What does RO remove from water?
The RO membrane blocks the passage of contaminants in water with an effective diameter larger than approximately 0.1 nm. Typically, it removes over 90% of ion contaminants, most organic compounds, and effectively all particles. The ability to remove non-ionic contaminants (with molecular weight [MW] <100 Dalton) is lower, but increases for substances with higher MW. Theoretically, molecules with an MW >300 Dalton, including particles, colloids, microorganisms, and large bioactive molecules, will be completely removed. Dissolved gases are not removed.
How does RO work?
Normally, water molecules move from an area with a lower concentration of salts/minerals to an area with a higher concentration of salts/minerals until equilibrium is reached (natural osmosis). When sufficient pressure is applied to force water from an area with high salt/mineral content to pass through a special membrane to an area with little or no salt/mineral content, this process is known as reverse osmosis.
During reverse osmosis, feedwater is pumped into the RO membrane under pressure (usually 4-15 bar or 60-220 psi) in a cross-flow pattern. Typically, 15-30% of the feedwater permeates the membrane, and the remaining portion stays at the inlet and exits as a concentrate stream containing most of the salts, organic matter, and virtually all particles.
RO membranes are typically made from thin polyamide membranes, which are stable over a wide pH range. However, they can be damaged by oxidative agents like chlorine and fouled by organic matter or colloids. Therefore, pre-treatment of water is required to protect the membrane. The membranes need relatively high pressure to overcome reverse pressure. To maximize the surface area, membranes are often wound into a series of spiral layers as shown in the diagram. If the recovery rate of water is too high, increasing concentrations of hard-to-dissolve salts will cause membrane fouling.
Benefits of RO Membranes
The greatest advantage of RO is its ability to remove a large number of contaminants, resulting in pure water that is almost entirely composed of H2O molecules.
In addition to reducing the load on subsequent filtration stages, RO also protects other filtration devices from contaminants and prevents fouling from large organic molecules, such as humic acids and fulvic acids, while completely eliminating microorganisms and viruses in the feedwater.