Reverse Osmosis

Reverse osmosis (RO) is an implementation of membrane technology where water is forced through a semipermeable membrane with pressure that overcomes the natural osmotic pressure of the processed water. With reverse osmosis, it is possible to remove all dissolved salts from water with up to 97–99.5% rejection. Typical use cases include removing fluoride, chloride and arsenic from water, production of potable and irrigation water from sea water (desalination) and various industrial processes from breweries to electric power plants.

In membrane filtration processes, water is always divided into permeate (filtrated water) and concentrate (residue water stream that includes the filtrated substances). In some cases, the concentrate can be collected and re-used by feeding it into the beginning of the process. There are also applications where concentrate is the final product instead of permeate. With reverse osmosis, the yield is typically between 70–80% with fresh water and 30–60% with brackish and sea water.

Equipment and system principle

A reverse osmosis unit consists of high-pressure pump(s) and housings/vessels with membranes inside, valves, sensors and piping. (Note that reverse osmosis unit normally requires pre-treatment of the water.) With reverse osmosis, the membranes are almost without exception spiral-wound. The process is usually controlled with a separate control system (programmable logic controller) which allows for a customised user interface, monitoring features and integrations with other systems.

Required information for system design

To complete a water treatment system design, a raw water analysis is needed together with information about the required water flow/capacity of the system and the required water quality after treatment/use of the water produced. If the equipment is being installed in an existing space, information about the available drainage and room dimensions is also good to know.