How the desalination process works

By now you probably know that we construct solar-powered desalination systems to produce clean and healthy drinking water from dirty and salty coastal water. However, unless you have studied chemistry or engineering, it is likely that you are wondering how the desalination process actually works. In this article, you will find out the steps that the water goes through to be transformed from unsafe to safe and what exactly the purpose of each step is.

Intake pump
Cartridge filter

The preliminary step, before any water treatment can occur, is that the water is pumped from deep under the earth’s surface into the desalination system. Once in the desalination system, the water runs through a multi-media and a cartridge filter. A multi-media filter is a vessel in which there are three layers of different mediums of varying porosity. Porosity essentially refers to the size of the gaps in a medium, for example, the size of spaces in between sand grains, or the size of spaces in between the fibers of any fabric. In the multi-media filter, the water first runs through the medium with the largest porosity (so the largest spaces), such that the largest particles in the water are removed first. Then the water runs through a second medium, with a smaller porosity, such that smaller particles are removed and finally the water runs through the medium with the smallest porosity, such that even smaller particles are removed. The multi-media filter is powered by pressure, to increase the flow of water. Once the water comes out on the other side of the filter, larger particles have been removed from the water, such as dirt and sand. Removing these larger particles first ensures that later treatment steps are more effective.

In the second step, the water runs through a cartridge filter. A cartridge filter is composed of many long tubular filters, set inside a long cylindrical housing. Water is run into the housing and cannot leave the housing unless it flows into the filters, from the center of the filters, the water can run out of the housing. Cartridge filters remove sediments, metals, and some microorganisms from the water.

Reverse osmosis system

Next, the water runs through a reverse osmosis system. You have probably heard of osmosis in your high school chemistry or biology class, it refers to a process where a solvent (e.g. water) passes through a filter such that the concentration of a solute (e.g. salt) is equal on both sides of the filter. The reverse osmosis system turns this process around by applying pressure on the side of the filter with a higher solute concentration, such that the water flows to the other side of the filter where the solute concentration is lower. The solute in this case is all the undesired particles in the water, such as sediments, salt, bacteria, and even viruses. The reverse osmosis system is highly effective and removes smaller contaminants than all of the other steps.

Remineralisation, UV treatment and activated carbon filter

The filtration process is so effective that even minerals beneficial to human health have been removed from the water, therefore the water now needs to be remineralized. The minerals added to the water include calcium, magnesium and sulfate. Remineralizing the water also improves the waters taste.

Next, the water runs through a UV filter. A UV filter can be described as a tube with a lamp inside of it, where the lamp emits a “germicidal” wavelength. This means that a lamp emits light at such a wavelength, that it deactivates any living organisms contained in the water. This wavelength is at 254 nm and disrupts the DNA of living organisms, such that they are unable to reproduce.

As a final step, the water flows through an activated carbon filter. Activated carbon has an incredibly high surface area, where 1 g has a surface area of 3000 m2. This is of relevance because the filter works via adsorption: the pollutants essentially stick to the surface of the carbon and are therefore removed from the water. These filters remove substances like chlorine, sediments, and volatile organic compounds. After this final step, the water is distributed to the community.

What makes the filtration process we use at EWF special?

What sets the desalination systems we use at EWF apart from others are three factors. Firstly, the systems are powered solely by solar energy, a sustainable source of energy, and therefore do not contribute to fossil fuel emissions. Secondly, the desalination process usually requires immense amounts of energy such that sufficiently high pressure can be utilized in the reverse osmosis step. However, the systems we use require 3 to 4 times less energy than conventional reverse osmosis systems. Less energy-intensive systems are an advantage in locations where the energy supply may be limited or unreliable. Part of this energy is saved by recovering energy from the brine. In our case using less energy allows the system to be powered solely with solar energy. Thirdly, a common issue with the desalination process is the discharge of the brine, which can have negative impacts on the ocean. The brine refers to the water which was not cleaned and contains the contaminants which were filtered out of the cleaned water. Conventionally, the concentration of salt in the brine can be 150% – 200% of the source water. Further, toxic chemicals such as chlorine and copper are often added to the water as antiscalants and antifoulants, further polluting oceans. To avoid these unsustainable practices the technology we use has a relatively low recovery ratio and no toxic chemicals are utilized in the desalination process.

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