Seawater Desalination - Water Purification

In this expert guide the water purification specialists at Water Treatment Services take a look at the use of seawater desalination processes, how they can be used to counter water scarcity by creating drinking water, what’s involved and some of the wider issues that need to be considered before investing in a processing plant.

The guide looks at the technologies involved including distillation, reverse osmosis, the use of RO membranes, forward osmosis and low temperature techniques. It goes on to review the practical stages involved in the desalination process, the costs involved and the potential environmental impact.

What is seawater desalination?

Wouldn’t it be great if the world’s drinking water problems could be solved by simply removing the salt from seawater to make it drinkable?

Well, the good news is that modern technological advances mean that the desalination of seawater is rapidly becoming a viable option.

Although in Northern Europe, and especially in the UK, our rainfall levels mean that we rarely have issues with water shortages, this isn’t the case across much of the developing world, parts of Southern Europe and the Middle East.

Many of the world’s driest regions are already experiencing significant scarcity of fresh water, with limited surface rivers, lakes and groundwater reserves available.

How removing salts can make seawater drinkable

There is one main hurdle to overcome in turning seawater into water that is suitable for drinking – removing the dissolved salts.

This can be done on a small, micro-level by simply boiling the seawater, capturing the steam and then condensing the water back into its liquid state, minus the salts.

However, on a larger scale this process can be very expensive, very energy-hungry and needs a lot of space to run the treatment plant.

Desalination is not a new idea

This idea of removing salt from seawater to make it suitable for drinking is nothing new.

Over 2,000 years ago, man was employing simple seawater desalination techniques using the evaporation and condensation method to provide fresh water on board ships.

Modern desalination plants

Desalination in the modern sense is often controversial.

The environmental impact of setting up and running a desalination plant can be considerable, and the benefits have to be weighed against the damage in constructing the plant and the ongoing costs in terms of energy and pollution.

A further obstacle in the desalination process is that saline water doesn’t always contain salt dissolved at the same rates.

When looking at levels of salinity, scientists express salt concentration in parts per million, or ppm.

Fresh water has dissolved salts at less than 1,000 ppm, and water defined as saline is anything between 1,000 ppm and 35,000 ppm.

Water drawn from the ocean has typically 35,000 ppm of salts.

Current desalination technology

Most desalination plants use a modernised process of distillation which rapidly evaporates the water, leaving the salts behind as a solid waste.

This process however is rapidly being left behind by the modern idea of reverse osmosis desalination.

The reverse osmosis (RO) technique involves pumping sea water through a series of special RO membranes which will also remove the salts dissolved in the seawater.

Reverse osmosis also requires a lot of power to force the water through these membranes.

However, scientists around the world are currently working on refinements to both current methods, such as ways of distilling water at a lower temperature, and forward rather than reverse osmosis techniques.

What is forward osmosis?

Forward osmosis combines special membranes to filter the water and thermal purification.

Thermal purification uses a solution of chemicals like ammonia and carbon dioxide which draws the water away from the salts.

The water still requires to be heated into a gas and condensed, but studies at Yale University have shown that this way of treating seawater is much more energy efficient than the alternatives.

What is low temperature desalination?

As the name suggests, low temperature desalination involves getting water to evaporate at lower temperatures, using less energy.

The water is pumped through a system of coils, which condenses it back into pure water.

The main benefit of this low temperature technique is that it can be operated using solar power or other types of renewable energy, hugely reducing the impact of desalination on the wider environment.

What are the main steps in the seawater desalination process?

Seawater desalination plants around the world will vary in exactly how they treat salt water, depending on their circumstances.

There are however some basic components which will be found in every traditional plant.

Seawater Intake System
The first part of the process is to extract the water out of the ocean and into the desalination plant so it can be processed.

A well might be drilled on a beach, or the water may be drawn in from deeper in the ocean.

The correct method will depend on the availability of land for the desalination plant, the local geology and weather conditions in each location.
Pre-treatment
Before the seawater is purified to make it suitable for drinking, it will be pre-treated to filter out larger particles.

Doing this stops any reverse osmosis membranes becoming damaged or clogged up prematurely.

The pre-treatment process involves water flowing through a range of filter layers which are typically filled with sand or carbon.

These filters effectively remove any microorganisms, or suspended solids from the water.
Reverse osmosis
Reverse osmosis involves using special semi-permeable RO membranes which filter out salts and other unwanted particles such as bacteria from the seawater.

A typical reverse osmosis filter process can remove 98% of the salts dissolved in sea water, bringing it down to a level suitable for drinking.
Chemical dosing
Water treated through a desalination process will still have to be treated chemically to ensure that it is safe for drinking.

The combination of chemicals used and the strength of the chemicals will be determined after laboratory analysis of the water quality.
RO membrane cleaning
The whole point of the RO membranes in a reverse osmosis system is to filter out anything which we don’t want in the purified water.

Over time, and even with careful pre-treatment to remove larger particles, membranes can become fouled with solid particles, biological material and clogged with salts.

Fouled membranes do not work efficiently, so all desalination plants will have a system of maintaining, cleaning and replacing the membranes when their performance begins to deteriorate.
Monitoring
No two intakes of seawater will be the same, as water composition can change according to the weather or environmental conditions.

All desalination plants should have a robust system in place to monitor how efficiently the process is working at every stage.

There should also be strategies for identifying issues quickly, and then ways of addressing these problems to keep the plant running.

How much does desalination cost?

The desalination of seawater isn’t always the answer to problems with fresh water shortages.

In some cases, it might be cheaper to load up tankers with fresh water from a reservoir and drive them to the shortage area when compared with setting up and running a desalination plant.

Factors such as the distance from the sea to the affected community, the cost of the fuel for running a desalination plant and the cost of construction, ongoing management and maintenance for the plant and equipment needs to be considered alongside some of the more traditional alternatives.

How does desalination impact the environment?

When it comes to environmental impact of desalination, the implications go wider than the energy costs and disruption of building the plant.

Extracting water directly from the sea could have a detrimental impact on the local environment.

It could cause damage to fish stocks, plankton and other tiny organisms, it could also have a devastating impact on sea creatures and plants living near the intake valves.

To minimise these shore and shallow water issues some seawater desalination plants choose to draw water from deeper into the ocean, channeling the extracted water into the plant using a series of pipes.

However, this strategy might be less environmentally damaging, but has significant cost implications.

Specialist water purification and desalination solutions

Water Treatment Services offer a comprehensive range of water purification solutions that includes the design, installation and management of reverse osmosis systems.

Our experts can provide advice and full support to help you identify the most appropriate strategies for the treatment and purification of water to meet your needs.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained water purification engineers and reverse osmosis specialists we offer cost effective environmental support solutions across the whole of the UK and internationally.