Reverse Osmosis Services
A reverse osmosis unit uses membranes with extremely small pores to separate ions and other particles as small as 0·1 nm from solution. Osmosis is the process by which water molecules will move through a selectively-permeable membrane into a salt solution of higher concentration.
Reverse osmosis is the application of high pressure to the more highly concentrated solution so that the process is reversed. Water or permeate is forced out of the concentrated solution and the ions and solids are concentrated in a residual solution. In a reverse osmosis unit, the solution to be treated is pumped at high pressure through a selectively-permeable membrane made from a polymer or composite and is arranged in layers or tubes. Spiral-wound designs are the most compact. and robust, but flat sheet plate and frame units may be applicable in wastewater treatment as they are more easily cleaned and sterilised. Some systems use a double membrane to improve the separation efficiency. Pretreatment, such as screening, filtration or softening, may be required before the liquid passes through the reverse osmosis unit. In certain cases, the liquid stream may have to be heated to prevent relatively insoluble ions precipitating on the membranes.
Reverse osmosis is used for desalination to provide drinking water on ships, on oil rigs and in arid areas. It is also used to provide ultrapure water for medical or industrial markets, for example in renal dialysis and silicon chip manufacture. In ultrapure water 90-99 % of the ions, colloids and viruses may have been removed. Reverse osmosis is used to remove salts and toxic molecules from landfill leachate to allow its safe discharge to watercourse and to separate metal ions from waste streams to enable the metal to be recovered electrochemically. Reverse osmosis also has great potential for recycling water as part of a waste minimisation programme.