Introduction to desalination systems
Understand how a desalination system works and what part pumps play in the process
Almost one-fifth of the world’s population lives in areas where freshwater is a physical scarcity. But for those living in coastal areas, the ocean offers a sustainable alternative.
With a desalination facility, seawater can be converted into potable water and delivered directly to the taps. In this module, we’ll have a closer look at how desalination works and what part pumps play in the process - from Intake to distribution.
The first step in desalination is collecting seawater for processing. There are many ways to design an intake system – and which method to choose depends on a range of factors, including the water demand and the quality of the seawater.
For smaller plants, it will often be a good idea to use beach wells fitted with submersible pumps. This will allow the sand to work as a natural filter, eliminating the need for pre-filtration in the plant.
For larger plants, or plants located near polluted coastal water, pump houses with dry-installed pumps and pipelines are often the better choice. Not only will a pipeline allow a larger intake of water, it will also enable you to collect water from beyond the polluted area.
Next step in the process is chemical pre-treatment. This can be done by adding chlorine to remove bacteria or flocculent to separate larger particles from the water, if necessary. This process relies heavily on precise dosing pumps to ensure that the exact amount of chemicals is added.
Once pre-treated, the water undergoes an initial filtration. Again, the choice of method depends on the water quality. The most common way is to pump the seawater through multi-media filters with layers of sand, pebbles or gravel to remove twigs, seaweed and other solid particles, before filtering the water in fine meshed cartridge filters. However, these filtration methods can also be replaced with ultra-filtration.
In this initial filtration process, feed pumps or end-suction pumps are essential to maintain the right pressure and ensure proper filtration. The pumps are also responsible for delivering sufficient inlet pressure to the high-pressure pump, which drives the reverse osmosis process.
Reverse osmosis is in many ways the heart of desalination. Here, high-pressure pumps force the seawater through semipermeable membranes to separate the water molecules from the larger salt molecules. The membranes allow a permeate of water to pass through the system, while expelling a salt concentration called brine.
The brine leaves the system at very high pressure. This energy can be recovered and reused in the reverse osmosis process by means of turbines or pressure exchangers. In fact, energy savings of up to 60% can be achieved by harnessing this energy.
The reverse osmosis membranes filter away everything but water molecules. Therefore, it’s necessary to regulate the PH value and add essential minerals afterwards, for instance by letting the water pass through lime stone filters. Again, precise dosing systems and dosing pumps are essential in ensuring that the end product is perfectly safe to drink.
The water undergoes a final disinfection before being transported to buffer tanks for distribution. What was once raw seawater is now clean and safe potable water.
As this brief introduction shows, there is no one-size-fits-all solution to desalination. A rule of thumb, however, is to always base the design on thorough water quality analysis. This will provide the information you need to choose the right system setup as well as the right individual components, such as pumps, membranes and filters.
Choosing the right pumps and systems will not only increase reliability and the quality of the water, it will also reduce operating costs, making desalination an economically viable solution.