Osmosis is a naturally occurring process found in (for example) human cell functioning, and is also the means by which water is conveyed to the leaves at the top of trees. If a solution of salts is separated from pure water by a thin semi-permeable membrane, 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 as the diluted solution.
The process can be reversed hence ‘reverse osmosis’ by applying a higher pressure to the salt solution. Pure (RO) water will then pass the other way through the membrane in a process that can be seen 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 natural salts can be purified without the need for chemical regenerants such as acid and alkali used in demineralisation plants.
Reject rates of salts is generally in the region of 95 – 99.5% dependent on the membrane 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 ROs are manufactured using the low energy membranes which will give a permeate water quality of approx. 10 micro-Siemens from an input water of between 500-700 micro-Siemens.
Arrangement and operation
The RO in its simplest form consists of a pressure pump, a housing and a 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 sent to drain with a proportion recycled back into the pump inlet. In this way the portion sent to drain is kept to a minimum, allowing a recovery ratio of approximately 75% – without significant fouling of the membrane. In addition the recirculation allows a higher flow rate through the pump, reducing the load on its bearings and keeping the pump to running cooler. The recirculation rate on all models 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 systems, to ensure the unit cuts out on; low pressure, high pressure, low conductivity, high conductivity and full permeate tank signal. It will also run various pre and post flush cycles to maximise the life of the membranes. The constant monitoring is automatic and the programming is pre-set 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 and de-chlorinated 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 oxidisation damage to the surface of the membrane. Membranes can also be fouled by iron, manganese, organics and micro-organisms. For boreholes and other private supplies a full water analysis is recommended before installing an RO. System UVEX can advise on a suitable pre-treatment.
Product tank and Delivery pump
A permeate (RO water) storage tank is recommended to collect the output and control production of the RO unit via level switches (supplied). A delivery pump (suitable to use with RO water) will be required to deliver the RO water from the tank to service. System UVEX can advise on and / or supply both the tank and pump.