The thesis that of all methods of purification of drinking water available to an average citizen, the most effective and reliable method is reverse osmosis is popular, but not obvious. On the one hand it is known that the “heart” of the reverse osmosis filter – a semi-permeable membrane – is a barrier which guarantees profound removal of all dissolved impurities and thus a high quality of purified water. On the other hand, deeply demineralized water is rarely found in wildlife, and experience with its use as drinking water has been limited to the past few decades. This leads to natural fears of users, which are fueled by the lack of unambiguous information and a generally accepted position of different specialists. Moreover, due to the active development of the market for domestic reverse osmosis, consumers and water treatment professionals have a large number of questions concerning the principles of operation, installation and proper operation of the relevant devices. This publication offers answers to the most frequent questions asked by attendees at WaterNet Water Treatment Academy seminars.
1. What is reverse osmosis and how can it be used to purify water??
Reverse osmosis is based on the widespread natural phenomenon of osmosis, known from the course of secondary school. Osmosis can be observed if we consider a system of two chambers with a salt solution and pure water separated by a semi-permeable membrane that allows water molecules through but is impermeable to water dissolved impurities (Fig. 1).
Fig. 1. Illustration of the principle of reverse osmosis
In such a system, water molecules will move through the membrane from the pure water chamber to the saline solution, diluting it and causing the liquid level in the latter chamber to rise. The phenomenon of osmosis is observed even when water and solution are under the same external pressure. The difference in the heights of the water and the salt solution is due to the force that causes the water molecules to pass through the membrane. This force is directly related to the osmotic pressure which depends on the concentration of the dissolved impurities in the water. If a pressure equal to the osmotic pressure is applied to the salt solution, the system will reach an equilibrium state and the osmosis process will stop.
If, in the system under consideration, a pressure greater than the osmotic pressure of the salt solution is applied to the salt solution, the movement of water molecules in the system will be directed from the salt solution chamber to the pure water chamber. In other words, due to the applied pressure, pure water will be “squeezed” out of the salt solution. It is precisely this phenomenon which lies at the heart of reverse osmosis water purification.
In reverse osmosis systems, thin film composite membranes are used, consisting of a barrier layer and auxiliary layers (Fig. 2).
Fig. 2. The structure of a thin-film composite membrane
For the reverse-osmotic separation process, a barrier layer consisting of a polymeric material (most often based on aromatic polyamide) that is permeable to water molecules but not to dissolved salts and other impurities is responsible. The exception is the gases dissolved in water – oxygen, carbon dioxide, and others, which pass freely through the membrane. The conventional pore size of reverse osmosis membranes is estimated at 0.0001 micron, which ensures retention of at least 99% of impurities by the membrane.
Under the influence of pressure a part of water passes through the membrane, being cleaned from impurities and forming a flow of the so-called permeate. The retained impurities and the rest of the water form a concentrate stream. The permeate, which is treated water, is delivered to the consumer, while the concentrate is discharged into the sewage system.
The ratio of permeate to raw water is called permeate recovery, and the higher the ratio, the better. However, increasing the permeate yield is not a simple task and requires taking into account a large number of factors – from the composition of raw water to the size of the unit. The search for and ensuring optimal conditions for increased “recovery” is at the heart of all membrane science and associated technologies.
2. What are the reverse osmosis systems and how do they differ?
For a long time practically the only application of reverse osmosis technology was in industrial water treatment, where there was a need for obtaining considerable amounts of high quality purified water with minimum consumption of energy and reagents and, what is important, with maximum use of raw water as raw material.
Industrial reverse osmosis systems in most cases are based on schemes using concentrate recycle which means return of a part of concentrate to the inlet of the unit and its mixing with initial water. The advantage of such schemes is high permeate yield – 50%-75% and in some cases up to 90%. However, such efficient use of water can only be achieved through the use of powerful pumping equipment, allowing high operating pressures of tens of atmospheres, appropriate piping and fittings, as well as instrumentation and automation. In addition, the industrial technology of reverse osmosis water purification includes a multi-stage pretreatment using various reagents, as well as mandatory membrane washing procedures.
Obviously, the domestic application of reverse osmosis excludes the use of such complex operations and equipment. Therefore, high permeate yields were abandoned for compactness, simplicity of design and operation, reliability, noiselessness and, not least, low cost when developing residential reverse osmosis systems designed for purification of small amounts of potable water. For this reason, for domestic reverse osmosis systems the permeate yield is low and is usually up to 10%.
3. What does domestic reverse osmosis consist of?
Water purification with a domestic reverse osmosis system, regardless of the system design, involves the following steps:
- Three-stage pre-treatment;
- Reverse osmosis separation at the membrane element;
- collection of treated water in the storage tank;
- Post-treatment or final treatment of water.
Let’s briefly consider the structure and principle of operation of a domestic reverse osmosis system. The filter is connected to the cold water supply line by means of the inlet socket 4 and the water supply tap 5 (Fig. 3).
Fig. 3. Wiring diagram of the reverse osmosis system in the basic configuration:
The red tube connects the water supply tap and the prefilter 10.10 1 of the filtration module. The incoming water first passes through three pre-treatment stages in the pre-filter 10.1, 10.2, 10.3. After undergoing pre-treatment, the water enters the reverse osmosis membrane 10.5, which is located in a special housing. The membrane housing has an inlet that connects to the prefilter 10 through an autoregulator.3, and two outlets: one for permeate and one for concentrate.
After the membrane, the water flow is divided into two parts – the concentrate is discharged into the sewage system and the permeate enters into storage tank 2 and is stored there. The tank is connected to the outlet of the membrane through the self-regulator and check valve built into the transition fitting, which is screwed into the outlet of the membrane housing. The autoregulator is followed by a T-piece, through which the tank is connected to the filter module with a yellow tube. On the upper connector of the tank there is a ball valve 6.
After the tank is filled, the automatic regulator shuts off the water supply from the pre-filters to the diaphragm and the purging process is stopped. After opening of the tap for treated water 3, water pressure in the storage tank drops, the automatic regulator automatically opens water supply from the pre-filters to the membrane and the tank is filled again. Concentrate is fed to the sewer through the outlet of the diaphragm housing, connected by a black colored tube to a drain clamp 8, which is mounted on the sewer pipe.
From the storage tank treated water through a tee 11 passes to the last stage of purification – post-filter 10.4. The post-filter is connected via a blue tube to the treated water tap 3, which is installed directly on the sink or kitchen countertop.
4. Why water pretreatment is needed and how it takes place?
The quality of water pretreatment before it reaches the reverse osmosis membrane is of great importance, because it determines the service life of the membrane element. Remember that the domestic reverse osmosis system is designed to purify tap water, pre-treated at centralized stations, or water treated at local water treatment plants. Household reverse osmosis systems are not designed for direct purification of water from a well or well, since in most cases such water contains significant amounts of various impurities – iron and manganese, hardness salts, natural organic substances and others.
All of these impurities are effectively removed by reverse osmosis, but very quickly put the most expensive element of the system – the reverse osmosis membrane – out of operation (Fig. 4). The treatment of untreated water with the domestic reverse osmosis (RO) is therefore not economically feasible in the first place.
Fig. 4. Contamination of the membrane of the domestic osmosis due to improper water treatment
Household prefiltration systems mainly use three cartridge filters (prefilters) (Fig. 5):
1. A polypropylene mechanical filtration cartridge with a filtration rating of 5-10 microns designed to remove all sorts of mechanical impurities found in tap water – grains of sand, rust and scale particles and other particles. Removing these impurities prolongs the service life of the subsequent cartridges and the system as a whole. The service life of a mechanical filtration cartridge depends on the turbidity of the water to be treated and averages 3-6 months. Untimely replacement of the cartridge, clogged with accumulated mechanical impurities, can lead to a decrease in water pressure at the inlet to the membrane element and thereby reduce the performance of the system.
Rice. 5. Types of filtration cartridges:
a) polypropylene mechanical filtration cartridge; b) granular activated carbon cartridge; c) carbon block cartridge
2. Granular activated carbon cartridge for removing natural organic substances and active chlorine from tap water that negatively affect the reverse osmosis membrane. Both coconut charcoal and bituminous activated carbon can be used as cartridge filling. The first refers to microporous charcoal and effectively removes impurities of active chlorine and organochlorine compounds from water, the second is mesoporous and is very effective in removing natural organic compounds found in surface water and tap water obtained from it. The life span of a granular activated carbon cartridge also averages 3-6 months. The real life of this cartridge depends on the quality indicators of treated water, such as oxidation and chromaticity. Failure to replace the cartridge at this stage in a timely manner can lead to clogging of the membrane element, development of microorganisms on the membrane surface due to slippage of nutritious natural organics through the depleted layer of activated carbon. In turn, this will shorten the life of the membrane element, and can worsen the quality of water purification.
3. Different cartridges can be used in the third stage, depending on the type of reverse osmosis system. In the case of low-cost systems, a polypropylene cartridge with a filtration rating of 1 or 5 microns is used at this stage to remove carbon particles that may leach from the previous cartridge, as well as possible residual mechanical contaminants. Higher class systems install a cartridge using briquetted activated carbon (carbon block), which provides not only mechanical filtration, but also additional purification from chlorine and organochlorines. The service life of a cartridge in the third prefiltration stage is 3 to 6 months. Thus, all three cartridges are replaced at the same time, which simplifies the use of the domestic reverse osmosis system.
5. Membrane elements for domestic reverse osmosis
A membrane element is a device comprising a reverse osmosis membrane and the components which ensure its effective use.
Domestic reverse osmosis systems use coiled membrane elements (Fig. 6) size 1812, which corresponds to an element diameter of 1.75 inches (44.5 mm) and a length of 11.74 inches (298 mm).
Fig. 6. The structure of the membrane element of a domestic reverse osmosis system
When a reverse osmosis system is in operation, water is fed into the membrane holder and enters the inlet of the membrane element from the end (Fig. 6). Water moves through the channels inside the membrane element. Water pressure forces water molecules to move through the membrane, forming a permeate flow on the other side of the membrane. Channels for water movement are formed by the so-called concentrate spacer, which is a mesh of polymeric material. A similar spacer, but thinner and with a smaller cell size, is used to create channels for permeate movement inside the membrane envelopes. To drain permeate from the membrane element, the membrane envelopes are attached with the open (unglued) end to a perforated catchment tube, which drains the permeate from the permeate channels.
A significant problem arising in reverse osmosis separation is the accumulation of trapped impurities in a thin layer of concentrate near the membrane surface. Due to this phenomenon, known as concentration polarization, both the quality of treatment and the performance of the membrane element are significantly impaired, as well as the service life. To minimize the negative effects of concentration polarization a concentrate spacer is used in the membrane element to provide flow turbulization and thereby quality agitation of the concentrate throughout its volume.
After the membrane element, the permeate flows into the storage tank. Concentrate is discharged to the sewer through a flow restrictor. A flow limiter is a special device that ensures a constant flow of concentrate and thus maintains a constant required water pressure in the membrane holder.
6. What are the membrane elements?
The membrane element is the main part of the reverse osmosis system which directly removes impurities from the water and for this reason strict requirements are imposed on its quality and effectiveness.
The most important characteristics of a membrane element for domestic osmosis are:
- the quality of the membrane web;
- quality of gluing of membrane sheets in the envelopes;
- number and length of membrane envelopes used to make the element;
- Accuracy of element manufacturing operations – envelope placement and fastening.
The quality of the membrane sheet directly determines both the efficiency of impurity removal and the performance of the membrane element. For example, the presence of patches on the surface of a membrane fabric used to correct fabric manufacturing defects can cause source water to leak into the permeate tube.
Another common quality problem with webbing is that it is chlorinated by the manufacturer. As mentioned above, active chlorine negatively affects the properties of the membrane, but this effect is ambiguous. When chlorine acts on the aromatic polyamide of the barrier layer, there is an increase in the performance of the membrane element and an improvement in the quality of water treatment. However, this effect appears for a short period of time at the beginning of operation, and then is quickly replaced by an even greater increase in performance and a dramatic deterioration in water purification quality. Chlorination of the membrane sheet is thus an easy way for an unscrupulous manufacturer to briefly improve the performance of his product, while drastically reducing its service life.
Often the need to chlorinate the web to ensure acceptable performance of the membrane elements, at least at the beginning of operation, is due to the manufacturer’s approach to bonding the web into membrane envelopes. Some companies practice manual application of the adhesive. An unpleasant consequence of manually gluing the sheet is the fact that the glue covers a large area of the sheet, thereby excluding that area from the water purification process. Also in this case there are inhomogeneities in the bonding of the web along the entire length of the envelope, which may lead to a possible violation of the integrity of the element. Thus, in practice, membrane elements with manually made adhesive joints are unreliable and have an underestimated performance.
A much more efficient option is to glue the sheet into envelopes on an automated robotic line, as Dow Chemical does. The application of adhesives using automated equipment creates, first, a homogeneous bonding line that provides maximum bonding strength and reduces the chance of leaks to a minimum, and second, it uses a minimum proportion of the membrane web area for bonding, thereby maximizing the performance of the membrane element.
The number and length of membrane envelopes used to make the element directly affects the performance of the membrane element and its service life. When creating any membrane element, a small number of long membrane envelopes or a large number of short envelopes can be used to provide the same membrane area.
The use of long envelopes simplifies the operation of securing the envelopes in the permeate tube. The disadvantage of such approach is the reduction of element productivity due to increase of permeate backpressure in the channels of long envelopes and increased fouling of such an element.
Dow Filmtec membrane elements (Tab. 1), which use short membrane envelopes, do not have these disadvantages. On the other hand, using more membrane envelopes in Dow cells is associated with their automatic production, which allows precision placement of the envelopes on the permeate tube at a uniform pitch. This significantly improves element reliability as compared to other manufacturers’ products, where membrane envelopes are not uniformly placed and are subjected to different loads during operation, which can lead to leaks.
Table. 1. Dow Filmtec TW30-1812 domestic membrane elements of different capacities
It should be said that different household membrane elements have the same standard size 1812, but they differ in capacity. Different productivity is caused by the different area of the membrane fabric. Consider the existing options of membrane elements conveniently on the example of the Dow Filmtec TW30-1812 line.
7. Why do you need a storage tank??
The need to use a storage tank in the reverse osmosis system is due to the low productivity of domestic reverse osmosis membranes. For example, if the system is equipped with a TW30-1812-50 membrane with a capacity of 7.9 liters per hour, a 200 ml beaker will be filled in more than a minute and a half. The accumulation of water in a membrane tank makes it possible to ensure a sufficient rate of delivery of drinking water from the tap to the consumer.
The tank is connected to the membrane outlet through a special device called an autoregulator and a check valve built into the fitting, which is screwed into the outlet of the membrane housing. After the autoregulator is installed T-piece, through which the tank is connected to the filter module with the yellow tube. When the tank is full, the automatic regulator switches off the water supply from the pre-filters to the diaphragm and the system is disconnected from the water mains. After opening the tap for purified water the water pressure in the storage tank drops, the autoregulator automatically opens the water supply from the pre-filters to the membrane and the process of water purification and filling of the storage tank begins again. Purified water from the storage tank is fed to the consumer through a special tap set next to the tap on the sink.
There are various sizes of storage tanks, allowing you to choose a complete system depending on water consumption. The reverse osmosis systems of most manufacturers are equipped with water-air tanks in which the purified water is forced out by the air pressure over the membrane when the faucet is opened. Water-water tanks can be found in some manufacturers. In their design, the displacement of the treated water is due to the pressure of the tap water. An advantage of this type of tank is the compact design of the. Disadvantages of such a solution – smaller effective volume of the tank, as well as the inability to withdraw purified water in case of a drop in pressure in the water supply, for example, in case of water outage.
8. How to reduce the discharge of concentrate?
The amount of concentrate discharged into the sewage system during cleaning is ensured by a flow restrictor. For proper operation of the system, the flow restrictor must match the size of the installed membrane. It would seem to follow from this that the discharge of concentrate to the sewer during operation of the reverse osmosis system is a constant value. However, the amount of water purified by the reverse osmosis membrane depends on the inlet pressure to the membrane element – the higher the pressure, the more permeate passes through the membrane barrier. On the other hand, the amount of water accumulated in the tank before the autoregulator shuts down depends on the air pressure in the corresponding section of the membrane tank – the lower the pressure, the more water is accumulated in the tank – and on the pressure of the source water in the water supply line. For reference: The recommended air pressure in the storage tank is 0.6-0.8 atm. Therefore, it is impossible to talk about any fixed value of discharge into the sewer per liter of treated water – too many parameters affect this value.
In order to assess the impact of the above-mentioned parameters on the ratio “permeate-concentrate” in a domestic reverse osmosis, it is useful to consider the data obtained in the operation of reverse osmosis system with a membrane element Dow Filmtec TW30-1812-50 rated capacity of 50 gallons, or 189 liters per day, at different values of tap water pressure and air pressure in the tank.
From the given in Tab. 2 data can be seen that at low values of water pressure in the water supply, the reverse osmosis system discharges significant amounts of concentrate, reaching almost 15 liters per 1 liter of purified water. Such system operation can hardly be considered rational. The concentrate discharge per unit volume of treated water decreases with increasing water pressure, reaching a minimum value of 7 liters at an inlet pressure of 5 bars.
Amazon | Ecosoft domestic RO filter installation (MO550ECOSTUS)
Table. 2. Test results of the domestic reverse osmosis system under different conditions
Are there any solutions, which allow to reduce the volume of discharged concentrate? Yes, such solutions exist, and one of them is the use of a reverse osmosis system with a pump. Since the concentrate flow limiter ensures a constant discharge into the drain that is nearly independent of pressure, using a pump allows more water to be pressed through the membrane. For one thing, the use of a pump will reduce the discharge to the sewer system. On the other hand, such intensification of cleaning will lead to faster contamination of the membrane element and prefilters.
Even the lowest achievable discharge value to the sewer system can seem high enough. However it should be remembered that domestic reverse osmosis is a way of additional treatment of drinking water used by a person in limited quantities – up to 3-5 liters per day. Therefore, the total discharge into the sewer system will be relatively small, commensurate with the overuse of water for a few extra toilet flushes per day. The compensation for this overspill is the high quality and safety of the potable water in the home.
9. Why post-treatment of permeate is needed?
Particular attention should be paid to the post-treatment or final correction of permeate. As part of domestic reverse osmosis systems, these stages are realized by means of various post-filters installed in the supply line of purified water from the storage tank to the tap. There are different versions of post-filters, which manufacturers complete household reverse osmosis systems, but all of them in practice perform three different functions:
- correction of water taste qualities;
- ensuring microbiological purity of drinking water;
- Remineralization and pH correction.
Let us consider in detail each of the tasks solved by the various options of post-treatment of permeate in household reverse osmosis systems.
Almost all domestic osmosis systems use post-treatment of permeate with activated carbon derived from coconut shells. For this purpose, the reverse osmosis system is completed with the so-called post-carbon, an encapsulated filter filled with high quality coconut activated carbon. When water passes through this filter, the important organoleptic indicators of water quality – taste and odor – are corrected. Post-carbon allows to improve the taste of water for those consumers who find permeate tasteless, and also allows to eliminate possible extraneous odors associated with the storage of water in the storage tank.
The issue of the necessity of ensuring microbiological purity of water after the storage tank in the domestic reverse osmosis occurred relatively recently. It is usually solved by regular maintenance of the system with flushing of the tank with decontaminating reagents. The problem of microorganisms presence in water can be also solved by means of certain post-treatment methods.
One such method is water disinfection with ultraviolet radiation. This physical method, which has been used for many years, is notable not only for its high efficiency, but also for the lack of negative impact on the chemical composition of purified water. Until recently, a wide spread of UV significantly hindered the high cost and energy intensity of the method, but today a wide range of lamps of different power, including LED (UV-LED) disinfection, allows its use in a variety of areas. For domestic water treatment, the UV lamp is a near-perfect alternative. Compact and ergonomic, they are easily installed in treatment systems, ensuring smooth operation and effective disinfection of water.
Another physical method of water disinfection which can be applied in local water treatment is ultrafiltration. The essence of water disinfection with ultrafiltration is that the passage of water through a semi-permeable membrane with a pore size of 0.001 to 0.1 microns detains various impurities: colloids, organic substances, algae and most microorganisms. More recently, this method has been used mainly for removing colloidal impurities and suspended solids on an industrial scale. Interest in its use for the removal of microorganisms in domestic water treatment has increased considerably.
To date, various companies produce compact, easy to use ultrafiltration cartridges, which on efficiency disinfection are not inferior to UV lamps. Cartridges of this type are recommended for use after the storage tank of household reverse osmosis systems for additional treatment of water from possible microbiological contamination.
The salt content of the osmosis permeate does not exceed 15-20 mg/l. In recent years, the world medical community has recognized that desalinated water is not harmful to human health. However, the taste of such water is significantly different from the usual. It is to provide the ability to choose the composition of purified water there is such an option for domestic reverse osmosis systems as a mineralizing post-filter, or mineralizer. Mineralizer in general is a filter filled with crumbs of various natural minerals. Reverse osmosis permeate characterized by pH 5.8-6 and low salt content, slowly dissolves in contact with such a crumb and is saturated with salts of calcium, magnesium, sodium and potassium to the level of 50-100 mg/l. Also at the same time there is a correction of permeate acidity – pH value rises to values of 6,5-7.
10. What determines the price of domestic reverse osmosis?
From the above information you can see that there are different variants of reverse osmosis systems with different configuration, different options and so on, which certainly affects their price. However, on the market you can often find products of different brands, which with the same appearance have a significant difference in price. Most consumers have a legitimate question – what is the reason for the difference in price and is it worth paying it??
As an example, consider a membrane element as the basis of the entire system. Inexpensive reverse osmosis systems, as a rule, are equipped with membrane elements of little-known manufacturers. For such membrane elements are typical of the problems discussed above with the chlorination of the fabric, patches and poor quality adhesives. All this does not make the system very reliable. They may also have lower capacity and selectivity than the leading manufacturers’ products.
On the contrary, in more expensive reverse osmosis systems it is more likely to find Dow Filmtec membrane elements which have the best performance/selectivity ratio and provide a higher quality of purification.
The situation is similar for prefilter cartridges. In inexpensive systems are most likely to find cartridges made in China, which often show a high filtration rate with poor quality of water purification from impurities. Consequently, with such cartridges the lifetime of the membrane is significantly reduced, the quality of purification in general deteriorates.
This creates a situation where the attractively low initial cost of the system leads to more frequent cartridge changes and, consequently, higher maintenance costs. And vice versa – more expensive solutions, complete with quality cartridges and membranes, require less maintenance and are cheaper.
Do not forget about the post-treatment, especially the remineralization of water. The efficiency of this process is determined by the nature of the minerals used and the time the water is in contact with them. Since remineralization of water is carried out at the final stage of the purification process, the purity of minerals used and their absence of toxic impurities are of extreme importance. In general, the post-treatment of water is the most critical stage in the domestic reverse osmosis, as it determines the safety of water supplied to the consumer. This should be kept in mind when selecting the manufacturer of the system you buy.
A separate point in the difference between cheap and expensive solutions is the question of system quality. As a rule, behind the higher price of components is not only their high efficiency, but also greater reliability of the reverse osmosis system. Thorough manufacturer inspection of its products for work under pressure, resistance to water hammer and other heavy tests, which the product passes in the quality control department – a guarantee of absence of problems with flooding of neighbors, loss of water and other troubles. And you have to pay for that, too.