How Energy Recovery Devices Improve RO Efficiency — From Turbines to Pressure Exchangers| Insights by AQUALITEK

Introduction

Seawater Reverse Osmosis (SWRO) systems are energy-intensive — with high-pressure pumps consuming up to 40–50% of total plant power.
To reduce energy costs, Energy Recovery Devices (ERDs) were developed to recover and reuse the hydraulic energy contained in the high-pressure brine stream discharged from RO membranes.

Today, ERDs are essential in every modern desalination plant, enabling specific energy consumption (SEC) as low as 2.5–3.0 kWh/m³ of freshwater — a major improvement over early systems requiring 6–8 kWh/m³.

The Principle of Energy Recovery in RO Systems

In RO desalination, feed seawater (typically at 55–70 bar) is separated into:

•Permeate (freshwater)

•Concentrate (brine) — still under nearly the same pressure

Instead of wasting this pressurized brine, ERDs capture its hydraulic energy and transfer it either:

•Back to the feed stream, or

•To assist the high-pressure pump

This process dramatically reduces the net energy input required for RO operation.

Evolution of ERD Technology

(1) Early Stage: Hydraulic Turbine (Pelton Wheel Type)

In the 1980s–1990s, RO plants used hydraulic turbines (e.g., Pelton wheels) to recover energy from brine flow.

Working Principle:
The high-pressure brine jet spins a turbine wheel connected to a booster pump shaft, helping pre-pressurize the incoming feedwater.

Advantages:

•Simple structure

•Proven mechanical reliability

Limitations:

•Energy transfer through a rotating shaft → mechanical losses

•Efficiency typically 75–80%

•Poor performance at variable flow rates

(2) Mid Stage: Turbocharger (Centrifugal Energy Recovery Pump)

Turbochargers (also called centrifugal energy recovery devices) combine a centrifugal pump and turbine on a shared shaft.

Principle:
The brine drives the turbine impeller, transferring energy directly to the pump impeller that pressurizes part of the incoming seawater.

Advantages:

•Compact and easy to integrate

•Fewer moving parts than Pelton wheels

Efficiency:
Typically 80–85%, depending on flow ratio and pump design.

Drawbacks:

•Still mechanical coupling → friction losses

•Limited energy recovery at low recovery ratios

(3) Modern Stage: Isobaric Pressure Exchanger (PX Type)

The isobaric pressure exchanger revolutionized energy recovery in desalination.
Invented in the late 1990s, it uses direct pressure transfer between brine and feedwater without mechanical conversion.

Core Principle:
A rotor with multiple cylindrical ducts alternately connects high-pressure brine and low-pressure feedwater.
As the rotor spins, the pressure from the brine is directly transferred to the feed stream in an isobaric (equal pressure) manner.

Advantages:

•No turbine or shaft losses

•Continuous, balanced pressure exchange

•Extremely high efficiency

•Compact and low-maintenance

Typical Efficiency:

•Hydraulic efficiency: 96–98%

•System-level energy savings: up to 60%

Leading Technologies:

•ERD PX® (Energy Recovery Inc.) — dominates seawater desalination markets

•DWEER (Dual Work Exchanger Energy Recovery) — by Flowserve

•iSave — by Danfoss

Comparison of ERD Technologies

These figures highlight how modern ERDs have made seawater desalination both economically and environmentally sustainable, transforming it from an energy-intensive process into a mainstream water supply technology.

Future Trends

•Smart ERD Controls: Integrating IoT sensors for real-time efficiency optimization

•Hybrid Energy Systems: Combining ERDs with variable-frequency drive (VFD) high-pressure pumps

•Compact PX Modules: Designed for modular containerized desalination plants

•Brine Concentration ERDs: Improving recovery in high-salinity waste RO systems

Conclusion

From Pelton turbines to pressure exchangers, the evolution of Energy Recovery Devices has been central to the advancement of reverse osmosis desalination.
Modern isobaric ERDs achieve over 98% hydraulic efficiency, drastically reducing energy consumption and operational costs.

As global demand for desalinated water grows, ERD innovation will remain the cornerstone of sustainable seawater desalination.