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Reverse Osmosis

Reverse osmosis system

Reverse osmosis (RO) is a water purification system or process that uses to separate ions, unwanted molecules, and larger particles from drinking water by a semi-permeable membrane. It is the most popular and older technology that is very useful for the purification of drinking water.

Reverse osmosis (RO) system diagram in water purification process by a semi-permeable membrane with definition, uses and differences with osmosis

In the reverse osmosis process, pressure is applied to overcome the osmotic pressure of the system.

Osmosis and reverse osmosis

Reverse osmosis is a process where osmosis occurs in reverse. Osmosis occurs naturally without requiring any energy but we apply energy to reverse the natural osmosis process.

What is osmosis?

Osmosis is a naturally occurring phenomenon that occurs when two solutions with different concentrations are separated by a semipermeable membrane.

In the natural osmosis process, a solvent will migrate from a low solute concentration (high water potential) to a high solute concentration (low water potential). Osmosis occurs when plant roots absorb water from the soil and our kidneys absorb water from our blood.

The reduction in the Gibbs free energy of the system is the driving force for the movement of the solvent. Osmotic pressure is generated due to the movement of solvent from lower to higher concentration regions.

What is reverse osmosis?

Applying an external pressure to reverse the natural flow of pure solvent is called reverse osmosis. In an RO water purification system, excess pressure is applied to the contaminated water side for the movement of pure water through a semipermeable membrane.

Reverse osmosis membrane

Reverse osmosis membranes are important units to remove salts and other pollutants from drinking water. These RO membranes are usually made of cellulose acetates, polyamides, polyimides, and poly-sulfones. After the development of polymer technology, various efficient membranes are developed and used in various RO machines.

The RO membrane should not allow to passes large molecules or ions through the pores or holes but it should allow to passes smaller components of the solution such as solvent or water molecules.

Osmosis membranes are sensitive to changes in the pH of water. Small concentrations of oxidized substances such as chlorine and chlorine dioxide, various organic compounds, algae, and bacteria can change the pH level of water. Therefore, careful pretreatment is needed in order to decrease RO membrane contamination and fouling.

How reverse osmosis works?

Before we know how reverse osmosis works, we should understand how osmosis works. In chemistry, osmosis is the process by which water passes through a semi-permeable membrane from a less concentrated solution into a more concentrated one.

Osmosis and reverse osmosis (RO) systems diagram, process and uses for purification of water

Reverse osmosis occurs when we use external pressure to run the osmosis process in the reverse direction. Therefore, it filters the contaminated or saline water to produce pure water.

Reverse osmosis system for home

Household drinking water purification systems commonly work by four stages of filtration including a RO step,

  1. Sediment filter: A sediment filter uses to trap different types of sediments or suspended solids including rust and calcium carbonate.
  2. Pre-carbon block: An activated carbon filter trap organic chemicals and chlorine that degrade the quality of the RO membrane.
  3. Reverse osmosis membrane: The RO membrane then trap the molecules heavier than water such as sodium, calcium, high levels of lead, mercury and arsenic, various minerals, fluoride, chloride, sulfate, etc.
  4. Post-carbon filter: A second carbon filter is used to capture the chemicals that are not removed by the reverse osmosis membrane

Uses of reverse osmosis systems

The systems may use to remove many types of dissolved and suspended chemical species and bacteria from contaminated water. It is used in both industrial and household production of potable drinking water. The most common uses of the RO process are given below,

Water purification system

  • We use the reverse osmosis step in household drinking water purification systems for improving the quality of drinking and cooking water.
  • Portable reverse osmosis water purifiers are sold and used for personal water purification in various locations of the world.
  • RO step is also used to remove pollutants and microorganisms in the production of bottled mineral water.
  • RO system can be used for the production of deionized water.

Uses in power plants

RO process is applied for the purification of water stored in drains and used in boilers of power plants. The pure water does not leave any deposits on the machinery or cause corrosion.

The deposits inside or outside the boiler may reduce its efficiency and result in poor steam production. Therefore, it decreases the production of electricity at the turbine.

Food industry

In the food industry, the reverse osmosis process is used for concentrating food liquids such as fruit juices. RO technology uses in the food industry due to its lower operation cost. It is also used to avoid conventional heat-treatment processes that destroy heat-sensitive substances such as proteins and enzymes present in food products.

In the dairy industry, it is extensively used for the production of whey protein powders and concentrated milk to reduce shipping costs. It is also used for concentrating alcohol in the wine industry.

Hydrogen production

For small-scale hydrogen production, it uses for the purification of water that is used in electrolysis. It helps to prevent minerals deposition on the surface of the electrode.

Benefits of RO system

  • It has the ability to remove most of the total dissolved solids (TDS) present in drinking water. Therefore it improves the odor, appearance, and overall taste of your drinking water.
  • Cooking with pure water obtained from the RO system dramatically impacts the flavor of food items such as coffee, tea, soup, etc.
  • It is the best method for softening your drinking water.
  • High-quality filtration of the RO systems can remove bacteria or heavy metals from our drinking water.
  • The energy used by RO systems for water purification is relatively low compared to other similar systems.
  • RO is space saving system and you would like to put it in a small space.
  • The maintenance of the RO system is very simple.
  • It is a totally automatic system and is designed to start and stop on its own.

Disadvantages of reverse osmosis

  • The RO system removes healthy minerals such as calcium, magnesium, potassium, and bicarbonates from drinking water.
  • The RO system which we use in our home wastes a huge amount of water.
  • It requires periodic maintenance.
  • The installation of an RO system in our home is costly.
  • The filter replacement cost of the RO system is very expensive.
  • The membranes used in the RO machine have limited pH and temperature tolerance.
  • The RO membranes can be degraded at temperatures greater than 35°C.
  • Reverse osmosis membranes may also be degraded by chlorine that is present in water which we filter by the RO system.