What is Desalination? This is a desalination membrane cutaway below. As you can see, the spiral wound membrane is quite a complex component. This is the heart of the desalination process. The most important components in a desalination system, are the membranes and the high pressure pump. The membrane, is where the actual process occurs. The fresh water or (permeate) is separated from the salt solution (feed water) and directed away  from the  membrane to be collected as the 'Product' . The high pressure pump creates the pressure required, to allow the membrane to function correctly. The salt-free water, or permeate, then becomes a usable source of fresh water. In theory, this water is potable, although for human consumption, it is highly recommended that the water be treated with an Ultra Violet lamp to kill any bacteria that may be present, prior to drinking. There are many pre-treatment and post-treatment methods for desalinated water. The most appropriate method, depends on the feed water source, and the intended use of the product water. Watch the short video below, to learn how the membrane is constructed. This will help you understand more about the importance of the membrane, within this type of desalination system. Reverse osmosis is explained, briefly, on page 2 of this article...

Desalination processes are divided into (i) thermal methods, which involve heating water to produce water vapour, and (ii) membrane processes, which use a relatively permeable membrane to move either water or salt to induce two zones of differing concentrations to produce fresh water. The main thermal method employed is distillation, where saline water is progressively heated in subsequent vessels at lower pressures. Brief descriptions of the main desalination processes are provided below.

1. Distillation Processes

Multistage Flash Distillation is the most widely-used desalination method worldwide. It involves heating saline water to high temperatures and passing it though vessels of decreasing pressures to produce the maximum amount of water vapour (fresh water).

Multi-Effect Distillation operates at lower temperatures but uses the same principles as multistage flash distillation.

Vapour Compression Distillation is generally used in combination with other processes, where the heat for evaporating water comes from the compression of vapour, rather than the direct exchange of heat.

2. Membrane Processes

Reverse Osmosis is a pressure driven process which forces saline water through a membrane, leaving salts behind.

Electrodialysis is a voltage driven process and uses an electric potential to move salts selectively through a membrane, leaving fresh water behind.

What is Reverse Osmosis? As the name implies, Reverse Osmosis is the opposite to osmosis. Osmosis is the process by which water is drawn into the stem of a plant by a pressure variation between the comparatively high pressure outside of the plant and the lesser pressure within the stem. The pressure differential that causes water to be drawn up into the stem of the plant is referred to as “osmotic pressure”.

Reverse Osmosis occurs when the same osmotic pressure is used to allow water to permeate through a membrane as a result of differing pressures outside and within the membrane. In this case, the process happens because the water pressure inside the membrane is higher than the pressure outside – the reverse of normal osmosis.The pore size of the membrane is so small that the majority of impurities and totally dissolved solids are too large to pass through it. They are therefore by-passed and discharged to drain, allowing only the pure water to pass through. The best known example of reverse osmosis is the seagull. The seagull drinks salt water and rejects the salt through a membrane on the top of the beak! Reverse Osmosis membranes typically filter to .0005 of 1 micron and are therefore fine enough to remove viruses, bacteria and parasites.They also effectively remove salts, heavy metals (lead, mercury, copper and iron etc) and most heavy molecule chemicals (insecticides, pesticides etc). .0005 is about 100,000 times finer than a human hair.