What is Reverse Osmosis?

FACTS ABOUT OSMOSIS AND REVERSE OSMOSIS

Osmosis is a physical and chemical phenomenon that occurs in a semi-permeable membrane with through the membrane until there is osmotic balance. If the membraneis permeable to fluids but not for the salts in the fluid (semi-permeable), the fluid can stream through the membrane until there is osmotic balance.

Reverse osmosis is based on using external pressure to assist water molecules to move through a semi-permeable membrane. The result is that the ions remain on one side of the membrane, while the water molecules pass through it. One exceeds the osmotic pressure. The method can be used for purifying sea water to drinking water on board ships and in arid regions. The result is pure water, osmotic water. The membrane is only permeable to water molecules. Water is transported in nature, for example in our bodies, by osmosis. The walls of our cells are natural osmosis membranes that separate impurities from clean water. With reverse osmosis the water is pressed using high pressure through a semi-permeable, synthetic membrane.

WATER TREATMENT WITH REVERSE OSMOSIS

Water treatment means the separation of unhealthy or unwanted amounts of organic and inorganic matter. Traditional cleaning is characterized by certain substances being separated from the water. Reverse osmosis is super filtration, where very small molecules and particles can be separated. A great advantage of reverse osmosis is that no chemicals are used in the treatment process.

PRINCIPAL DRAWING OF TYPICAL RO INSTALLATION

 

The untreated water is pumped up from the sea, lake, river or a cistern (1) through a mechanical pre-filter (2) which removes large particles such as mud, vegetation and similar impurities. Followed with pre-filter of 5 micron to remove particles even further (3) to protect the RO membrane (4).

A high preassure pump pushes the water through the membrane in the osmotic pipe (4). The cleaned water is then moved through a miniralistion filter to add back some minerals (5) after that the water is moved on to a collection tank/cistern which has a level sensor which determines when the system needs to be filled with more water (6). As long as the level keeps itself between the min- and max levels the pressure pump does not start. Fresh water is then pumped out for distribution (7). Depending on the distance and height difference a smaller pressure tank might be is positioned close to the fresh water line’s main valve. It’s highly recommended to have a UV lamp for disinfection of the stored water to stop any bacteria to be distributed with the fresh water.

The water that is separated in the osmotic pipe (waste) is sent back as overflow water (8).

A CLOSER LOOK AT THE MEMBRANE

A reverse osmosis membrane is a highly effective purification method to remove salts and ions to produce pure water.
A spiral membrane is constructed with a number of membrane envelopes wound around a perforated central tube. The water is forced through the semi-permeable membrane which rejects the salt molecules leaving pure water on the other side. This pure water is called the permeate and spirals inward to the central tube for collection.

 

WHY ARE SOME RO SYSTEMS MUCH CHEAPER THAN OTER?

The selection of what type of RO system to be used is very much dependent on what salinity content the water has. There are in princip four different type of RO’s (some variations might exist) and they are:

  • Fresh water RO
  • RO for salinity level of max TDS 2 000 (Well water with some salinity penetration)
  • RO for salinity level of max TDS 7 000 (Baltic Sea north of Öland)
  • RO for salinity level above TDS 7 000 (Rest of the sea water)

The principal of Reverse Osmosis is the same in all  cases above but the requirement of the required water pressure of the pump differs a lot between the systems as well the material they can be built with.

So it is of great importance to have the source water analysed at a lab prior purchase of a system.

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