How does reverse osmosis work?

Osmosis is the spontaneous passage of a liquid from a dilute to a more concentrated solution across a semi-permeable membrane. This allows the passage of water through the membrane but not the dissolved salts. Water flows from the less concentrated side of the membrane to the other. Flow continues until pressure in the concentrated side is high enough to prevent any further transfer of water. This pressure is the osmotic pressure. Applying pressure to water on the concentrated side causes one of three conditions:

  1. Water will still cross the membrane towards the more concentrated side if pressure is less than the osmotic pressure.
  2. If pressure is equal to the osmotic pressure, no net transfer of water will occur in either direction.
  3. If pressure is greater than the osmotic pressure, pure water passes from the concentrated side to the less concentrated side. This is reverse osmosis (RO).

Reverse osmosis is a method for purifying water. The process uses a membrane separation process. For each raw feed stream, two discharge streams develop. The product stream passes through or “permeates” the membrane. This stream is pure water is free of colloidal and microbiological (such as bacteria) species. The stream is also low in the dissolved matter.

The other stream is smaller and contains species that could not pass through the membrane. This stream is the reject stream. The reject stream species concentration is much higher than the feed stream.

What factors affect reverse osmosis performance?

Water flow through the membrane depends on the applied pressure. As water pressure increases, the flow rate through the membrane increases. However, the pressure cannot increase too high because the pressure will rupture the membranes. The amount of salt crossing the membrane depends entirely on the concentration of the dissolved salts in the feed water.

Problems with reverse osmosis units

One problem with reverse osmosis units is that the membranes can become fouled with various contaminants in the feedwater. These various contaminants known as foulants may be:

  1. Metal oxides, such as iron.
  2. Inorganic salts, such as calcium carbonate.
  3. Silica, normally as colloidal silica.
  4. Microorganisms which tend to grow in areas of low flow.
  5. Organic compounds.

All these foulants coat the membrane and act to prevent or reduce water flow through the membrane. Furthermore, chlorine also has a detrimental effect on the membranes in that it attacks the membrane structure. Therefore, the water pretreatment system prevents or reduces the chance of fouling or degradation of the membranes by removing specific contaminants from the water before the RO modules. It is, however, still possible for the membranes to become fouled. When this occurs, the membranes are using a chemical solution. The type of fouling and required cleaning will determine which chemical is used. This blog provides a good overview of RO cleaning recommendations.

For example, microorganisms and organic fouling require sodium hydroxide (caustic); metal oxide or hardness scale requires hydrochloric acid or citric acid. Silica removal requires citric acid mixed with ammonium bifluoride (NH4HF2).

To decide if the membranes should be cleaned weekly, bi-weekly, or monthly, several parameters should be monitored and evaluated periodically. For example, these parameters include system flows and conductivities, module rejects pressure, module product conductivity, module rejects conductivity, and module silica concentration.

FCS provides maintenance and training services on reverse osmosis and other plant-related processes.