Full Installation Service on all equipment
Reverse Osmosis
The Basics
Reverse osmosis is a process that is used to remove a wide range
of salts to give water of a high purity. Osmosis is a natural
process involving fluid flow across a semi-permeable membrane
barrier.
It is the process by which nutrients feed the cells in our
bodies and how water gets to leaves at the top of trees. If you
seperate a solution of salts from pure water using a basic thin
semi-permeable membrane like a sausage skin, the pure water
passes through the membrane and tries to dilute the salt
solution.
If the salt solution is connected to a vertical pipe then
the progressively diluted solution will fill the pipe until the osmotic
pressure drawing the pure water through the membrane is the
same head pressure as the diluted solution.
This process can be reversed, hence ‘reverse osmosis’ - by
applying a higher pressure to the salt solution. Pure water will
then pass the other way through the membrane in a process
that is easy to visualise as ‘filtration’ where the filter will only let
through the small water molecules and retain almost all of the
other molecules. This means that water containing a high level
of natual salts can be purified without the need for chemical
regenerants such as the acid and alkali used in demin plants.
Reverse osmosis is therefore considered a much safer route of
producing pure water for many commercial and industrial
applications, and additionally the plant doesn’t need to be
taken out of service for regeneration as a demin plant does.
Rejection rates of salts from water is generally in the region of 95-
99.5% dependant upon the membrane type used and the raw
water feed quality.
Reverse osmosis systems, in their most basic form, consist of a pressure pump, housing and membrane. Water is
forced into the housing under pressure and the pure water (or permeate) is collected and passed to service.
Reject water (or concentrate) is collected from another outlet and routed to drain, with a portion of the
concentrate water recycled back to the inlet of the pump. This means that the portion of water sent to drain is
kept to a minimum, allowing a recovery ratio of approx 75% to be achieved without significant fouling of the
membrane.
