Reverse Osmosis

Reverse Osmosis is the 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 the leaves at the top of trees.

If you separate a solution of salts from pure water using a basic thin semi-permiable 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 as the head pressure of 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. This means that water containing a high level of natural salts can be purified without the need for chemical regenerants such as the acid and caustic 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 does not 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 membrane type used and the raw water feed quality. RO systems can be designed to utilise the wide range of membranes available, which will give different permeate water qualities. Standard designed RO's are manufactured using the low energy membranes which will give a permeate water quality of approx. 10 microsiemens from an input water of between 500 - 700 microsiemens.

RO System Management

Reverse Osmosis systems in their basic form consist of a pressure pump, housing and the 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 form another outlet and routed to drain, with a portion of the concentrated water recycled back to the inlet 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. The re circulation allows a higher flow of water through the pump reducing the load on its bearings and keeping the pump running cooler. The re circulation on all units is adjustable.

The controller used on the RO system constantly monitors the quality of the permeate water and is also linked with safety controls on the system, to ensure the unit cuts out on low and high pressure, high & low conductivity and full permeate tank signal. It will also run various pre and post flush cycles to maximise the lifetime of the membrane. The constant monitoring is automatic and the programming is all preset to ensure protection of the system at all times and to maximise the quality of the pure water.

RO plants must be supplied with softened, de-clorinated or de-chlorinated anti scalent dosed water. A duplex softener is recommended for continuous operation. Utilising softened water for the feed to the RO will reduce the scaling potential on the membrane and therefore lengthen its working life. De-chlorination of the feed will reduce oxidation damage to the surface of the membrane. High output reverse osmosis plant offers considerable advantages over traditional deionisation systems, with no acid/caustic consumables nor problems with COSHH compliance. If softened service water is needed elsewhere on the same installation site, concentrate water can be returned to a softened water holding tank, eliminating water wastage.

Specifying and sizing

The size of the RO and choice of membrane will be determined by the permeate quantity required, feed water salinity and permeate quality expected. Low energy membranes allow the units to run at pressures around 150 - 200 psi, and as such the pressure booster pumps required to generate the pure water are smaller, and the power consumption is reduced significantly.

These membranes will produce water quality of approx. 10 microsiemens form an input water of between 500-700 microsiemens. If higher permeate quality is required a different range of membranes with higher pressure pumps can be used.

RO units are normally built and used as single units producing the quantity required during the working day. If circumstance demand, the RO units can be duplexed with a central control panel being used to allow manual or automatic switching of the RO plant in service, and will also allow the units to run in parallel when the demand for water is higher, this doubling the permeate production.

Also for very high purity waters the RO can be manufactured in "Double pass mode" - This means the permeate water from the first unit is fed as raw water into the second pass of the RO - This already high quality water will then be improved and a very pure water is produced, typically less than 1 microsiemen.

When sizing the pre-treatment system the quantity of water available on the raw water feed side needs to be checked carefully as the RO system needs a higher feed flow than it's permeate output, as the recovery of the units is approx . 75% of the pure water. The 25% concentrate that is rejected by the RO can be used for any application where soft water is required such as wash water, gray water for toilet flushing or in some cases cooling tower make-up. The use of this water minimises any waste from the feed supply.

RO Plant size & technical specification