Have you wondered, why you are asked to gargle with salt water, when suffering from a strep throat? It is because of the osmosis process of brine with the affected tissue of your throat. So what happens in osmosis? The fluid flows from a more solvent concentrated region to a low solvent concentrated region, passing by a semipermeable membrane, and this process is termed as osmosis. Hence, salt water dehydrates the bacteria in your throat by this process. In reverse osmosis, precisely the opposite of osmosis happens. For instance, imagine there is a semipermeable membrane separating freshwater and concentrated aqueous solution on the other side. Ideally the freshwater will cross the membrane and dilute the concentrated solution by the process of osmosis. However, if enough pressure is exerted on the concentrated aqueous solution, then the water molecules start moving to the freshwater side, under the osmotic pressure. This is known as reverse osmosis, which is extensively used in commercial applications like water filtration.

Reverse Osmosis Filtration System

In a reverse osmosis (RO) water filter system, dissolved inorganic salts are separated from tap water to make it potable. In simple words, the tap water is pushed across a semipermeable membrane under osmotic pressure. The membrane, which is more like cellophane, allows only water to pass through.

Cold water line valveWater source for the system
Pre filterThis is a sediment filter, with an additional carbon filter used for removing sand, silt, chlorine, dirt, and other sediments.
Reverse osmosis membraneThis is the heart of the RO system and comes as two options, chlorine tolerant (cellulose tri-acetate) and non chlorine tolerant (Thin film composite).
Post filterThis is a pure carbon filter which removes tastes and odors from the water
Automatic shut-off valveAn automatic shut off valve stops any water from passing further through the membrane, when the storage tank is full.
Check valveThis valve is located at the end of the membrane. This is used for preventing backward flow of water from the storage tank. (a backward flow can damage the membrane)
Flow restrictorTo maintain the flow rate, a flow control is needed to maintain pressure on the inlet side of the membrane. This is located in the drain line tubing.
Storage tankThis is the storage tank with a capacity of 2.5 gallons of water. This tank is further equipped with a bladder, which keeps the water pressurized until it is full.
FaucetEvery RO unit is equipped with its own faucet, that is installed in the kitchen sink. It is the opening valve for the drinkable water.
Drain lineThen there is this drain line which runs from the outlet of the reverse osmosis casing to the drain. It is used for disposing off impurities found in the water source.

So, it is apparent as to how this system uses a semipermeable membrane to purify the water. The normal pressure provided by a standard residential water system is about 40 psi, which is sufficient enough to force the water through the membrane. The contaminants which are huge are rejected and are flushed down the drain line. This quality check is enforced by the cross flow mechanism of the membrane, which self cleanses itself, from time to time. In this water filter system, the incoming water pressure, temperature of the water, type number, content of the dissolved compounds in tap water, and quality of filters and membranes used, play a major role in assessing the performance of RO for water purification.


For installing this system, you would require a reverse osmosis system kit, a drill, an adjustable wrench, and a utility knife.
  • First figure out a suitable place for installation. Under the sink, in an adjacent cabinet, or in the basement would be an appropriate location to install your system.
  • Now, connect the drain line of the RO kit to the sink waste pipe, which will drain out the contaminants and other dissolved compounds in the water source.
  • Connect the inlet pipe (water source) to the cold water source, using a saddle valve. Make sure there is a shutoff valve inserted in the connection, which stalls water flow when the storage tank is full.
  • Install the faucet, connecting this entire reverse osmosis casing properly.
  • Last but not the least, install a refrigerator line, which connects to the faucet.
When you go for an entire house installation, make sure you are well aware of the rejection rate (rate of rejecting contaminants) and efficiency rate (producing enough drinking water for every member of the family). There are two kinds of membranes, cellulose triacetate and thin composite film, as mentioned above. Of these, CTA needs to be disinfected from time to time, pertaining to its organic nature. Also if CTA is being used, make sure that the water source is chlorinated. On the other hand, TCF is highly bacteria resistant and if used for purifying chlorinated water, it must be paired with carbon filters.

RO system is an excellent water purification system that makes even salt water potable. Although the only drawback is that, less drinking water is produced. Say, if the input is about 3-10 gallons of untreated water, then about 1 gallon of purified drinking water is obtained. But all said and done, with the advantages outweighing the disadvantages in the long run, it is best to have this system at home, as it would surely assure you toxin and chemical free water to drink!