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What is Reverse Osmosis and How Does It Work?

What is Reverse Osmosis and How Does It Work?

People may have heard the term “reverse osmosis” in passing reference to seawater treatment plants or saltwater aquariums. Still, very few people understand how this mechanical process actually works or the various ways it is used in day-to-day human activities. So what is reverse osmosis, and how does it work?

Reverse osmosis is a mechanical filtration process where water is passed through semi-permeable membranes that filter out ions along with most molecules and particles. Reverse osmosis results in water that is one of the purest forms of water on Earth, both pH neutral and free of trace minerals.

Reverse osmosis is used in both household and industrial applications. Keep reading to find out more about how reverse osmosis works and how you can use it to your advantage.

Who Invented Reverse Osmosis?

The discovery of the physical principle of osmosis goes back to Jean Antoine Nollet, a peasant who was educated with the clergy and who eventually became both a priest and a scientist of renown for his time. In 1748, he discovered the process of osmosis, which eventually led to the discovery of reverse osmosis.

This physical process wasn’t utilized in practical applications much for the next two centuries until it was used to filter water in a Thayer University student project by inventors Chris Miller and Dean Spatz. This experiment became the basis for a multimillion-dollar water filtration industry used in homes and water treatment plants.

This student experiment eventually went on to win government contracts from the Department of the Interior to develop reverse osmosis filtration systems. This, in turn, led to its applications from straining maple syrup at the industrial level to kidney dialysis.

Is Reverse Osmosis the Same as Water Softening?

Some people might accidentally confuse reverse osmosis water purification with water softening, but the two processes are not the same. Water softening is the process of filtering water to remove hard minerals that increase the water’s pH and make the water alkaline. This can have a negative effect on water’s ability to be used in cleaning and other applications.

Reverse osmosis does soften water to a neutral pH. Still, it goes beyond water softening filtration to filter out a much higher percentage of contaminants than those removed by water softeners.

While water softening might increase a water’s quality by making it easier to bathe with or positively affecting its flavor, it can’t stand up to the purification that reverse osmosis provides in comparison.

How Does Reverse Osmosis Work?

In its simplest terms, reverse osmosis uses a membrane to filter pure water from a less pure solution. In the normal osmosis process, a solvent naturally moves from an area of high concentration to an area of low concentration. The movement of the solvent in its attempts to move to an area of lower concentration is the cause of osmotic pressure.

The effectiveness of reverse osmosis is dependent on ambient temperatures and external pressure. While reverse osmosis can be used at lower temperatures, the higher the temperatures utilized, the more reverse osmosis water is produced.

What Are Reverse Osmosis Membranes?

Reverse osmosis membranes are the part of the reverse osmosis system that is responsible for the bulk of the filtration performed by the reverse osmosis system. The material used as a reverse osmosis membrane in most reverse osmosis systems is a thin-film composite membrane (TFC).

TFC membranes are made up of a very thin but dense barrier layer (0.2 millimeters) on top of a microporous polysulfone support layer. The benefits of TFC membranes for use in the process of reverse osmosis are as follows:

  • Higher operating flux
  • Lower operating temperature
  • More stable than other chemicals
  • Highly salt-resistant
  • Non-biodegradable
  • High rejection of contaminants such as nitrate, silica, and organic particulates

Without the reverse osmosis membrane, the process of reverse osmosis would not be able to purify water to nearly the level that materials technology currently allows it to.

What Chemicals Are Used to Clean Reverse Osmosis Membranes?

To function properly, the membranes in a reverse osmosis filtration system have to be periodically cleaned to remove any accumulated mineral scale or other debris. Here are some of the various contaminants that can build up on reverse osmosis membranes:

  • Iron deposits
  • Calcium carbonate deposits
  • Oil and other organic deposits
  • Biofilms

Reverse osmosis membranes are cleaned with a combination of antiscalants to remove any mineral deposits and antiseptics to clear any biofilm or other organic materials. Ensuring that the reverse osmosis membranes are cleaned regularly is crucial to extending the life of the reverse osmosis membrane. It plays a large part in maintaining the system’s overall performance.

Alkaline cleaners are typically used to address biological materials that need to be cleared from the system, while acid solutions are used to clear away mineral deposits. Sediment filters and carbon filters should be replaced roughly every twelve months, while reverse osmosis membranes have an operational life of 2-3 years depending on workload and regular maintenance.

Reverse Osmosis System Stages of Filtration

The process of reverse osmosis typically involves four separate stages. Here are the basic stages of filtration in reverse osmosis:

  • Sediment pre-filter: The sediment pre-filter in the reverse osmosis process is the filter that catches the largest particulates and other debris in raw water. Not only does the sediment pre-filter perform the most rudimentary step of the reverse osmosis filtration process, but it also prevents larger particulates from getting deeper into the reverse osmosis system.
  • Carbon block filter: Carbon blocks were first put into application in water filtration systems in the seventies, but in a reverse osmosis system, the first carbon block filter acts to both remove any taste of chlorine and also filter out a finer level of contaminants from the water that was purified by the sediment pre-filter.
  • Reverse osmosis membrane (typically thin-film composite material): The most intricate part of the filtration process occurs at the reverse osmosis membrane, where the water is passed through a millimeter-thin material via osmotic pressure to both remove ions and strain any remaining contaminants out of the water.
  • Carbon block post-filter: This filter is also known as a “polishing” filter and is incorporated into the reverse osmosis system to make sure that any remaining odors or flavors are removed from water during the purification process, leaving a finished product that tastes and smells like nothing at all.

These stages are present in almost all reverse osmosis systems. But, some RO systems include a fifth stage of filtration, which involves adding harmless trace minerals back to the water once the real contaminants have been removed. This remineralization stage is to combat the argument against reverse osmosis removing vital trace minerals that people need for health purposes.

Why Reverse Osmosis Water Storage Is Necessary

All reverse osmosis water systems include a water storage system between the reverse osmosis membrane and the post filter.

The purpose of this storage system is to hold water that is generated until it is ready to be used. Reverse osmosis systems accumulate water until the pressure reaches a certain level because reverse osmosis systems filter water relatively slowly compared to other less-intricate forms of water purification.

Because the reverse osmosis system can only generate so much reverse osmosis water in a certain amount of time, reverse osmosis water is stockpiled in the storage tank until it is needed. This ensures that there is RO water available for use when necessary and helps keep the pressure in the reverse osmosis system consistent.

What Contaminants Does a Reverse Osmosis System Remove?

Reverse osmosis water purification is one of the most intensive forms of water purification available and can eliminate almost all contaminants from source water. Here are some of the contaminants that are removed from tap water through the process of reverse osmosis:

  • Sodium: Excess sodium levels in the diet lead to various human ailments, including hypertension, heart disease, and increased risk of stroke.
  • Chloride: Chloride (or chlorine, as it is more commonly known) is a type of chemical used to treat wastewater and make it potable for human consumption. Chlorine is used in waste management to sanitize wastewater and is also used extensively in swimming pools for the same purpose.
  • Copper: Copper enters water sources through pipe systems and is safe to consume in small amounts, but large amounts of copper can lead to copper toxicity. Copper toxicity is defined as more than 140 micrograms of copper per deciliter of blood. Copper toxicity can lead to liver failure, heart failure, and brain damage.
  • Chromium: Chromium is another heavy metal sometimes found in tap water that can lead to toxicity if consumed in large amounts. Chromium poisoning can result in various symptoms, from respiratory distress to ulceration of the skin and mucous membranes, such as sinus cavities.
  • Lead: Lead used to be one of the most commonly used heavy metals in America, and subsequently, Americans discovered that it is also one of the most dangerous and deadly heavy metals that humans are exposed to. Even in the twenty-first century, thousands of areas in the United States are known for lead exposure. Unlike other types of poisonous heavy metals such as copper, there is no acceptable safe level for lead in the blood. Cumulative exposure leads to neurological problems, especially in children.

Along with the chemical and mineral contaminants that reverse osmosis removes from tap water, reverse osmosis also can remove many kinds of parasites and bacteria. Here are some of the dangerous living things that reverse osmosis can remove from tap water:

  • Cryptosporidium
  • Giardia
  • Campylobacter
  • Salmonella
  • Shigella
  • E-coli
  • Enteric
  • Hepatitis A
  • Norovirus
  • Rotavirus

All of these parasites, bacteria, and viruses are capable of causing real damage to humans if ingested, especially those who are immune-suppressed, such as children or the elderly. Using a reverse osmosis system eliminates the threat of accidental exposure to a wide array of pollutants that would otherwise end up ingested by the person drinking the water.

What Contaminants Can’t Be Removed by Reverse Osmosis?

While reverse osmosis can remove most of the contaminants found in tap water, up to and including radioactive fallout, there are still some contaminants that can’t be removed by reverse osmosis:

  • Pesticides: Pesticides can’t be filtered out of the water through reverse osmosis because the molecular composition of most pesticides is actually smaller than water, allowing them to pass freely through the reverse osmosis filtration system without getting caught up at any stage of the process.
  • Herbicides: Herbicides can’t be removed by reverse osmosis filtration for the same reason that pesticides can’t—they are present in molecules that are too small to get caught by the reverse osmosis membrane.
  • Fungicides: The types of fungicides used in industrial agriculture on a large scale are usually impossible to remove from water for the same reason that herbicides and pesticides are—their molecules are too small to filter out.
  • Dissolved gases: Gases such as hydrogen sulfide cannot be removed through the process of reverse osmosis.

Because powerful pesticides and herbicides are used regularly near sources of water, the threat of run-off of pesticides into drinking water is a very real danger that reverse osmosis is unfortunately not equipped to handle. This is one of the reasons why the regulation of pesticides and herbicides is so important for environmental protection.

Uses for Reverse Osmosis Water

There are many different uses for reverse osmosis water. Here are some of the ways reverse osmosis water is used in everyday applications:

  • Hydroponic agriculture: Because the water in a hydroponic culture needs to be as close to pH neutral as possible and free of any minerals and contaminants, reverse osmosis water is a good choice for hydroponic applications as long as other trace minerals and nutrients are added back into the system. Reverse osmosis water makes hydroponic systems much easier to control on a chemical level.
  • Crafting beer and wine: Since the flavor of the water used in a brewery or winery can significantly impact the final flavor of a vintage or craft beer, using reverse osmosis water can give brew masters the most neutral palate possible to build their flavors on.
  • Household drinking water: With water scares like the Flint River water crisis, more and more people turn to bottled or otherwise purified forms of water to prevent themselves from accidentally ingesting contaminants like heavy metal. Reverse osmosis water systems in the home can act as a more environmentally friendly alternative to plastic water bottles.
  • Saltwater aquariums: Saltwater aquarium enthusiasts often end up with a reverse osmosis water purification system. RO water is the only water suitable for adding into a saltwater system to emulate a real oceanic or brackish environment. The chlorine and other contaminants found in water from the waste treatment plant are not suitable for marine life.
  • Ice rinks: Increasingly, reverse osmosis water purification systems are being used to construct indoor ice rinks because they produce clearer and more aesthetically pleasing ice. Reverse osmosis is also used to wash rink ice and create a new layer on top.
  • Concrete production: The by-product water produced by reverse osmosis water treatment plants is incorporated into stronger concrete production.
  • Detailing cars: The most common cause of water spots found on a vehicle after getting the car washed is mineral deposits in the water. This cause is eliminated by using reverse osmosis water, which can rinse a car to leave it shining and spotless.

Along with the day-to-day uses of reverse osmosis water that an individual can achieve, here are some of the industrial applications of reverse osmosis water:

  • Reducing particulates and solids in industrial boiler water
  • Producing pharmaceutical grade water for the drug industry
  • Metal finishing

Whether it’s used at the home faucet or in a factory, reverse osmosis water has many useful life applications that affect several different industries. Without it, many of the technologies we depend on today would be much less efficient.

What Are the Benefits of a Reverse Osmosis Filtration System?

There are many benefits of reverse osmosis filtration that have been put to use throughout various human industries. But as it pertains to water, here are some of the advantages associated with reverse osmosis:

  • Reverse osmosis improves how water tastes. One major reason people install reverse osmosis water purification systems in their homes is that RO water doesn’t have the distinct mineral flavor of unfiltered water from the water table or local water sources. This is because the reverse osmosis process removes all of the contaminants from water that would otherwise impart it with a flavor.
  • Reverse osmosis water has fewer heavy metals. Many trace heavy metals such as arsenic and lead that commonly pollute waterways and human water sources can be stripped out by reverse osmosis, leaving water that is both cleaner and safer to drink. There are very few substances that a reverse osmosis system cannot filter out of drinking water.
  • Reverse osmosis is a low-energy filtration process. And as technology advances, reverse osmosis is only getting more energy-efficient all the time.
  • Reverse osmosis systems are low maintenance. Home-based reverse osmosis systems usually only need to be serviced every half a year or so, making them a good choice for those families dependent on other sorts of household water purification systems that require more weekly or monthly maintenance.
  • Reverse osmosis is cost-effective. Having a reverse osmosis system in the home can save hundreds of dollars each year in bottled water that the household no longer needs to buy. On the industrial level, the desalination of seawater in coastal towns via reverse osmosis-based water treatment plants is significantly cheaper than trying to outsource that same water source.
  • Reverse osmosis removes water parasites. Reverse osmosis can remove parasites from water, such as Cryptosporidium, which can cause various associated health problems with the digestive and respiratory systems. If cheap reverse osmosis systems could be fashioned for humanitarian applications, scientists could eliminate a large percentage of the world’s waterborne illnesses in the third world.

Reverse osmosis has been around as a mechanical concept since the mid-sixties. Still, it is just now coming to prominence as a twenty-first-century technology, with new applications for it being discovered all the time.

The Argument Against Reverse Osmosis

People make a few claims against reverse osmosis, although some of these arguments are based on misconceptions:

Wasted Water

One of the arguments against reverse osmosis is that it puts off a lot of wasted water as a by-product of the filtration process. For example, one gallon of reverse osmosis water requires four gallons of water to produce. This means that overall, reverse osmosis is only 25% efficient with regards to water production.

This might sound wasteful out of context, but it needs to be quantified with a few sidebars. The first is that many other processes in running a household, from washing vegetables in a sink to drawing a bath, wastes water. And not just 75% of it, but every single gallon that is used.

This means that while reverse osmosis may seem wasteful if you look at its water efficiency statistic alone, it is still significantly less wasteful than most household water practices. And in doing so, it provides the added benefit of a purified source of drinking water in the bargain.

The water that is “wasted” by reverse osmosis water treatment plants isn’t really wasted either. This water that is a by-product of purified reverse osmosis water is still usually within safe drinking standards. At worst, it can be safely used in agricultural irrigation and other water-saving practices.

Environmental Impact

Unfortunately, while reverse osmosis does provide many human benefits, there are some areas where reverse osmosis can cause negative environmental impact, especially in its application in coastal desalination water treatment plants.

Here are some of the negative environmental impacts identified by the National Research Council in their 2008 analysis of reverse osmosis desalination plant operation:

  • Removal of water from natural coastal habitats
  • Impact on environments from the disposal of concentrate liquids
  • Impact on increased greenhouse gas emissions as the result of increased fossil fuel use and electricity generation
  • Impingement (wildlife being trapped and killed by water plant intake)
  • Entrainment (fish, fish larvae, and fish eggs being accidentally taken up into the plant’s water treatment process and destroyed in the first part of the filtration process)

While these negative environmental impacts are comparable to similar technologies being utilized, there is still concern about the long-term impact of desalination plants on the marine environment.

Luckily, several advances are being made in the operation of desalination plants to reduce their impacts on native habitats and environments.

Reverse osmosis has a somewhat checkered environmental reputation, but objectively it is no worse than other similar industrial practices regarding environmental impact. Future developments in reverse osmosis technology also have the potential to make the process more efficient and less dangerous for local wildlife populations.

Health Concerns

Most people get into reverse osmosis water purification to ensure that their water doesn’t contain any waterborne illnesses or chemical contaminants. Still, there are some negative health issues associated with reverse osmosis water as well. Whether these negative health issues are a true cause for concern or not is a matter of some debate between reverse osmosis enthusiasts and skeptics.

While it is true that reverse osmosis removes all potential contaminants from water, leaving it as close to pure as humanly possible, the downside of this process is that there are many trace minerals stripped out of the water that is necessary for human health.

Tap water contains many trace minerals that modern people are not very consistent about getting in their diets, such as potassium, magnesium, and calcium. With reverse osmosis water, people are not only not getting these vital minerals from their food, but they’re also not getting them through their liquid intake, either.

This can lead to cumulative health problems related to dietary malnutrition, such as hypokalemia or hypocalcemia. To avoid complications due to trace mineral deficiency, people who use a reverse osmosis system for their water intake should be sure to either take a multivitamin or monitor their food intake to make sure that they get all of the vitamins they need to survive.

Final Thoughts

While most people might not realize the effect that reverse osmosis has on their daily life, most people are touched by the process in some way throughout the day, whether it’s when they take prescription medication that has been created using reverse osmosis purified water or they’re observing an aquarium maintained with it.

The process of water purification through reverse osmosis does pose some environmental challenges, but hopefully, as the technology advances, more reverse osmosis plants can be put into place to help remove dangerous contaminants from our drinking water.