Thermodynamics

Reverse Osmosis 

The process of reverse osmosis is a special type of filtration that uses a semi-permeable, porous membrane, that allows only pure water to pass through it. Reverse osmosis is a water purification process. This process of reverse osmosis uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome pressure. This pressure is osmotic pressure. It is a colligative property that we get by chemical potential differences of the solvent, a  thermodynamic parameter.

Reverse Osmosis

                                                                                                    Reverse Osmosis

Introduction to Reverse Osmosis

The process of reverse osmosis can remove different types of dissolved and suspended chemical species as well as biological ones’ alike bacteria from water. This process is useful in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is passed to the other side. In particular, this membrane does not allow large molecules or ions through the holes. Though it allows smaller components of the solution, for instance, solvent molecules like water \(H_2O\) to pass through it freely.

In normal osmosis, the solvent naturally moves from an area of low solute concentration through a membrane, to an area of high solute concentration. Basically, it goes from high water potential to low water potential. The driving force for the movement of the solvent is because of the reduction in the Gibbs free energy of the system when the difference in solvent concentration on either side of a membrane is reduced.

By reducing it generates osmotic pressure due to the solvent moving into the more concentrated solution. By applying an external pressure to reverse the natural flow of solvent is actually reverse osmosis. The process is similar to other membrane technology applications ever in use.

Reverse Osmosis Principle

Reverse osmosis works by reversing the accurate principle of osmosis.  The salt solution is subjected to pressure against the semi-permeable membrane. Remember that the applied pressure is greater than the osmotic pressure. Thereby, the molecules move easily from a highly concentrated solution to a less concentrated solution.

Reverse osmosis differs from filtration as if the mechanism of fluid flow is by osmosis across a membrane. The removal process in membrane filtration is straining, or size exclusion. Here, the holes are approximately 0.01 micrometres or larger. Thus, the process of osmosis can theoretically get perfect efficiency regardless of parameters such as the solution’s pressure and solution’s concentration.

Reverse osmosis involves solvent diffusion. It is across a membrane that is either without holes or it uses nanofiltration with holes of 0.001 micrometres in size. The removal mechanism is from differences in solubility and the process dependent on pressure, solute concentration, and many other conditions.

Drinking Water Purification by Reverse Osmosis

Reverse osmosis is most commonly in use in drinking water purification from seawater, removing the salt and other sewage materials from the water molecules.

Generally, household drinking water purification systems, including a process of reverse osmosis. It is commonly in use for improving water for drinking and cooking.

Such systems typically include a number of steps as follows:

  1. A sediment filter traps particles such as rust and calcium carbonate.
  2. Optionally, it has a second sediment filter with smaller pores.
  3. It contains an activated carbon filter to trap organic chemicals and chlorine, this will attack and degrade some types of thin-film composite membrane.
  4. A reverse osmosis filter, which is a thin-film composite membrane
  5. An ultraviolet lamp is in use for sterilizing any microbes that can escape filtering by the reverse osmosis membrane
  6. Also, a second carbon filter to capture those chemicals that are not removed by the reverse osmosis membrane

In some systems, the carbon prefilter is discarded and a cellulose triacetate membrane is placed. Cellulose triacetate is a paper by-product membrane bonded to a synthetic layer. It is made to allow contact with chlorine in the water. These require a small amount of chlorine in the water source to prevent bacteria from forming on it. The rejection rate for cellulose triacetate membranes is 85–95%.

The cellulose triacetate membrane is made to rotting unless protected by chlorinated water. The thin-film membrane is prone to breaking down under the influence of chlorine. A thin-film composite membrane is made of synthetic material, and need chlorine to be removed before the water enters the membrane.

To protect the thin-film composite membrane elements from chlorine damage, carbon filters are in use as pre-treatment in all residential reverse osmosis systems. thin-film composite membranes have a higher rejection rate of 95–98% and a longer life than cellulose triacetate membranes.

Portable Reverse Osmosis Water Processors

Portable reverse osmosis water processors are made for personal water purification. To work properly, the water feeding to these units are under some pressure of 280 kPa (40 psi) or greater is the norm. Portable reverse osmosis water processors are in use by people who live in rural areas as if without clean water. This is the water that is far away from the city’s water pipes.

Rural people filter river or ocean water themselves, as the device is easy to filter the water. Many travellers on long boating, fishing, or island camping trips, or in countries where the local water supply is polluted or substandard. The use of reverse osmosis water processors is coupled with one or more ultraviolet sterilizers.

FAQs on Reverse Osmosis

Question 1: What are the disadvantages of reverse osmosis filtration?

Answer: Few advantages of reverse osmosis are:

  1. A lot of energy is requiring for the entire process of reverse osmosis filtration.
  2. There is a lot of pressure needed so that deionization can occur for further filtration.
  3. During the process, the water might become acidic because it has been deionised of all its mineral content.
  4. It is not advisable to drink water from the process because naturally, the water must have some minerals that help in the functioning and malnutrition of the body.

Question 2: What are the advantages of reverse osmosis?

Answer: Few advantages of reverse osmosis are:

  1. Reverse osmosis is the best method for water softening. As a matter of fact. It performs two functioning that is water softening and water purification.
  2. No ion particles are allowed to enter through the semipermeable membrane.
  3. Maintenance is done when the machine is still operational.
  4. It is easy to adapt and efficient in its working for filtration purpose.

Question 3: How is osmosis different from reverse osmosis?

Answer: Osmosis is the process by which the molecules of a solvent pass through the semi-permeable membrane from a region of lower concentration to a higher concentration. While reverse osmosis is the process by which the molecules of a solvent pass through the semi-permeable from a region of higher concentration to lower concentration when pressure greater than the osmotic pressure is applied.

Osmosis is a natural process that occurs along the potential gradient. Whereas, Reverse osmosis is an artificial process that occurs against the potential gradient. Osmosis is observed during the opening of stomata and absorption of water from the soil by the roots. Reverse osmosis is in use in water purification systems.

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