How reverse osmosis works: the principle of operation of fine water purification devices
Among household cleaning systems, a new version has recently appeared, which is called reverse osmosis. This set of filters and a special membrane costs a lot, but it can give a head start to almost any analog.
Does it make sense to fork out? To get an answer to this question, you need to find out how reverse osmosis works, and then correlate costs and results.
What is osmosis and how does it work?
The process of osmosis is based on the property of water to level the level of impurities in solutions separated by a membrane. The holes in this membrane are so small that only water molecules can pass through them.
If in one of the parts of such a hypothetical vessel to increase the concentration of impurities, water will begin to flow there until the density of the liquid in both parts of the vessel is equalized.
Reverse osmosis gives the exact opposite result. In this case, the membrane is used not to equalize the density of the liquid, but in order to collect pure water on one side, and on the other hand, a solution that is maximally saturated with impurities. That is why this process is called reverse osmosis.
All these chemical features are of little interest to customers, especially those who are not very well versed in science. It is enough for them to understand that the center of the reverse osmosis system is a special membrane, the pores of which are so small that they do not miss anything that exceeds the size of the water molecule, and this is a significant part of the pollution contained in tap water.
Alas, the water molecule is not the smallest on earth, for example, chlorine molecules are much smaller, so they can also leak through the membrane.In addition, contact with large suspensions of this membrane is contraindicated. With such an impact, its small pores will quickly become clogged, and this element will have to be replaced immediately.
To prevent this from happening, three additional filters are added to the reverse osmosis system, with the help of which the water undergoes preliminary preparation. The membrane separates partially purified water into two unequal parts. About a third of the incoming volume is pure water, which then enters the collection tank.
Another two-thirds of the volume of water is the part in which the pollution is concentrated. This concentrate is discharged into the sewer. There is usually a small container between the tank and the crane. Here they install a cartridge that is designed to improve the quality of already purified water, for example, to saturate it with useful minerals.
Schematically, the principle of action of reverse osmosis can be described as follows:
- Water flows from the plumbing system to pre-filters.
- Then the liquid undergoes the procedure of reverse osmosis.
- Purified water enters the storage tank.
- The concentrate containing filtered contamination is transferred to the sewer.
- Clean water from the storage tank enters the tap for clean water directly or through additional devices.
Thus, the reverse osmosis system is a set of devices that provide the ability to get drinking water with a high degree of purification. Until recently, such systems were used mainly in industry, at catering establishments in recreational facilities, etc.
But because of the growing demands on the quality of tap water in recent years, reverse osmosis systems for household use have become popular. They differ in configuration, performance, size of storage capacity, etc. Filters and membrane need to be replaced periodically.
How to determine that the membrane needs to be replaced? As the exploitation of its pores clogged, and there comes a time when the water simply does not pass into the storage tank. Such a membrane will have to be replaced in any case. But experts recommend performing a replacement much earlier.
To determine the quality of water purified using a reverse osmosis system, use an electronic device - TDS-metr. With it, determine the level of salt in water.
For tap water before purification, this indicator can be 150-250 mg / l, and after purification using reverse osmosis technology, the salt content in the range of 5-20 mg / l is considered the norm. If the amount of salts in purified water is more than 20 mg / l, it is recommended to replace the membrane.
Individual elements of the system
The most expensive and basic element of the reverse osmosis system is the membrane. It is a microporous material twisted in one or more layers around a perforated plastic core. On top of the membrane is closed with a plastic protective cover, which is fixed with sealing rings.
Water enters the membrane housing and passes through a porous filler. In this case, the molecules of pure water penetrate through the porous core and then move into the storage tank. But pollution with some water cannot overcome the membrane barrier. They exit the opposite end of the membrane unit and are disposed of.
As previously mentioned, direct contact of the membrane with ordinary tap water can be disastrous. The fact is that its pores are very small - only 0.0001 microns. The membrane cannot be removed, rinsed and re-installed, as is the case with some mechanical pre-filters.
The costly item has to be completely replaced every two to three years. The term may vary depending on the operating conditions of the membrane: pressure in the plumbing system, the amount and nature of contamination, water temperature, etc. In some cases, the membrane may last for five years or even longer, and sometimes it is necessary to replace it after the first year of operation.
An important factor here is the presence of pre-filters: two mechanical and one carbon. The first mechanical filter cuts off all insoluble contaminants that are larger than 0.5 microns. These can be grains of sand, particles of rust and other such “mechanics”.
Then the water passes through a carbon filter that holds the molecules of various chemicals: chlorine compounds, which are invariably present in tap water, as well as heavy metals, pesticides from the soil, dissolved iron and other inclusions of organic or inorganic origin.
After that, another mechanical single micron filter completes the preliminary water purification. As a result, the membrane receives water that does not contain impurities that can damage the porous filter or overcome it. These filters need regular replacement every few months.But they are several orders of magnitude cheaper than the membrane, and timely replacement can significantly extend its service life.
For collecting the purified water use an accumulative tank. Its capacity can vary within 4-12 liters, it depends on the performance of the system. For the production of such tanks is usually used high quality steel. Outside the tank is covered with a layer of durable enamel. Inside install silicone gasket that divides the tank into two parts.
On one side of the gasket there is collected clean water, on the other - air. The amount of air in the chamber can be changed using a nipple located on the air side of the tank. The air creates additional pressure inside the storage tank, which promotes the movement of water to the faucet with the necessary pressure. Of course, on the tank there is an outlet for connecting a hose, through which drinking water moves to the tap.
Usually this faucet is installed at the sink. Even if the water supply through the water supply system is for some reason stopped, there will remain some of it in the tank, which will make the problem less acute.In this situation, to completely empty the storage tank, it makes sense to increase the pressure by pumping air through the nipple connection.
The degree of water purification using a reverse osmosis system brings it closer to distilled. In this case, the membrane passes molecules of oxygen dissolved in water, which enhances its quality. To improve the taste, water can be passed through a mineralizer or other modules. But it is suitable for drinking and immediately after passing through membrane cleaning. No need to boil it for drinking.
As an additional module that improves the quality of drinking water, reverse osmosis systems can be supplemented with a mineralizer or a bioceramic cartridge. The mineralizer is used to enrich water with useful substances such as calcium, sodium, magnesium, etc.
All these minerals have a positive effect on the human nervous and cardiovascular systems, prevent the occurrence of many dangerous diseases, stimulate the normal level of blood acidity, etc.
The bioceramic cartridge works differently.As part of its “filling” there are balls consisting of baked clay and pieces of tourmaline. This mineral is able to provide extremely beneficial effects for water. It helps to change the structure of water, which ultimately has a beneficial effect on the endocrine system and strengthens the immune system.
Over time, the resource of such additional modules is exhausted; they require periodic replacement. Therefore, it is more convenient to use a faucet with two valves. Water is supplied to one valve directly from the reservoir, and to the second - through the mineralizer. Enriched with beneficial substances, water is used only for drinking, and it is prepared in water without minerals.
Advantages and disadvantages of reverse osmosis
A high degree of purification and guaranteed quality of drinking water is the main advantage of reverse osmosis systems. It is estimated that in the water purified by this method, the content of foreign substances is ten times lower than the minimum permissible rate. Features of the membrane design exclude accidental ingress of contaminants into the stream of purified water.
Such water can be safely consumed for drinking and cooking, it can be given to children and pets. For health, water obtained by reverse osmosis technology is much more useful than boiled tap water. Aquarists use this water to supplement the volume of aquariums without settling.
Despite the more complex design compared to conventional household filters, the installation of such systems is performed without any problems. Everything you need for installation is usually supplied. Almost all elements or their modifications can be purchased separately.
The system does not take up too much space, most often the tank and a set of filters with a membrane are fixed directly under the sink. A compact tap for drinking water, installed at the sink, usually fits perfectly into the interior.
The main disadvantage of reverse osmosis systems is the high initial cost of the kit. Further system maintenance will also require the cost of replacing filter cartridges, but they cost significantly less.
Every few years, you will have to replace the membrane, the price of which can be about $ 50. But calculations show that as a result, the cost of clean water will cost the family less than buying third-party drinking water.
Another feature of the reverse osmosis system, which can hardly be considered a drawback, is low productivity. Purified water seeps through the membrane very slowly, the standard membrane capacity is about 150-300 liters per day.
At the same time, more than half of the water coming from the water supply system goes down the drain, which to some extent affects the amount of utility bills. But if the volume of the storage tank is chosen correctly, then problems may arise for a short time only when the system is started up immediately after installation or after it has been idle for a long time with the storage tank emptied.
We recommend an article on the topic: Reverse osmosis: harm and benefits of membrane cleaning of tap water.
How to choose a suitable model?
The dimensions of the elements of reverse osmosis systems usually meet a certain standard, but not always. Using a system with standard parameters gives a wider choice of replaceable elements. Practice shows that domestic products, which are cheaper than their foreign counterparts, differ little in quality from the latter.
Of course, it is better to make a purchase directly from the manufacturer, from a verified dealer or in a reliable store. Good reverse osmosis systems have a technical passport, official warranty, detailed instructions for installation and operation, as well as quality certificates.
The size of the storage tank depends on the needs of the family, as well as the size of the space in which the device will be installed. The capacity of the maximum size will be appropriate for a home in which a large family lives, the most demanded tanks are about 8-10 liters.
An important indicator is the working pressure in the plumbing system. The optimum indicator is 2.8 bar. If the pressure is too low, it makes sense to buy a reverse osmosis system with a built-in pump, which will provide sufficient drinking water pressure. If the pressure in the water supply is much higher, you will need to install a pressure reducer.
The membrane of the reverse osmosis system requires the closest attention. This element must be consistently high quality and with suitable performance. For a small family, an optimum indicator is about 7 liters per hour.
It makes no sense to overpay for a more productive membrane, if it is not necessary.For comparison: a device with a capacity of 15 liters per hour can easily meet the clean water needs of a small catering establishment.
The standard is a set that includes five basic cleaning steps: three pre-filters, a membrane and a post-filter. The content of this last item may vary. The mineralizer and bioceramic cartridge have already been mentioned. Installation of a cartridge containing coconut shell, activated carbon and silver ions is also considered a popular solution.
These substances disinfect water, in addition it is cleaned and give it a pleasant taste. Another interesting option for post-processing water is an ultraviolet disinfectant. Some models of reverse osmosis systems allow the use of several different cartridges at the same time.
Conclusions and useful video on the topic
A general overview of the principles of functioning of reverse osmosis systems can be found here:
This video describes in detail the operation of such a system using the example of the “Geyser Prestige M” model:
This video allows you to compare the features of the flow-through filters and reverse osmosis systems:
Reverse osmosis systems are a reliable and convenient way to provide your house with enough clean drinking water. Yes, the prices of such devices remain high, but they are more than paying for themselves, since clean drinking water reduces the risk of many diseases and improves the quality of life.