Selecting High-Pressure Pumps in RO Design Based on Feed Water TDS

Introduction

Choosing the high-pressure (HP) pump in reverse osmosis (RO) design based on the total dissolved solids (TDS) of the feed water is a critical aspect of the system design. 

The TDS affects the osmotic pressure, which in turn influences the required operating pressure of the pump.

Key Considerations:

• Osmotic Pressure and Feed Water TDS
• Operating Pressure 
• Pump Selection Based on TDS
• Flow and Recovery Rate
• Pump Efficiency and Energy Recovery
• Material Considerations

1. The osmotic pressure (π) 

increases with higher TDS. It is calculated using this formula:

π = 0.0831 × T × TDS

Whereas:

T = The temperature in Kelvin 

TDS = Total Dissolved Solids in mg/L.

0.0831 = derived from the universal gas constant

2. Operating Pressure:

The HP pump must overcome: 

• the osmotic pressure 
• The pressure drops across the membranes 
• The pressure drops across the piping

Brackish Water RO: Typical pressures range from 100-300 psi (6.9-20.7 bar) depending on TDS. 

Seawater RO: Typical pressures range from 800-1,200 psi (55-83 bar) due to higher osmotic pressure.

3. Pump Selection Based on TDS 

Brackish Water RO (TDS <10,000 mg/L):

– Pumps with moderate pressure capabilities (100–300 psi) are suitable.

– Energy efficiency and flow rate are key parameters.

Seawater RO (TDS >30,000 mg/L):

– Pumps with high-pressure capabilities (up to 1,200 psi) are needed.

4. Flow and Recovery Rate, Pump Efficiency, and Energy Recovery 

The required pump flow rate depends on: 

1- system recovery rate 

2- feed flow. 

• Higher TDS feed water usually implies a lower recovery rate to prevent scaling and fouling.
• HP pumps must be capable of handling the required feed flow at the system’s design recovery. 

5. Material Considerations 

Pumps handling high-TDS water require corrosion-resistant materials like stainless steel (316L or duplex stainless steel) to withstand the corrosive environment.

Summary of Guidelines