Question

Why did we not take osmotic pressure in account for microfiltration and ultrafiltration? Perform a representative calculation to prove your answer.

Answer 1

Solution

In microfiltration and ultrafiltration, the membrane can only retain very large particles, while the smaller ions can freely pass through pores and equilibrate on both sides of the membrane. As a result, larger particles are the only contributor to the molar concentration difference across the membrane, which will be very small due to large molecular weight of these particles. Since osmotic pressure is purely generated by molar concentration difference across membrane, it is likely negligible compared to the positive pressure applied in the case of MF and UF.

Representative calculation In seawater, the salt concentration is 3.5 wt%, which generates an osmotic pressure of 25.2 atm against pure water in RO applications. In UF applications, the typical molecular weight is 1 to 1000kDa. Assume a molecular weight of 100 kDa and the same solid concentration as seawater. The molar concentration in feed is,

     

Assuming negligible solid concentration on permeate side, the osmotic pressure at 

 

Typical transmembrane pressures for UF application are about 1 to 10 atm. The osmotic pressure generated in this example is less than 0.1% of the applied TMP. Therefore osmotic pressure can be safely ignored in UF and MF applications.