What is the expected differential pressure (ΔP) in the first and second stages of a reverse osmosis (RO) system if the feed pressure is 8kg/cm2 (113.8 psi)? What range of differential pressure is acceptable for these stages, and how do changes in differential pressure impact system performance?

Expected Differential Pressure (ΔP) in RO Systems

There is no correlation between feed pressure and differential pressure. Therefore, the expected differential pressure cannot be derived from the feed pressure.

Differential pressure in RO systems is determined by factors such as:

• Flowrate
• Feedwater temperature
• Membrane element feed spacer height

There is no standard “suitable” differential pressure value. The expected differential pressure is calculated based on specific plant design and operating conditions. These values are displayed in the membrane projection reports created by engineers during the design phase of your RO system.

 

Factors Affecting Differential Pressure (ΔP)

Differential pressure will increase with:

• Increased flowrate
• Decreased feedwater temperature
• Narrowing of the membrane feed channels due to scaling or fouling.

Differential pressure will decrease with:

• Decreased flowrate
• Increased feedwater temperature

For example, if you reduce your % recovery, the concentrate flowrate will increase, resulting in an increased differential pressure across all stages. If you maintain the same overall % recovery but throttle first stage permeate production, the average feed/concentrate flowrates will increase for both stages. However, this will result in a significantly higher differential pressure in the second stage due to the lesser number of pressure vessels in that stage. When such a shift occurs due to first stage fouling, the resulting differential pressure increase is often misinterpreted as second stage scaling due to the higher increase in differential pressure in the second stage.

 

Diagnosing Differential Pressure (ΔP) Changes

Normalize your operating data to correct for changes in flowrate and temperature. This will allow you to determine if a differential pressure increase is due to membrane fouling or scaling.

• First stage differential pressure increase often indicates biological fouling or suspended solids loading.
• Second stage differential pressure increase is commonly suggestive of scaling.

However, that isn’t always the case. To confirm the cause, a definitive diagnosis requires performing a membrane autopsy.

 

Impacts of Differential Pressure (ΔP) Changes

 A dramatic increase in your differential pressure can cause:

• Membrane telescoping
• Membrane element feed spacer protrusion

Both issues can cause irreversible damage to the active membrane surface, resulting in decreased salt rejection and compromised system performance.