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Reverse Osmosis FAQ

Many reverse osmosis concepts are complex and often misunderstood. Let us walk you through the main tips and tricks of this desalination technology

1. how does the reverse osmosis process work

Reverse Osmosis (RO) is a pressure-driven membrane separation technique used to separate dissolved solids from solutions. Semipermeable membranes allow water and some dissolved solids to pass through, retaining the majority of dissolved solids.

Rejection is defined as the degree to which dissolved solids are retained by the membrane. It is expressed in terms of percent of feed concentration and typical values for RO membranes are 98% or 99%. That means that 98% to 99% of dissolved solids are retained by the membrane.

The higher the concentration of dissolved solids, the greater the osmotic pressure of the solution and the driving pressure required to force the water flow inside the membrane in the reverse osmotic direction. As a consequence, typical working pressures for low salinity waters are 10-20 Bar, while typical working pressures for seawater RO are > 50 Bar.

2. Why is the electrical conductivity of water used to measure its salinity?

Water salinity comes as a combination of concentrations of all the dissolved ions present in water. It is also called TDS (Total Dissolved Solids).

Measuring salinity or TDS involves collection and evaporation of a discrete water sample at 105°C followed by weighing of the solids remaining after evaporation. At the same time salinity is a critical value that requires continuos monitoring for many water treatment process and in particular for reverse osmosis. 

Therefore, in practice, TDS concentration is often monitored continuously by measuring the electrical conductivity (EC) which reflects the water’s ability to conduct electricity. Conductivity is expressed in microsiemens per centimetre (μS/cm) or decisiemens per metre (dS/m).

The ratio between TDS and EC in water depends on the type of dissolved ions and the temperature and usually varies between 0.67 and 0.70.

3. What is the difference between seawater and brackish water from an RO perspective?

Every 100 mg/L of dissolved salts represents 0.1 bar of osmotic pressure. Accordingly, sea water with 35,000 mg/L of dissolved salts will create 35 bar of osmotic pressure while a well water source with 6,000 mg/L of dissolved salts will create only 6 bar.

4. Can reverse osmosis remove dissolved gases from water?

No, reverse osmosis cannot remove dissolved gases in water.  In fact that is the reason why reverse osmosis permeate is sometimes acidic, as dissolved CO2 in the feed water permeates as product and dissociates into carbonic acid. 

5. What are typical salinity levels for brackish water and seawater?

Typical salinity levels for water found in oceans, lakes and rivers are:

  • 500 mg/L is considered the top salinity limit for potable (fresh) water.
  • Rainwater has a typical salinity of 20 mg/L
  • Seawater has an average TDS concentration of 35,000 mg/L, with an actual range from 33,000 to 36,000 mg/L at various locations and depths.
  • Groundwater (lakes, rivers and underground) presents a high TDS variability. Being a consequence of the terrain’s nature and structure typical TDS values range from 200 mg/L to 15,000 mg/L. 


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