Reverse osmosis (RO) allows the finest form of filtration currently known. RO is capable of rejecting particles from approximately 0.0001 to 0.001 microns in size. Removal includes bacteria, salts, metal ions, minerals, sugars, proteins, particles, dyes, and other organic constituents that have a molecular weight of greater than 150-250 daltons.

Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified to pass through it under pressure, while rejecting the contaminants that remain. Most reverse osmosis technology uses a process known as crossflow to allow the membrane to continually clean itself by sweeping away the rejected particles.

The separation of ions with reverse osmosis or `hyper filtration', as it is sometimes known, is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected.

Reverse osmosis is commonly used to purify fluids such as drinking water, ethanol and glycol. RO will typically remove salts, chemical contaminants, and heavy metals (such as lead) in order to improve the colour, taste or properties of the fluid.