The Impact of Feed Water Salinity on Reverse Osmosis Systems

Feed water salinity, usually expressed as total dissolved solids (TDS), is the sum of the concentrations of all inorganic salts (such as sodium, calcium, magnesium, chloride, sulfate, etc.) in the water. It is one of the most critical parameters in the design, operation, and economic efficiency of a Reverse Osmosis system, and its impact is comprehensive and interconnected.

1. Direct Impact on Osmotic Pressure and Operating Pressure

The essence of the RO process is to overcome the natural osmotic pressure of water, forcing water molecules through a semi-permeable membrane, while retaining salts. According to the van der Hoff equation, the osmotic pressure of a solution is directly proportional to its molar concentration (directly related to TDS).

Osmotic pressure ≈ 0.8 bar / (1000 mg/L TDS). This is a simplified empirical formula, but very intuitive. If the feed water TDS is 1000 mg/L, its theoretical osmotic pressure is approximately 0.8 bar. If the feed water TDS increases to 5000 mg/L (such as brackish water), its theoretical osmotic pressure rises to approximately 4 bar. For seawater, the TDS is approximately 35,000 mg/L, with a theoretical osmotic pressure as high as approximately 28 bar.

To generate effective permeate driving force (net driving pressure), the operating pressure of the Ro System must be much higher than the osmotic pressure. Therefore, for every order of magnitude increase in feed water salinity, the required operating pressure increases almost linearly. The operating pressure for seawater desalination RO is typically 55-80 bar, while for ordinary brackish water RO it is only 10-25 bar. This implies higher power consumption and higher requirements for equipment and materials.

2. Impact on System Desalination Rate

The rejection rate of RO membranes for different ions is relatively stable. However, changes in feed water salinity affect the system's desalination performance on two levels.

Apparent Desalination Rate Decrease: The desalination rate is calculated as (1 - permeate salinity / feed water salinity) * 100%. Assuming a constant absolute salt permeability (the amount of salt passing through per unit time), when the feed water salinity increases sharply, even if the membrane's physical properties remain unchanged, the calculated "desalination rate" will decrease.

More importantly, high salinity means a greater salt concentration gradient across the membrane, which increases the driving force for salt diffusion from the membrane surface to the permeate side (exacerbating concentration polarization), thus leading to an increase in actual salt flux. At extremely high salinity, the membrane's desalination rate does indeed show a substantial decrease.

3. Impact on Permeate and Recovery

At a fixed operating pressure, net drive pressure = operating pressure - osmotic pressure. As mentioned above, high salinity leads to high osmotic pressure, directly reducing the net drive pressure.

Permeate flow is directly proportional to net drive pressure. If the operating pressure remains constant, an increase in feed TDS from 1000 mg/L to 5000 mg/L may cause a 3-4 bar decrease in net drive pressure, resulting in a 20%-30% or greater decrease in permeate flow.

When the system recovery rate (permeate/feed) increases, the salinity of the feed at the membrane element's end will be concentrated exponentially. To avoid osmotic pressure spikes and scaling risks due to excessively high TDS at the end of the feedwater system, high-salinity feedwater systems must employ more conservative recovery rates.

Ordinary municipal water RO recovery rates can reach 75%. High-brackish water RO recovery rates are typically designed at 60%-70%. Seawater RO recovery rates are further limited to 40%-50%. Low recovery rates mean more concentrate discharge and lower water utilization.

4. Increased Membrane Fouling and Scaling Tendency

High salinity itself does not directly cause fouling, but it creates an environment more prone to fouling and scaling.

Increased Concentration Polarization: When high-TDS water is concentrated at the membrane surface, the concentration of salts and other contaminants (such as silicates, sulfates, and organic matter) is much higher than in the bulk water flow. This significantly increases the risk of slightly soluble salts (such as CaSO₄, CaCO₃, BaSO₄) exceeding their solubility product and precipitating out, leading to scaling.

Colloidal stability degradation: High ionic strength compresses the electrical double layer of colloidal particles, disrupting their stability and making them more prone to depositing on the membrane surface, forming colloidal fouling.

Difficult cleaning: Once fouling occurs in a high-salt environment, the fouling layer is typically denser and more difficult to clean, shortening the membrane's lifespan.

5. Decisive impact on energy consumption and economy

The overall impact is ultimately reflected in operating costs. In reverse osmosis systems, electrical energy is primarily consumed by the feed pump. Operating pressure is generally proportional to energy consumption.

Desalinating 1000 mg/L brackish water to the same permeate volume may only consume 0.8-1.2 kWh/m³. However, desalinating 35000 mg/L seawater typically consumes 2.5-4.0 kWh/m³, 3-4 times more.

The high energy consumption is the main reason why seawater desalination costs significantly more than brackish water desalination. In addition, high-pressure operation places higher demands on the materials of pumps, pipelines, and diaphragm housings, which also increases the initial investment.