In this module, we’ll have a closer look at how reverse osmosis works and how you can optimise the process to create energy savings in desalination systems. But first a quick definition:

Osmosis is a natural process in which a liquid passes through a semi-permeable barrier until equilibrium has been reached. This could be freshwater seeping into seawater until the amount of water molecules is the same on both sides of the barrier.

Reverse osmosis is a desalination technology that reverses the process.

By means of high-pressure pumps, seawater is forced through a membrane to separate water molecules from minerals, bacteria and other impurities. The result is perfectly clean and potable water.

In order to achieve reverse osmosis, the pump has to be powerful enough to overcome the osmotic pressure – a property that depends on the composition of the water and describes the amount of pressure needed to prevent osmosis. This takes a lot of energy and has traditionally made reverse osmosis an expensive process.

To make desalination an economically viable solution to the world’s water scarcity challenges, it’s necessary to optimise the reverse osmosis system. The best way to do this is to harness the energy of the concentrated brine that is expelled from the membranes at high pressure and reuse that energy in the process.

Let’s have a look at two options:

The first method includes installing a turbine-driven pump to support the high-pressure pumps. The turbine-pump is powered solely by the brine and consumes no additional energy. It will, however, enable you to reduce the power input to the main pumps significantly and reduce energy costs by up to 33%.

In the pressure exchanger, the high-pressure brine drives a rotor, which subsequently increases the pressure of the input seawater. Because the pressure exchange is hydraulic instead of mechanic, as is the case with the turbines, the pressure loss is only 2%, leaving 98% of pure energy to support the reverse osmosis process.

This takes quite a load of the high-pressure pumps. In fact, in a system with a 40% water recovery rate, the pressure exchanger will deliver 60% of the energy needed, drastically reducing operating costs. Depending on the place of operation, the payback time for such a system is approx. 6 months.

So let’s recap:

Seawater reverse osmosis has traditionally been an expensive way of converting seawater to potable water, because the process relies on high-pressure pumps to overcome the osmotic pressure. A large amount of energy is consumed during this process.

The hydraulic energy in the highly pressurised reject brine can be re-used with the help of energy recovery devices, such as turbine-driven pumps or pressure exchangers. This will reduce energy consumption significantly.

A pressure exchanger is by far the most efficient method. It transfers 98% of the energy from the high-pressure brine to the input seawater, recovering as much as 60% of the energy needed to drive the process.