Reverse Osmosis is a very popular process for bringing water back to “life” suitable for human consumption. Just like the name implies, the process happens in reverse of the process called Osmosis. The osmosis and reverse osmosis processes occur naturally in our bodies. During the normal Osmosis process, water, or any other liquid, moves from a low concentrated solution on one side of a semi-permeable membrane, through the membrane to the side with the higher concentration until both sides reach equilibrium. In a reverse osmosis filter, water is forced under pressure in the opposite direction of osmosis through the semi-porous membrane. The holes in the membrane are big enough for water to pass through but small enough to stop most other elements and impurities from passing through. Pure water passes through the membrane to the side with the lower pressure. Water that did not get purified and did not pass through the membrane is flushed out to the drain along with impurities. Learn more about the technical aspect of reverse osmosis here.
History of Reverse Osmosis
Osmosis through a semi-permeable membrane was first documented by a French scientist Jean-Antoine Nollet in 1749. The first semi-permeable membrane is rumored to be a pig intestine. In early 60’s the first artificial membranes were conceived in UCLA and were made from cellulose acetate. Nowadays all the membranes are made from a polyamide thin-film composite aka TFC which has proven itself to be more reliable.
You might be surprised to learn why there was so much interest in Reverse Osmosis for Water Purification in the 1960s. When the space age began there was one problem that needed to be solved along with many others. That problem was — water being very heavy and space consuming, but necessary for human survival. An average person in the US goes through 75-100 Gallons per day, every day. That includes water for drinking — the smallest amount, showers, laundry, toilet flushing, dishwashing etc. Just to remind you, proper hydration is crucial for your health and well-being. In space, you may not consume 75-100 gallons per day, but for long sustainable missions more water is needed then we can bring from earth. A four-person crew on the space station would require 40,000 pounds or 4792 gallons to be exact. In space, our astronauts are only using 3.32 gallons per day. It’s amazing how efficient we can be in extreme situations. So brilliant minds of that time looked back at history for answers. Scientist knew that the water we drink today was the same exact water dinosaurs drank. The solution was obvious recycling and reuse of the water.
Reverse Osmosis is the only process also known as hyper-filtration that can turn wine and other liquids back into the water. This level of water purification is needed for closed loop water systems such as space station. We are not going to dive into specifics of such system in this post. You can learn more about a space station water system here.
RO and Desalination
Another very popular application of Reverse Osmosis, also known in short as RO, is desalination. Huge Desalination stations are utilized on cruise-ships, in water-deprived nations, military stations and emergency relief situations. The natural process for desalination is evaporation/distillation of water in the oceans, which in turn comes down as rain. The process of distillation requires huge amounts of energy, that’s why it’s NOT the best choice for desalination or water purification. RO is much more energy-efficient when compared to distillation, but still requires a lot of energy when it comes to desalination. The reason is that the more contaminants in the water the more pressure is required for pure water to be squeezed through the membrane. For example, ocean water is anywhere between 30,000 to 40,000 Parts Per Million, or PPM in short, of Total Dissolved Solids, or TDS in short. TDS is a measurement of all dissolved molecules, ions and microorganisms in the water or any other liquid. 40, 000 PPM in lamest terms means that in a million water molecules there are 30,000 to 40,000 of other matter. Following is a table of water classification Per Water Quality Association:
- Fresh: <1,000 ppm TDS
- Brackish: 1,000-5,000 ppm TDS
- Highly Brackish: 5,000-15,000 ppm TDS
- Saline: 15,000-30,000 ppm TDS
- Sea Water: 30,000-40,000 ppm TDS
- Brine: 40,000-300,000+ ppm TDS
In household water treatment water rarely goes beyond the “Fresh” scale and almost never exceeds Brackish Limits. US EPA regulates TDS as a secondary contaminant and sets the limit at 500 PPM of TDS. In fact, EPA recommends perfect drinking water at 0-50 PPM TDS and an acceptable limit of 50-500. For my personal taste once water is over 400PPM it starts to taste somewhat salty. There are a few nations in the world that are implementing RO for desalination on a huge scale. For example the Tampa Bay Water Treatment Facility which produces 25 million gallons of delicious desalinated water a DAY! Even at this number, which seems like a very big number, it only satisfies 10% of the region’s need. RO water is considered to be very pure and high quality, that’s why all municipalities mix it with water from other sources to get the numbers they need in terms of gallons and of course to keep the cost down. Florida is one of the few states that is incorporating more and more RO plants to satisfy the population growth. Mostly because the groundwater in Florida is becoming increasingly salty. As demand for water goes up and a growing amount of water is pumped from wells, the supply of fresh water shrinks, thus allowing sea water to seep through porous Florida grounds into aquifers close to the shores and saltwater canals.
RO water filters
RO water filters for household use started to gain ground in the late 80′s just about the time bottled water started to gain popularity. Bottled water is very convenient when you are on the go. Grab a bottle on the way to the gym or on the way out. On the other hand bottled water is very inconvenient for cooking or use in the home. If you are cooking a soup, for example, making a pot of tea or coffee, you have to empty out more than 2-3 water bottles. This is where a good reverse osmosis system comes in very handy. Having a virtually unlimited supply of water ready to use is a huge step up from the bottled water. Speaking from personal experience, I can say that it’s a big pain having to go to the store just to pick up some bottles of water.
There is also a lot of unnecessary waste associated with bottled water, even if you are subscribed to a bottled water delivery service. Just recently I saw a booth in Costco promoting a bottled water delivery service. Being very interested in the subject and very thirsty I could not resist but to grab a sample glass of water. I have to admit that water tasted very good, ( just like most of the stuff being promoted in Costco) probably because I was very thirsty. After I finished chugging down the small sample of water, I started a conversation with the salesperson. He told me that his company operates two water purification plants in Florida, one in Orlando and second in West Palm Beach. 5 Gallon water bottles had a picture of a beautiful spring running through the green valley. But, just as I suspected the source of the water was municipal supply – exactly the same water you and I have coming out of the faucet purified with Reverse Osmosis. The Costco I was in is two hundred miles away from the West Palm Beach Plant. Now just think about it, bottled water travels 200 miles to a local distribution center. From there another 10-20 miles to your home. A lot of gas, time and effort multiplied by profit margins, equals dollars being wasted.
You can have exactly the same quality water, even a little bit more fresh right at your fingertips with quality reverse osmosis system. RO water purification systems consist of multiple filtration stages:
- Pre-filtration stage – Particle Filtration up to 5 microns: for sediment and large particles.
- Pre-filtration stage – Carbon Filtration: for chlorine removal.
- Purification stage – Reverse Osmosis membrane: for removal of everything and anything.
- Post-filtration – Carbon Filtration: For final polishing and taste enhancement.
Water passes through the first stage of sediment filtration, where all the particles larger than 5 micron or the size of red blood cells get filtered out.
Second and sometimes consequent third stages are Carbon filtration. The main purpose of carbon filters is to remove chlorine from water. Chlorine, even below EPA drinking water standard which is maximum of 5 PPM – perfect swimming pool water, is so strong that it burns through the membrane and completely destroys it. Carbon filtration works through adsorption – adhesion of atoms to solid surfaces. This means that there is a limit of how much chlorine can be taken out. That’s why all RO water system manufacturers recommend changing filters every six to eight months. If you do not replace your filters in time you are taking a chance of ruining the reverse osmosis membrane, which leads to poor water quality. The same carbon filtration is used in Refrigerator filters. If you think your refrigerator filter is giving you the purest drinking water possible, you are deeply mistaken.
Next, water is forced through the RO membrane under pressure. Just like we described before, water is split into pure (permeate) and waste (concentrate.) Permeate or Pure water is collected and stored in 3, 4 or 5 Gallon tanks. But do not be confused, because the tank also contains air to maintain pressure, the actual amount of pure water in the storage tank is anywhere from 1.5 – 2 Gallons. This means that at any given time you will have one and a half to two gallons of water available for immediate use. Depending on the production rate of the membrane, (50 Gallons Per Day is the most popular size,) it would take 1-2 hours to refill the storage tank when you take into consideration the backpressure build up in the holding tank.
The speed of water production is directly related to the size of the membrane measured in GPD – gallons per day and water pressure. Most reverse osmosis systems are designed to operate at 40 PSI – Pounds Per Square Inch.
Final stage is polishing post filter. Final polishing post filter is also based on carbon filtration. It removes any unwanted tastes and odors from the water. Where are the tastes and odors are coming from you might ask? The answer is simple – storage tank or at least it’s so believed. The first RO systems were designed 20-30 years ago, and design has not changed much. There is no evidence that there is any taste or odor in the water coming from the storage tank. How do I know? I serviced many reverse osmosis units and tasted the water. So I know this from firsthand experience. The role of the post filter is starting to change from unwanted odor and taste prevention to taste enhancing. But, the storage tank has all the properties of the petri dish – it’s dark, warm and filled with pure water starving for life. Also, it’s impossible to properly disinfect the holding tank after years of service.
Alkaline Post Filters
Within the past few years, many different post filters have appeared on the market. Many of them are pure nonsense – for example Ceramic balls that give water positive “CHI” energy as it passes through it. Energized “CHI” water is claimed to cure a wide range of symptoms. I do not believe in claims not backed by science. It’s not a big surprise that these types of post filters originate in Asia. On other hand post filters with minerals such as calcium and magnesium are a big hit. Because these minerals enhance the taste of the water, by adding a trace of the minerals and raising the pH to an alkaline level over 7. This type of post filter still contains one-third carbon in its media as a precaution for elusive taste and odor problems.
We’ve reviewed the history of Reverse Osmosis, Uses and finally the design of Point Of Use Reverse Osmosis System. Please drop a comment below and let me know if you enjoyed this article or if you have any questions.