Reverse Osmosis - What is it and How Does it Work?

Reverse Osmosis is an advanced filtration technique used to purify water in a wide range of industries as well as for use at home. First demonstrated in the late 1950's, reverse osmosis technology has progressed considerably, resulting in the high-performance purification systems available today.

At the core of every reverse osmosis system is a semi-permeable membrane: a thin polymer barrier lined with pores of extremely small size - while a typical filter cartridge may be rated at 0.5 to 5 microns, the pores of a reverse osmosis membrane are on the order of 0.0005 microns. This allows the semi-permeable membrane to operate on the molecular level - while a water molecule is small enough to pass through the pores, larger molecules cannot.

A household reverse osmosis system does not require electricity. It operates solely on mains water pressure, which pushes water through the membrane. This process results in clean, pure water on one side of the membrane, while larger contaminants are retained on the other side. A minimum mains pressure of 40 psi is typically necessary for normal operation.

This exceptional purifying ability allows a reverse osmosis system to remove a huge range of contaminants and impurities in your tap water. This includes sediments, colloids, bacteria, cysts, viruses, dissolved ions, heavy metals, fluoride, chlorine, herbicides, and pesticides.

To ensure that the contaminants in the water do not build up on the membrane and block water flow, the membrane is continuously flushed with water during operation, and the water carrying the flushed contaminants is then discharged. For every litre of pure water produced, 2 to 3 litres is flushed out, which can either be drained, or collected and used for other purposes such as watering the garden. This self-cleaning ability gives the membrane cartridge an extensive service life, several times that of a normal filter cartridge.

The smaller, ultra-fine pores of a semi-permeable membrane results in a flow rate of between 3 and 10 litres per hour. While this may appear to be low, most reverse osmosis systems come with a pressurised storage tank, which provides on-demand access to plenty of water, even for a whole family.

Aside from the reverse osmosis stage, a complete system also features two or more additional stages. A sediment filter is placed before the membrane, reducing the load on it by removing larger particulates. A carbon filter is also placed after the membrane, as it can chemically bind to and trap smaller, volatile molecules such as chlorine compounds which the membrane may have missed.

In conclusion, Reverse Osmosis is considered to be by far the best filtration method in terms of purity, running cost, efficiency, practicality, and convenience, compared to other filtration methods, such as activated carbon, ion exchange, KDF, and distillation.