Reverse osmosis, also introduced as (RO), is a process for purifying water that uses a semi-permeable membrane and exerts pressure to filter out ions and pollutants from drinking water. But first, we need to know what osmosis is to understand the procedure of reverse osmosis.
Osmosis is a natural phenomenon in which the net movement of water occurs from a saline solution of high water potential to a saline solution of low water potential. For example, if there were two buckets of liquid, one with a low salt concentration and one with a high salt concentration separated by a semi-permeable membrane, water from the low salt concentration will migrate to the bucket with high salt concentration. Very common examples of osmosis are how plant roots absorb water from the soil, or kidneys absorb water from our blood.
A semi-permeable membrane allows very few tiny particles like atoms or molecules to pass through it but withholds anything larger than the openings in the membrane, like insects or contaminants.
Reverse Osmosis Technology
In the process, water that contains the pollutants and ions is on the concentrated side, and through the pressure, water flows to the less concentrated side, by getting rid of the contaminants. The less concentrated side is the clean drinking water and is referred to as permeate. The concentrated side with the water and contaminants is called brine. So, the opposite of osmosis happens here, hence the name, reverse osmosis.
Osmosis occurs naturally, but reverse osmosis requires external energy which applies the pressure to let the water pass through. The partially permeable membrane of reverse osmosis filters allows water molecules to pass through, but it blocks out dissolved mineral salts, pathogens, bacteria, and organic materials.
The pressure required for reverse osmosis to occur must be greater than the natural osmotic pressure since the process is the opposite of osmosis. Reverse osmosis works by increasing the pressure on the saline side of the RO and pushing the water past the semi-permeable RO membrane, leaving practically all of the soluble substances in the contaminated stream (about 95 percent to 99 percent). The amount of pressure required is determined by the incoming water’s salt concentration. The higher the feed water concentration, the more pressure is needed to overcome the osmotic pressure.
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Contaminants That Reverse Osmosis Removes
The reverse osmosis system contains at least 3 to 5 types of filtration before the water is marked safe for drinking. The semi-permeable membrane present in reverse osmosis water systems can diminish up to 98% of the dissolved contaminants in the water by filtering them. All reverse osmosis water systems contain two other filters apart from the partially permeable membrane, they are –
- Sediment filter – removes sedimentary particles like dust, dirt, and rust
- Carbon filter – removes organic compounds, also known as volatile organic compounds (VOCs), removes chlorine and other chemicals that might make the water have a bad aftertaste or an unpleasant odor
Apart from these, the reverse osmosis membrane removes a huge number of other particles as well, such as:
- Herbicides, and
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Stages Of Reverse Osmosis Technology
These filters above are referred to as pre-filters and post-filters, depending on their placement in the process. Typically this is how the reverse osmosis water system works.
- First comes pre-filtration. This stage includes both the carbon filter and sediment filter. This helps remove any large particles like chlorine, or sediments that might clog the semi-permeable membrane.
- Now, this water passes through the reverse osmosis membrane where the dissolved pollutant particles, even if microscopic, are eliminated.
- Then, this water flows to the tank, and it is stored there for use. Whenever the water level in the storage tank is reduced due to use, the process repeats instantly.
- Lastly, for use, when the consumer turns on the faucet for water to come through, post-filtration occurs before the water comes out of the tank.
Benefits of using this system
Even though the process seems short and easy, this is one of the most costly procedures for filtering water. That seems fair since the process eliminates 98% of pollutants, which ultimately makes the water healthier. Reverse osmosis filter –
- Dissolves damaging pollutants in water
- Reduces sodium content
- Removes bad after-taste
- Removes unpleasant smell
- Is more eco-friendly
- Has an easy installation process
- Has easy and affordable maintenance
- Is compact, so it can be placed under sinks
- Has a storage tank
- Takes sixty seconds to filter out two to three ounces of clean water
Some contaminants are so small, that they are invisible to the naked eye, but they might be very detrimental to our health. Reverse osmosis reduces the workload for our kidneys, by providing us with already filtered water. However, the process also removes advantageous minerals like calcium and magnesium, while filtration.
However, this thorough filtration process wastes water. About four gallons of contaminated water leaves through the sewage system, for every one gallon of clean and filtered water produced. However, the contaminated water or brine is not technically wasted, since that can be used as well for other purposes. To reduce the wastage some additional equipment can be installed, for example-
- Permeate pump – this will increase the efficiency of the system and cut down wastage by 75 to 80%. Some reverse osmosis filters have this built-in.
- An automatic shut-off valve can be installed, which stops the flow of water to the sewage when the tank is full.
- About using the brine for other purposes, the rejected water can be used in lawns, gardens, and landscapes. The components in that water might be harmful to humans but are safe for plants.
The reverse osmosis filters are long-lasting. They have a shelf-life of about 10 to 15 years. However, the filters require frequent changes, since they need to be clean to trap the pollutants and dissolved solids. Both the pre-filters and post-filters should be changed within six months to a year. The RO membrane which is the semi-permeable membrane requires a replacement every two to four years.