Reverse Osmosis Membranes

Reverse Osmosis (RO) is a separation technique that is suitable for a wide range of applications, especially when salt and/or dissolved solids need to be removed from a solution. Reverse Osmosis water purifier uses a semi-permeable membrane to separate water from the dissolved salts. Accordingly, RO can be used for seawater and brackish water desalination, to produce both waters for industrial application and drinking water. It can also be applied for the production of ultrapure water (e.g. semiconductor, pharmaceutical industries) and boiler feed water. In addition, RO membrane systems are used for wastewater and water reuse treatments

What is the membrane in reverse osmosis?

In the reverse osmosis process, the membrane allows molecules of certain to pass through it. The osmotic pressure is created by water at different concentrations.

What is osmotic pressure?

Osmotic pressure is the pressure caused by water at different concentrations due to the dilution of water by dissolved molecules (solute), notably salts and nutrients. Osmotic pressure is closely related to some other properties of solutions, the colligative properties. These include the freezing point depression, the boiling point elevation, and the vapor pressure depression, all caused by dissolving solutes in a solution. The osmolarity is often determined from vapor pressure depression or freezing point depression, rather than from direct osmotic pressure measurements. The osmolarity is the concentration necessary to observe these phenomena.

A solution placed in a sealed container with a source of pure water will gain water because its vapor pressure is lower than that of the water. This situation is formally equivalent to osmosis, where the semipermeable membrane is the intervening air between the two surfaces. Thus osmotic pressure and vapor pressure depression are perfect predictors of each other because essentially they are the same phenomenon.

 

Principle

Osmosis is a natural phenomenon that can be defined as the movement of pure water through a semi-permeable membrane from a low to a high concentration solution. The membrane is permeable to water and some ions but rejects almost all ions and dissolved solids. This process (movement of water) occurs until the osmotic equilibrium is reached, or until the chemical potential is equal on both sides of the membrane. This is the working principle behind reverse osmosis water purifiers.

A difference of height is observed between both compartments when the chemical potential is equalized. The difference in height expresses the osmotic pressure difference between the two solutions. Reverse osmosis is a process which occurs when pressure, greater than the osmotic pressure, is applied to the concentrated solution. Water is forced to flow from the concentrated to the diluted side, and solutes are retained by the membrane.

The performance of an RO membrane is defined by various parameters. The important parameters are defined below.

Flow Rate

In RO device there are three streams. The feed stream is separated by RO membrane into permeate and concentrate streams. Flow rates of these streams are usually expressed in cubic meters per hour (m³/h) or in gallons per minute (gpm). Feed flow rate is defined as the rate of water entering the RO system. Permeate flow rate is defined as the rate of water passing through the RO membrane, and concentrate flow rate is defined as the rate of flow which has not passed through the RO membrane, and comes out from the RO system with rejected ions.

Element Construction

In RO device there are three streams. The feed stream is separated by RO membrane into permeate and concentrate streams. Flow rates of these streams are usually expressed in cubic meters per hour (m³/h) or in gallons per minute (gpm). Feed flow rate is defined as the rate of water entering the RO system. Permeate flow rate is defined as the rate of water passing through the RO membrane, and concentrate flow rate is defined as the rate of flow which has not passed through the RO membrane, and comes out from the RO system with rejected ions.

How Reverse Osmosis Filtration Works

Pre-Filtration

The first step in purifying water with reverse osmosis is meant to protect the membrane. It removes larger sediment, including some dissolved solids, and helps reduce chlorine.

This first cartridge is referred to as the sediment filter or carbon block filter. It helps conserve the membrane, which can get clogged by excess sediment or damaged by exposure to too much chlorine, which you’ll find in municipal water.

Reverse osmosis works best when you start with good water and then make it great. That’s why you should never use a reverse osmosis system with hard water unless it is under 10 grains per gallon. If your water is too hard, start with one of our other water treatment solutions.

The Reverse Osmosis Membrane

Your water is forced through the semi-permeable membrane under pressure. The membrane is a synthetic plastic material that allows the passage of water molecules. However, sodium, chlorine, and calcium as well as larger molecules like glucose, urea, bacteria and viruses cannot pass. We have reverse osmosis drinking water systems that are tested and certified for reduction of:

  • Lead
  • Arsenic
  • Copper
  • Nitrates and Nitrites
  • Chromium (Hexavalent & Trivalent)
  • Selenium
  • Fluoride
  • Radium
  • Barium
  • Cadmium
  • Cyst (Cryptosporidium)
  • Total Dissolved Solids (TDS)

Typical Rejection Characteristics Of Reverse Osmosis Membranes

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