Reverse osmosis

Reverse osmosis (RO) is the process of water purification by which water is passed through a semipermeable membrane under pressure to remove contaminants, ions, large particles and other impurities. This is in fact a process opposite to osmosis which is a naturally occurring phenomenon.

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Osmosis naturally occurs when two solutions of different solute concentration are separated by a semi permeable membrane. On the side of the higher solute concentration, solutes interact with the solvent molecules (for example water), causing a reduction of free solvent molecules on the solute side of the membrane. Therefore, the molecules of the higher solvent concentration move towards the side of the membrane with lower solvent concentration (but higher solute concentration), until there is an equal concentration of solvent molecules on both sides.

Looking at the left image in Figure 1, the "U" shaped tube has two solutions on each side that are separated by a semi permeable membrane in the middle. A semi permeable membrane refers to a material that allows certain substances to pass through such as water and forms a barrier to larger size particles such as solutes. There is fresh water with a low salt concentration on one side and salt water with a high salt concentration on other sideof the membrane. The water with lower salt concentration will begin to move towards the other side with higher salt concentration because nature strives for en equilibrium of free solvent molecules. This causes the water level on the side with higher salt concentration to rise. It is ‘pushed’ up by the purer water. This is called osmotic pressure. It is the pressure needed to prevent net movement of solvent molecules across the membrane.

Working principle of osmosis revere osmosis

Figure 1: Osmosis and reverse osmosis

Reverse Osmosis

As mentioned, reverse osmosis is ‘Osmosis in reverse’ and therefore is not a natural process. Hence, for reverse osmosis to occur, pressure needs to be applied to the higher concentration solution. This leads to the passage of water molecules towards the lower concentration region through a semi permeable membrane. This pressure should be higher than the natural osmotic pressure in order for pure water to pass through, leaving behind a majority of contaminants or salts.

Figure 1 shows on the right side a U-shaped tube with reverse osmosis in action. The applied pressure on the higher salt concentration water leads to water moving across the membrane, thereby purifying water in the process.

Reverse Osmosis systems

A simple reverse osmosis system consists of a high-pressure pump which increases the pressure of the feed water with salts, ions or impurities. This forces the water across the semi permeable RO (Reverse Osmosis) membrane, leaving behind all the dissolved salts. The required pressure is directly proportional to the salt concentration of the inlet water. The higher the concentration, the higher pressure is required to overcome the osmotic pressure.

Working principle of a simple reverse osmosis system

Figure 2: A simple Reverse osmosis system

The RO membrane lets the cleaner water through and this is called permeate water. The salts and other contaminants which are stopped by the membrane come out as a reject stream. This can be drained out or recycled if possible through the RO system to save water.

The RO systems remove up to 99% of dissolved salts (ions), particles, pathogens and bacteria from feed water.

Solenoid valve for RO water purifier

Normally solenoid valves for RO water purifiers (reverse osmosis systems) are of 2/2 way (1 inlet, 1 outlet of the open/closed type), ideal for water ON/OFF control. There are three types of solenoid valves:

Direct operated fluid solenoid valves

Direct acting solenoid valves have a compact design. Figure 3 shows that the solenoid valve plunger rests directly over the flow path or orifice and controls the on/off function of the solenoid valve. As it does not use a membrane, the flow is restricted according to the orifice size.

Schematic representation of a NC direct operated solenoid valve

Figure 3: Schematical representation of a direct operated solenoid valve (2/2-way, normally closed).

Normally this is between 1mm to 5mm. As the type of valves is direct acting, they do not depend on system pressure differences to function. This makes them ideal for the low flow, small, medium and high-pressure systems up to 150 bar. Port sizes range from 1/8” to 3/8”.

Semi-direct operated solenoid valves

This type of design uses a hung membrane and is pilot operated as shown in Figure 4. The internal solenoid valve membrane is attached or hung from the plunger assembly. The energized plunger thereby lifts the membrane and keeps it open mechanically. At the same time the plunger opens a pilot orifice. As a consequence, the pressure of the medium supports the opening of the membrane.  Hence this type of solenoid valve operates without a pressure differential between the inlet and outlet. It is ideal for low pressure, closed loop, gravity feed and water suction systems.

Schematic representation of semi-direct operated solenoid valves

Figure 4: Schematical representation of a semi-direct operated solenoid valve (2/2-way, normally closed).

Indirect operated solenoid valves

Indirect solenoid valves (Figure 5) use a membrane that is not attached to the solenoid plunger. Hence it remains floating and uses a pressure differential between inlet and outlet to operate. This pressure differential is controlled by a solenoid actuated pilot circuit. The solenoid coil/plunger is mounted over outlet channel port which connects the outlet and the area above the membrane.  When energized, the pressure above the membrane is released into the outlet port via this pilot channel. This causes a pressure differential which lifts the membrane and allows medium to flow through the valve. When the pilot circuit is closed, the inlet pressure builds up above the membrane forcing it to close and stop flow of medium.

It is ideal for medium to high pressure, open to atmosphere and pump fed systems.

Schematic representation of a servo operated solenoid valve

Figure 5 Schematical representation of an indirect operated solenoid valve (2/2-way, normally closed).

Other valve types

Feed valves

Feed valves are used to control the supply of feed water to RO systems. They have an ON/OFF function. Often, they are solenoid valves. They are offered in various styles and suitable for many plumbing scenarios.

Ball Valves

Two-way ball valves shut off the flow of water to and from equipment with a quarter turn of the handle. These valves can be manually, pneumatically or electrically actuated. They can be used in intermediate positions to stop water flow to a system, tank or components for easier servicing and extended non-use periods. Depending on the material they usually are more robust than solenoid valves.

Automatic shutoff valves

Automatic shutoff valves shut off or turn on depending on the tank levels. These are membrane operated valves which work in conjunction with pre pressurised tanks and float valves to automatically turn the feed to the RO-system off when the tank is full and back on when the tank level drops below a certain pre-set point. They help to save and conserve water. They operate mechanically and no electricity is required.

Float valves

Float valves have a float that senses the level of water beyond a certain level and automatically stop the flow of water into a tank or reservoir. When the water level reaches the float position, the buoyancy of the float causes the arm to elevate, thereby closing the valve. When used in conjunction with a membrane type automatic shut off valve. The back pressure created by closing the tank valve will trigger the shutoff valve to turn off the water feed to the RO-system. When water is used and the level drops, the valve will open, disengaging the shutoff valve and allowing feedwater to flow to the RO-system again.

Check Valves

Check valves are connected to the water tubes of the RO membrane or the permeate outlet port of the membrane housing. These valves prevent back flow to the membrane. When there is back pressure in the system, especially when the tank is full, the spring-operated valve is engaged and stops the flow of water. Flow is not resumed until the back pressure drops below the feed pressure.

These valves are required for systems operating with pressurised tanks and/or automatic shut off valves.

Required characteristics of solenoid valves

Material Selection:

Since RO system valves handle water with a high content of salts, there are good chances of degradation of valve materials due to corrosion. This occurs from chloride rich water and chemicals used. Hence corrosion resistant alloys are commonly used. Plastics such as polyamide and stainless steels are a more common choice for RO systems to resist pitting and crevice corrosion. Brass is also a traditional choice, but normal brass cannot be used with chloride solutions or with purified water due to the dezincification process.

Valve type:

  • 2-way direct operated valves are used for low flow, small, medium and high-pressure systems up to 150 bar. Port sizes from 1/8” to 3/8”. They do not rely on pressure differences between inlet and outlet in order to operate.
  • 3-way direct operated valves are used when three ports are needed to achieve two switching states. These are ideal for actuator and cylinder control or diverting flow.
  • 2-way indirect operated solenoid valves are ideal for atmospheric, medium pressure and high flow applications. The pressure difference needs to be greater than 0.5 bar.
  • 2-way semi-direct operated valves are ideal for low pressure and closed loop high flow pipe systems.


Normally closed

2-way valves are closed when de-energized and open when energized.

When 3-way valves, are de-energized, the pressure port is closed and cylinder port is connected to exhaust port. When energized, the pressure port is connected to cylinder port and the exhaust port is closed.

Normally open

2-way valves are open when de-energized and closed when energized.

When 3-way valves, de-energized, the pressure port is connected to the cylinder port. When energized, the pressure port is closed and the cylinder port is connected to the exhaust port.


This modification allows the valve to be connected in either the Normally Closed or Normally Open position to select one of two fluids or to divert flow from one port to another.


Specifying the correct voltage to ensure it is available at the installation location is important. Common voltages are:

Port Size & Thread Type

Ensuring the thread types and port size are the exact same ensures they will thread together properly and form a good seal, to prevent any leakage. Common issues arise due to different thread standards. For example, a 1/4 inch BSPP water solenoid valve will not work properly if the input port has 1/4 inch NSP threads.

Water Hammer

A water hammer can occur due to solenoid valves being quick acting. Therefore, for certain applications it is important to get a valve with an adjustable closing time, like these slow closing water solenoid valves.

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