We’ve listed the top 20 asked questions about reverse osmosis with answers below.
Simply click each question for the answer:
What is reverse osmosis?
Reverse osmosis (RO) is a separation process that uses pressure to force a solution through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but it has also been used to purify fresh water for medical, industrial and domestic applications since the early 1970s. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions.
Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as “osmotic pressure”, since it is analogous to flow caused by a pressure differential.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.
How does it work?
In modern home units water, driven by normal city water pressure, flows first through a sediment pre-filter which removes any dirt and small particles that are in the water, next a carbon pre-filter, which removes organic contaminants including chlorine and its by-products. Then, it enters the reverse osmosis membrane, a very tight, sheet-like filter, which allows water to pass but rejects dissolved solids like sodium and impurities like lead and arsenic. Some of the water entering the unit is used to cleanse the membrane surface and flows to the kitchen drainpipes. The purified water is stored in a small storage tank until it is needed. When the faucet mounted on the sink is opened, the purified water is forced by air pressure through another carbon filter, which gives it a final polish and from there to the faucet. (This is a simplified description of a four-stage RO unit. The simplified description omits a few very essential parts like flow control devices, check valves, and an automatic shutoff device that stops the inflow of water when the storage tank is full.)
What is the best RO system?
Essentially all RO membranes produce similar, highly purified water. However not all systems offer the same features. For example: The FMRO5-MT model is perfect for city water applications. This unit has five stages, meaning the water passes through a 5-micron sediment polypropylene prefilter, then two solid extruded carbon block cartridges for chlorine removal, then the RO membrane to remove the smallest of contaminants, and finally another carbon filter to polish the taste. The FMRO4G-ERP has water conservation in mind with a high efficient design. This model produces more water in a shorter period of time, with less water going to the drain than a traditional 50 gpd RO. This is due to the non-electric permeate pump that utilizes the drain flow to reduce the backpressure from the storage tank allowing a more consistent working pressure across the membrane. The WQC4RO13 model is designed for the consumer that would like a low maintenance premium RO. This model is designed with a designer water-dispensing faucet, low profile polypropylene storage tank, and manifold filter heads that are able to pivot 180 degrees with built in shut off valves to make filter replacements as easy as a 1/4 twist of the filter to replace.
Is a reverse osmosis unit like a distiller?
Both effectively reduce “dissolved solids” content of water, but the processes are quite different. RO filters water through a very tight semi-permeable membrane. A distiller is like a big teakettle: it boils water, catches the steam, condenses it, and captures the resulting water. Most impurities are left behind in the boiling chamber. Both distillers and reverse osmosis systems rely heavily on carbon filtration for chemical removal. (Cheap distillers often have little or no carbon filtration and are, therefore, of limited effectiveness.)
But isn’t distilled water purer than reverse osmosis water?
Distillers typically remove a few parts per million more of common mineral constituents like sodium. However, distillers don’t do a good job with volatile chemicals with a low boiling point. Chloramines, for example, which many cities now use instead of chlorine as a disinfectant, aren’t removed well by distillers. Reverse osmosis, with the carbon filters that accompany it, does a very good job with chloramines. Unless volatile chemicals like chlorine are removed by carbon filtration before they enter the distiller, they will be released into the room air or they will end up in the distilled water. But in general, distilled water is very pure, as is reverse osmosis water.
A friend told me reverse osmosis units waste a lot of water, is that true?
It depends on what you mean by waste. A home RO unit uses water to clean itself and wash away impurities. It’s like a lot of other water-using appliances. We use water to wash clothes, to wash dishes, to wash cars, to flush toilets. A reverse osmosis unit uses more water in its operation than you actually consume, but it doesn’t use enough that you’ll notice it on your water bill. It uses water only while it’s filling its storage tank. When the tank is full, the whole unit shuts down and no water runs to drain. It is typically like two or three extra toilet flushes a day.
What is the annual maintenance costs?
Only the prefilters and post filters need to be replaced at least on an annual basis to ensure proper performance. Ultimately the quality of the water and the amount of use will determine the frequency of filter changes. Depending on the point-of-use RO model, the annual cost is less then $0.30 per day. The reverse osmosis membrane itself will normally last between two and five years.
What Is The Importance Of Water Pressure To An R.O. System?
It is the pressure of water that forces the water through the membrane for purification and flushes the rejected solids away. Low water pressure will result in reduced production and premature fouling of the membrane. The ideal pressure for operating an R.O. system is 60 PSI. Pressure below 40 PSI is generally considered insufficient, and should be boosted using a pressure booster pump.
Can I hook the reverse osmosis unit to my refrigerator/icemaker?
Yes, if you can reach it with a 1/4″ tube from the under sink RO unit. Pressure is a consideration with some refrigerators, so it’s a good idea to check with the manufacturer. The pressure you’ll get from the RO unit is about 2/3 of the incoming line pressure.
How long will a reverse osmosis unit last?
Virtually forever if you service it regularly and replace parts that wear out, like the storage tank and the faucet. Typical membrane life is about 2 to 5 years, depending on the nature of the water that it’s processing.
A filter salesman showed me a chart that said reverse osmosis doesn’t remove chlorine. Is that true?
Technically what he said was true, but for practical purposes it’s an out-and-out lie. It’s true that the reverse osmosis membrane doesn’t remove chlorine. It doesn’t have to, because it has a couple of high quality carbon filters with it that do the job. In fact, if the first carbon filter didn’t remove all the chlorine, the membrane would get eaten alive in no time.
The same salesman told me that reverse osmosis units remove minerals that are essential to health. Is that true or is he again twisting the truth?
It’s true that RO units remove minerals about 95% of the mineral content anyway, but he isn’t really telling you the whole story. The mineral issue is probably the most controversial question in drinking water purification. Experts on both sides of the issue speak convincingly. Our own view, after reading much of the expert opinion, is that the mineral content of water either high or low isn’t nearly as important as they would have you believe. That is, minerals in water are inorganic and hard for your body to use. You get most of your minerals from food, which provides organic, easily assimilated minerals. The human body is a sophisticated instrument capable of adapting to a wide range of circumstances and capable of thriving in areas having water of high or low mineral content. As long as water is palatable, it’s within the body’s acceptable range. The main issue with water is chemicals, not minerals. Whether water contains 30 or 3 parts per million calcium isn’t really significant, but the difference between 0.5 and 5 parts per million chloroform is of life or death importance.
Do reverse osmosis units need electricity?
No, they run on water pressure. You need electricity only if you add an electric pressure-boost pump or an ultraviolet lamp. Standard units have neither and normally don’t need them.
Why are reverse osmosis units so popular?
Because they produce great-tasting, very pure water at a very reasonable cost (compared to buying bottled water) and in a trouble-free, fully automatic format. And the most frequent comment we get is: “I didn’t think my water could taste this good, we drink so much more water than we used to.”
Will A Water Softener Harm The Reverse Osmosis (R.O.)?
No. Calcium and magnesium (limescale) are two of the hardest minerals for the R.O. membrane to remove. Sodium (added to the water by the softener) is much easier on the membrane and it will reject 98% of all sodium in the water. A water softener will help extend the life of the membrane.
What is a water filter?
A water filter is a device which removes impurities from water by means of a fine physical barrier, chemical process and/or biological process. Filters are used to cleanse water for irrigation, drinking water, aquariums, and swimming pools.
What is drinking water?
The human body is anywhere from 55% to 78% water depending on body size. To function properly, the body requires between one and seven liters of water per day to avoid dehydration; the precise amount depends on the level of activity, temperature, humidity, and other factors. Most of this is ingested through foods or beverages other than drinking straight water. It is not clear how much water intake is needed by healthy people, though most advocates agree that 67 glasses of water (approximately 2 litres) daily is the minimum to maintain proper hydration. Medical literature favors a lower consumption, typically 1 liter of water for an average male, excluding extra requirements due to fluid loss from exercise or warm weather. For those who have healthy kidneys, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) it is dangerous to drink too little. People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication (hyperhydration), which can be fatal. The “fact” that a person should consume eight glasses of water per day cannot be traced back to a scientific source.[There are other myths such as the effect of water on weight loss and constipation that have been dispelled.
An original recommendation for water intake in 1945 by the Food and Nutrition Board of the National Research Council read: “An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.” The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 2.7 liters of water total for women and 3.7 liters for men. Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated. According to the Institute of Medicinewho recommend that, on average, women consume 2.2 litres and men 3.0 litresthis is recommended to be 2.4 litres (approx. 9 cups) for pregnant women and 3 litres (approx. 12.5 cups) for breastfeeding women since an especially large amount of fluid is lost during nursing. Also noted is that normally, about 20 percent of water intake comes from food, while the rest comes from drinking water and beverages (caffeinated included). Water is excreted from the body in multiple forms; through urine and feces, through sweating, and by exhalation of water vapor in the breath. With physical exertion and heat exposure, water loss will increase and daily fluid needs may increase as well.
Humans require water that does not contain too many impurities. Common impurities include metal salts and/or harmful bacteria, such as Vibrio. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes. The single largest freshwater resource suitable for drinking is Lake Baikal in Siberia, which has a very low salt and calcium content and is very clean.
What is Ultraviolet Disinfection?
Ultraviolet disinfection is a form of wastewater treatment. It is commonly used in garden pond filtration systems to kill algae. UV is also used for treatment of water after it leaves a Reverse Osmosis Holding Tank.
Large scale urban UV wastewater treatment is performed in cities such as Edmonton, Alberta.
One method for gauging UV effectiveness is to compute uv dose. The U.S. EPA publishes UV doseage guidelines
Doseage involves the following parameters:
- flow rate (reflecting contact time)
- transmittance (reflects light reaching the target)
- turbidity (“cloudiness”)
- lamp age (reflects reduction in UV intensity)
- lamp fouling
- % active lamps (reflects lamp outages in each lamp bank)
What is Water Purification?
Water purification is the process of removing undesirable chemical and biological contaminants from raw water. The goal is to produce water fit for a specific purpose. Most water is purified for human consumption (drinking water) but water purification may also be designed for a variety of other purposes, including to meet the requirements of medical, pharmacology, chemical and industrial applications. In general the methods used include physical process such as filtration and sedimentation, biological processes such as slow sand filters or activated sludge, chemical process such as flocculation and chlorination and the use of electromagnetic radiation such as ultraviolet light.
The purification process of water may reduce the concentration of particulate matter including suspended particles, parasites, bacteria, algae, viruses, fungi; and a range of dissolved and particulate material derived from the minerals that water may have made contacted after falling as rain.
The standards for drinking water quality are typically set by Governments or by international standards. These standards will typically set minimum and maximum concentrations of contaminants for the use that is to be made of the water.
It is not possible to tell whether water is of an appropriate quality by visual examination. Simple procedures such as boiling or the use of a household activated carbon filter are not sufficient for treating all the possible contaminants that may be present in water from an unknown source. Even natural spring water – considered safe for all practical purposes in the 1800s – must now be tested before determining what kind of treatment, if any, is needed. Chemical analysis, while expensive, is the only way to obtain the information necessary for deciding on the appropriate method of purification.
According to a 2007 World Health Organization report, 1.1 billion people lack access to an improved drinking water supply, 88% of the 4 billion annual cases of diarrheal disease are attributed to unsafe water and inadequate sanitation and hygiene, and 1.8 million people die from diarrheal diseases each year. The WHO estimates that 94% of these diarrheal cases are preventable through modifications to the environment, including access to safe water. Simple techniques for treating water at home, such as chlorination, filters, and solar disinfection, and storing it in safe containers could save a huge number of lives each year.
What are Nitrates?
Trinitrate and Nitrogen Trioxide redirects here. See also glyceryl trinitrate. For the film stock see nitrocellulose.
An electrostatic potential map of the nitrate ion. Areas colored red are lower in energy than areas coloured yellow. The oxygen atoms carry the majority of the negative charge.
The structure and bonding of the nitrate ion. The N-O bonds are intermediate in length and strength between a single bond and a double bond.
In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms (NO3-). In organic chemistry the esters of nitric acid and various alcohols are called nitrates.
The nitrate ion is a polyatomic ion with the empirical formula NO3- and a molecular mass of 62.0049. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by bcanonical structures of the nitrate ion are shown resonating below:
Almost all inorganic nitrate salts are soluble in water at standard temperature and pressure.
In organic chemistry a nitrate is a functional group with general chemical formula RONO2 where R stands for any organic residue. They are the esters of nitric acid and alcohols formed by nitroxylation. Examples are methyl nitrate formed by reaction of methanol and nitric acid, the nitrate of tartaric acid, and the inappropriately named nitroglycerin.
Nitrates should not be confused with nitrites, (NO2-) the salts of nitrous acid. Organic compounds containing the nitro functional group (which has the same formula and structure as the nitrate ion save that one of the O- atoms is replaced by the R group) are known as nitro compounds.
High levels of nitrates, most often through occupational exposure in adults, are toxic to humans. Nitrates oxidize the iron atoms in hemoglobin from Ferrous Iron (2+) to Ferric Iron (3+), rendering it unable to carry oxygen. This condition is called methemoglobinemia and can lead to a lack of oxygen in tissues. Methemoglobinemia is treated with methylene blue. Infants, in particular, are especially sensitive to methemoglobinemia as a result of nitrate exposure. This is most often caused by high levels of nitrates in drinking water.
In freshwater or estuarine systems close to land, nitrate can reach high levels that can potentially cause the death of fish. While nitrate is much less toxic than ammonia or nitrite, levels over 30 ppm of nitrate can inhibit growth, impair the immune system and cause stress in some aquatic species. However, in light of inherent problems with past protocols on acute nitrate toxicity experiments, the extent of nitrate toxicity has been the subject of recent debate.
In most cases of excess nitrate concentrations in aquatic systems, the primary source is surface runoff from agricultural or landscaped areas which have received excess nitrate fertilizer. These levels of nitrate can also lead to algae blooms, and when nutrients become limiting (such as potassium, phosphate or nitrate) then eutrophication can occur. As well as leading to water anoxia, these blooms may cause other changes to ecosystem function, favouring some groups of organisms over others. Consequently, as nitrates form a component of total dissolved solids, they are widely used as an indicator of water quality.
Nitrates are also a by-product of septic systems. Specifically, they are a naturally occurring chemical that is left after the break down or decomposition of animal or human waste. Water quality may also be affected through ground water resources that have a high number of septic systems in a watershed. Septics leach down into ground water resources or aquifers and supply near by bodies of water. Lakes that rely on ground water are often affected by nitrification through this process.