compounds, for example, large organic molecules, on the contrary, are removed almost 100%.

"Slippage" occurs for three reasons:

1. "Mimicry". If there is something in the water that is similar in chemical behavior to a water molecule, it can form bonds with the membrane material and “pass along”.

2. Diffusion, we will talk about it in more detail later.

3. Damage or poor quality of the membrane.

About mimicry. Imagine a line of workers passing bricks down a chain. If several bricks are replaced with something very similar, that is, "heavy and rectangular-parallelepiped", it is unlikely that anyone in the chain will notice the substitution.

Any membrane allows some amount of dissolved substances to pass through, which is why measurements of salinity (and in fact, electrical conductivity) using a TDS meter show results that are very low, but not zero.

Diffusion is a parallel process

* Diffusion is a non-equilibrium process of moving a substance from an area of ​​high concentration to an area of ​​low concentration,

leading to spontaneous equalization of concentrations throughout the occupied volume.

Simultaneously with the process of transfer of water molecules through the membrane, the process of diffusion of dissolved substances through it also takes place. The larger the concentration gradient, the greater the diffusion. Of course, the result of this process is also determined by the nature of the diffusing substances: some of them are more "nosy", some less. Other things being equal, large organic ions diffuse worse than small, "nimble" alkali metal ions.

Compared to the main transport of water molecules through the membrane, the amount of diffusible substance is small and can be neglected in household water treatment. However, it is for this reason that the selectivity of the membrane is not 100%.

The result of diffusion is usually noticeable in the first portion of water after a long stagnation - filter downtime. During this time, the concentration of salts on both sides of the membrane has time to level off. The "advanced" filters have special tricks to deal with this problem.

Any material is subject to diffusion. Do you think polyethylene is airtight? Gases pass through it with a whistle, albeit quiet. Helium diffuses much faster from a balloon.

What does not clean even a reverse osmosis filter?

There are substances that easily deceive the membrane. Among them are boron/borates. At neutral pH, boron is in solution in the form of an H3BO3 molecule and, in some properties, is very similar to water in a membrane. This allows the boron to pass through the membrane for 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. As an anion, boron is already perfectly cut off by the 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 by a carbon prefilter. The membrane is not designed to remove gases, in particular, hydrogen sulfide. Residents of megacities 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 instilled into children's ears.

Membrane “health” factors

Reasons why the membrane fails:

1. Physical damage

2. Loss of ability to hydrate due to exposure to chlorinated water or other oxidizing agents (ozonation) or due to membrane drying. The process can become irreversible, therefore, drying of an already worked membrane should not be allowed. And in order to prevent damage to the membrane due to chlorine, the reverse osmosis filter necessarily includes carbon pre-filters.

3. Deposition on the surface of insoluble salts and mechanical impurities present in the water (poor / insufficient pre-filtration)

4. Insufficient flow of water to the drain or “saving” of drain 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 using drainage water under the auspices of planting on the “needle of operating costs” is also recorded as a minus. We have compiled a small FAQ on these and similar issues.

1. Is water “dead”? (our favorite)

What lovers of the term “dead” water mean is not completely clear to us. From the point of view of official science, there is neither living nor dead water. Almost every H2O molecule on the planet has once been in a raindrop and in the waste products of some organism. There are no fabulous properties of water - there is its circulation in nature, as well as a great way to clean the aqueous solution from all alluvial with a membrane. We propose to consider fairy tales only as a cultural resource, because our body needs H2O, the rest is divided into two groups:

1. optional, as it comes with food;

2. unhealthy in the short or long term.

The absence of impurities in drinking water, and with them the so-called “useful minerals”, does not stand up to nutritional criticism. Calcium is practically not absorbed by our body from water, since it is found there in the form of inorganic salts. Even if this calcium were absorbed, it would be difficult to drink 17 liters of Moscow water of medium hardness to satisfy the daily requirement for this element - about 1000 mg. Only magnesium can be well absorbed from water, but we talked about this separately.

2. Is it expensive to maintain?

Given the 1/6 loss to drainage, one can easily calculate the huge difference in final cost between getting clean water at home and drinking water from the store, and the quality of water in plastic is similar, if not worse.

3. Why is reverse osmosis water not a distillate?

Water for us is not food and not a way to get "bricks" for the construction of the body. This is the environment in which the chemical and physical processes of the body take place. Moreover, she herself is quite inert and almost never participates in these processes. We do not split it into hydrogen and oxygen; the process of electrolysis does not take place in the body.

Understanding this role of water, you can also drink distilled water, which does not contain “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 rid water of impurities of organic substances, the boiling point of which is below 100C.

There is a huge difference between water after a reverse osmosis membrane and distilled water. Reverse osmosis does not completely cut off dissolved salts, but during distillation, it is the salts that completely remain in the distillation cube. On the other hand, organic volatile substances during the distillation process move with steam into the distillate, while the membrane removes them well. In addition, earlier distillers had rubber tubes, which added to the "tastelessness" of the resulting water.

4. Why is reverse osmosis water not an electrolyte?

Sharing our find:

"Osmosis-purified water could be compared to distilled water, if not for one small,

but an important “but”: as a result of purification, water inevitably becomes slightly acidic, since the molecules of alkaline salts

and alkaline earth metals and their constituent ions freely pass through the filter.

As a result, we drink a very, very weak...electrolyte every day."

An electrolyte is any liquid that conducts electricity. For example, soup or compote. That is, everything liquid that conducts electric current due to the movement of ions.

Electrolytes are a delicious lunch.

5. Does the membrane need flushing?

Membrane flushing is indeed done. However, this applies to industrial membranes. To wash them, depending on which particles “stuck” on the membrane, a whole arsenal of special compounds is used: alkaline, acid detergents, surfactants, and so on. In the case of industrial membranes, everything is known about these particles and the composition is selected individually.

Watching videos on youtube about how membranes are boiled in citric acid is a bit sad, because before our eyes people are wasting time in vain - the membrane loses its properties from high temperatures.

What did we want to say?

1. Historically, reverse osmosis membranes are good because they can perfectly desalinate water, for this they were created. This property relieves suffering from those whose drinking water is either very hard or just salty.

2. Household membranes can filter out the maximum of harmful impurities with which the modern world enriches water. This ability of membranes exceeds the capabilities of sorption filters. With ion exchange cartridges, sorption filters can also reduce hardness. However, there are a number of annoying limitations in terms of efficiency and resource.

3. There is no harm to water after the osmotic filter. The purer the water, the easier the metabolic processes in the body.

4. 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 bottles from the store, since the water in plastic is purified using 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 for pure water from our laboratory sounds like this:

Water should not be visible in tea.