Reverse Osmosis Desalination Process

The core process of desalination is the Reverse Osmosis Process. It consists of a High pressure pump followed by an Energy Recovery device and the Reverse Osmosis Membranes

High pressure pump

Before entering the seawater reverse osmosis membranes, clarified seawater is pressurized by the High Pressure Pump typically between 55 and 85 bars, depending on the temperature and the salinity of the water.

Main Seas and Oceans Water composition

Calculate you required applied pressure

Energy Recovery Device (ERD)

The pressure drop over the RO membranes is about 1.5 to 2 bar, depending on the number of element per pressure vessel, so the concentrate is released at high pressure.

Thanks to Energy Recovery Devices, it is possible to reuse the energy from the concentrate flow. The concentrate is directed to the ERD, where it directly transfers its energy to part of the incoming feed water.

There are two main energy recovery concepts:

- Energy Recovery Turbine (ERT)

Courtesy of FedCo

- Pressure Exchanger (PX)

Courtesy of Grundfos

Example:

Seawater Flowrate: 100 m3/h
Applied Pressure: 75 bar
R.O. Recovery: 40%
Permeate Flowrate: 40 m3/h

Power required without energy recovery devices: 300 kW
Specific Energy: 300/40 = 7.5 kWh/m3

Power required with Energy Recovery Turbine: 177 kW
Specific Energy: 177/40 = 4.4 kWh/m3

Power required with Pressure Exchanger: 140 kW
Specific Energy: 140/40 = 3.5 kWh/m3

Desalination costs analysis

Spiral Wound Seawater Reverse Osmosis modules

The most common RO membrane used in desalination are spiral wound Thin Film Composite. They consist in a flat sheet sealed like an envelop and wound in a spiral.

Spiral wound Reverse Osmosis membrane

There are 3 typical membrane diameters : 2.5", 4" and 8". Seawater RO membranes have a maximum permeate flowrate ranging from 1.4 to 37.9 m3/d, therefore many membranes are often required to meet the plant permeate production requirements.

Seawater RO membranes in Thin Film Composite are manufactured by Dow Chemical (Filmtec), Hydranautics, Osmonics (Desal) and Toray.

The membranes are enclosed in series in pressure vessels and the number of membrane element per pressure vessels can vary from 1 to 8:

Pressure vessels are then arranged in parallel to satisfy the membranes flow and pressure specifications as well as the plant production requirements:

The total number of membranes and pressure vessels required and their arrangement (the array) depends on permeate flow and applied pressure, i.e., salinity and temperature.

Reverse Osmosis Pressure vessels Rack

LENNTECH engineers design and size the most cost-efficient Seawater Reverse Osmosis Process based on your water specifications.

Desalination Key Issues

Seawater Intake

Desalination Pretreatments

Desalination Post-treatments

Desalination Polishing steps

Reverse Osmosis Desalination Process

Desalination and Energy Costs

Brine Disposal

Membranes cleaning (CIP)

Storage and Distribution

Containerization

Instrumentation and Control (SCADA)