How does Reverse Osmosis Work?

Reverse Osmosis (RO) is a water purification process that uses pressure to force water molecules through a semi-permeable membrane. Through this process, water flows from a lower concentration to a higher concentration level. This method is highly effective in removing contaminants and sediments from water. It can be used to improve water quality for both drinking and cooking.

How Does Reverse Osmosis Work?

The semi-permeable membrane is very effective at blocking unwanted impurities. It has small pores designed only to allow the water molecules to pass through. Usually, during osmosis, the transfer of water takes place until an equal level of concentration is reached on both sides. Here, however, the impurities are limited to one side while the pure water gets collected on the other.

The end result of reverse osmosis is brine and permeate. Brine is the concentrated leftover wastewater that needs to be discarded. Permeate is the clean water formed after the filtration process. There are usually three steps involved in this process. They are detailed as follows:

Step 1: Pre-filtration

During the pre-filtration process, the reverse osmosis system is connected to the waterline. A high-pressure pump is attached, which helps to push water through the system. The water is then ready to pass through a series of filters that are in place to catch all the impurities. These filters are usually carbon-based. Here, the sediments and deposits are captured before the rest of the water passes on to the reverse osmosis chamber.

Step 2: Reverse Osmosis

In the reverse osmosis chamber, we have a semi-permeable membrane. This is a tightly wound material with plenty of pores in place to collect all the contaminants. Each pore is only about 0.0001 microns thick. In comparison, a strand of human hair is about 75 microns thick. This membrane is effective in removing impurities like salts, particles, colloids, organic bacteria, and protein-based contaminants.

Step 3: Draining and Storing

The third process is fairly straightforward. The reverse osmosis drains out all the brine water, whereas all the permeate is stored in a small tank.

Stages of Reverse Osmosis System

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Now that we have a general understanding of a reverse osmosis system, we can look into the stages of a reverse osmosis system. This system can be summarized in about five stages and will help you understand how your RO filter supplies you with clean and fresh water.

Stage 1: Sediment Removal

As we discussed, the first process is pre-filtration. During this stage, a sediment filter in your RO works to remove all the larger particles from the contaminated water. This can include contaminants like stones, dust, or clay. Usually, it has a 5-micron filter that is efficient at removing impurities like calcium carbonate and rust.

Furthermore, it protects the more vulnerable filters inside by acting as a barrier between them and the harsh contaminants. This filter will, thus, need to be replaced more frequently, at least about once every six months.

Stage 2: Carbon Filtration

This stage is also a part of the pre-filtration process. This step includes an activated carbon filter. It is highly effective at filtering organic chemicals, chlorine, and iron. It is also great at removing oils that may damage the upcoming RO filters. Generally, there are two levels of carbon filters, the second one more compact than the first, which ensures a proper pre-filtration process.

Stage 3: RO Filtration

The RO filtration stage efficiently removes 99% of the impurities present in your water. These can include both organic as well as inorganic compounds. This covers arsenic, lead, fluoride, and so on. It is highly effective at targeting even the smallest deposits. The fine semi-permeable membrane works to remove any contaminant with a molecular weight above 200.

Stage 4: Remineralization

RO filtration will result in pure and clean water. However, there is a possibility that it might still retain some acidic properties. In this case, there is often a remineralizing filter. This helps balance the pH by introducing healthy minerals such as magnesium and calcium in the water.

This means the resulting water after this stage is not just pure but also full of beneficial minerals. Some RO filters also pair a UV light filter, which can help further target microorganisms like bacteria and viruses from your water.

Stage 5: Storage

Once the water has gone through these stages of purification, the pure water or permeate is then stored in a separate tank. This is the final stage of water purification through reverse osmosis. However, there are some RO filters that offer extra filters, such as a TDS deionization filter or an additional final carbon filter. Depending on your requirements and water quality, you may or may not choose to add these filters.

Reverse Osmosis Benefits

Effective Removal of Contaminants

RO filters are known to be extremely efficient and boast of removing 99% of the impurities from your water supply. It can target chemical contaminants like sodium, chloride, lead, copper, and also reduce harmful substances like arsenic, radium, sulfate, and so on. This simple investment can provide you with the pure and clean water that you need.

Targets Inorganic Impurities

Inorganic impurities include heavy metals such as nitrates and phosphorus, charcoal, inorganic salts, and so on. Many ordinary filters miss these components. However, with the extremely fine pores of the semi-permeable membrane present in RO filters, you can get assured that these impurities will be targeted and removed.

Odor-free Water

Most regular water filters may be good at removing impurities but cannot really target bad odor. The activated carbon filters in this system aim to remove odor as well. These filters capture all the contaminants that are responsible for causing a bad-odor. This ensures that your resulting water not just looks like pure water but smells like it too.


A typical reverse osmosis system should last you about two to five years. This can be extended exponentially if the parts are regularly taken care of and replaced. The filters don’t need replacement too often, and the entire system requires very low-maintenance. As such, you can be sure you will be receiving clean water for a long time.

To sum up,

Reverse Osmosis is an excellent filtration system that targets both organic as well as inorganic impurities. The pre-filtration, reverse osmosis membrane, along with additional post-filtration filters, should make sure you receive pristine water supply. An RO filter can thus be an excellent investment due to the multitude of benefits it brings along with durability.

Frequently Asked Questions

1. How often should the RO membrane be replaced?

The RO membrane itself needs to be replaced every 2-3 years. However, it is a good idea to change them more often if your local water conditions are of poor quality than the average conditions. The pre-filtration filters will need to be replaced more often as they are the first barrier for the contaminants. You should replace these about once every 6 months.

2. Do they have any negative impact on the environment?

Once the water goes through the filtration process, a certain amount of wastewater is generated. This could be counted as a negative environmental impact. Another concern is that there may be some water wastage during the filtration process. However, we can avoid this by utilizing this water for other purposes.

3. Is reverse osmosis system similar to other water distillers?

No, these are two different appliances altogether. A water distiller works to filter water by boiling the water and then condensing the steam. An RO system uses a membrane to purify water. So while they are not similar, they aim to provide the same end result. Read the clear differentiation on reverse osmosis and distilled water here.

4. Is reverse osmosis water bad for you?

The main reason this has become a concern is that the resulting water from RO filtration is quite devoid of any minerals. However, many manufacturers have already solved this problem by adding a remineralization filter to ensure a proper pH balance.

5. What elements are not removed by reverse osmosis?

Though reverse osmosis is a highly effective filtration process, nothing can really provide you with a 100% purified water. Similarly, the RO filters are unable to remove some pesticides, solvents as well as some volatile organic chemicals.