Reverse Osmosis Membranes

Reverse Osmosis Membranes

reverse osmosis water purifiers

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?

Reverse osmosis differs from filtration in that the mechanism of fluid flow is by osmosis across a membrane. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution's pressure and concentration. Reverse osmosis instead involves solvent diffusion across a membrane that is either nonporous or uses nanofiltration with pores 0.001 micrometers in size. The predominant removal mechanism is from differences in solubility or diffusivity, and the process is dependent on pressure, solute concentration, and other conditions.

How does the membrane work 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.


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.

reverse osmosis water purifiers

Flow Rate

reverse osmosis water purifiers

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.

Permeate flux describes the quantity of permeate produced during membrane separation per unit of time and RO membrane area. The flux is measured in liters per square meters per hour (lmh) or in gallons per square feet per day (gfd).

Transmembrane pressure (TMP or ΔP) is defined as the difference in pressure between the feed side and the permeate side of the membrane. This pressure is usually measured in bar or psi, and is the driving force for membrane separation and permeate production. In general, an increase in the transmembrane pressure increases the flux across the membrane.

Salt rejection is a percentage which describes the amount of solute retained by the RO membrane.

The recovery rate is defined as the fraction of the feed flow which passes through the membrane. It is usually expressed in percentage.

The pressure drop is the difference between the feed and concentrate pressure during water flow through one or more RO membrane elements.

Element Construction

reverse osmosis water purifiers

Most commonly used RO membranes are typically composed by a thin film composite membrane consisting of three layers: a polyester support web, a microporous polysulfone interlayer and an ultra think polyamide barrier layer on the top surface. Thin film composite membranes are packed in a spiral wound configuration. Such element contains from one to more than 30 sheets, depending on the element diameter and element type.

In membrane systems the elements are placed in series inside of a pressure vessel. The concentrate of the first element becomes the feed to the second element and so on. The permeate tubes are connected with interconnectors (also called coupler), and the combined total permeate exits the pressure vessel at one side of the vessel.

The membranes are wound around a perforated permeate pipe. Two membrane sheets are glued together on three sides, only with an opening towards the permeate pipe. The feed water flows across the membrane surface from one side to the other. Due to the high pressure in the vessel, a part of the water penetrates the membrane and this permeate water can only leave the PV through the permeate pipe, while the rest of the water - now more concentrated - leaves on the other side of the membrane , just flowing across the sheet.

How Reverse Osmosis Filtration Works


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.

reverse osmosis water purifiers

The Reverse Osmosis Membrane

reverse osmosis water purifiers

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

Arsenic 92-96%
Barium 95-98%
Cadmium 95-98%
Calcium 94-98%
Chloride 85-92%
Cyanide 84-92%
Fluoride 85-92%
Iron 94-98%
Lead 95-98%
Manganese 94-98%
Magnesium 94-98%
Mercury 95-98%
Nickel 96-98%
Nitrate 60-75%
Phosphate 96-98%
Potassium 85-95%
Selenium 94-96%
Sodium 85-94%
Sulfate 96-98%
Zinc 96-98%