How is Reverse Osmosis Used to Purify Seawater

Osmosis seems to be a big word, especially if you are not the kind who is not really into science. But basically, it is the naturally occurring process of the movement of water from an area of low salt concentration to one wherein there higher salt concentration. And guess what, you don’t have to go to a laboratory to find proof that this process is natural and commonplace because it happens right inside our bodies, such as when our kidneys absorb water from our blood. 

Now, what is reverse osmosis? Is it more complicated than plain osmosis? Well, not really. In layman’s terms, reverse osmosis is the process wherein saltwater is made to pass through a semipermeable membrane, which technically works as a filter so that only water molecules–which are small–can pass through and larger salt molecules are left behind. The resulting desalinated water can then be used for the manufacturing of electronics and drugs, agricultural irrigation, and even drinking. 

Sea Water RO Systems : Hitachi Aqua-Tech Engineering Pte Ltd
Image source: hitachi-aqt

What You Will Need

To purify seawater through reverse osmosis on a large scale, you must have an RO system with the following parts: feed water tank, feed pump, sand filter, activated carbon filter, precision filter, RO system, dosing system, intermediate tank and pump, booster pump, energy recovery system, and water producing tank.

Image source: pureitwaterpurifier

On the other hand, smaller reverse osmosis systems that can be used at home must have an RO module, feedwater valve, pre-filters, automatic shut-off valve, membranes, drain valve, post-filter, bladder storage tank, tubings, fittings, and a faucet. 

In this article, we are going to delve into each step in the process of purifying seawater through reverse osmosis on a larger scale. Do not worry, pretty much the same concept applies to reverse osmosis systems that you can buy in the market and install in your own house. 

Source: O.K. Buros, et. Al., The USAID Desalination Manual, Englewood, N.J., U.S.A., IDEA Publications.

Steps to Use Reverse Osmosis to Purify Seawater

1. Intake of Water

Seawater that will be purified–also called feedwater–will enter the system through a large channel. In some plants–especially smaller ones–saltwater that goes in ends up in a beach well first. The water in beach wells is pretreated through slow filtration and often has less solid particles, microbes, and other contaminants. 

2. Pretreatment

The first step in reverse osmosis for desalination is called pretreatment since this is the part where you prime or prepare the feedwater. The importance of this process is to ensure that the feedwater is going to be highly compatible with the membrane. The membrane has a spiral design, and so lack of pretreatment can lead to the accumulation of materials that cannot be easily removed from the membrane and reduce the efficiency of the system. The pretreatment process has four steps: 

The first is the elimination of solids. This removes particles and other solids that you don’t want in your feedwater. This reduces the risk of membrane fouling and damage to high-pressure components of the system. 

Next is cartridge filtration. The cartridge filter functions as an additional filter to protect the unit’s membranes. This removes smaller particles, those that are around 1–5 µm wide. 

The third is called dosing. In this step, oxidizing biocides are added to kill the microbes. Following this, another chemical that will cancel out the effects of the biocide will be added. Biofouling inhibitors, which prevent microbial growth in the system will also be added. 

The last step is pH adjustment. Here, the feedwater pH is adjusted to around 5.0 to 6.5 to reduce hydroxide and carbonate scaling. 

3. Pressurization

A pump is used to increase the pressure of the filtered feedwater such that it smoothly flows through the reverse osmosis system. Ideally, the pressure in an RO system, if saltwater is the feedwater, is about 800 to 1,800 psi. This requires quite a huge amount of energy. 

4. Separation

This is when the water and salt are separated. What happens is that water is allowed to pass through while salt particles are not. Two streams, namely a freshwater stream and a concentrated brine reject stream are made. The brine rejects stream refers to the wastewater that is produced during the desalination process. 

5. Stabilization

Stabilization of desalinated water is essential to protect pipelines and storage. In this step, a substance such as lime, which promotes corrosion control and disinfection, is added. It is also during the stabilization stage that remineralization may be done. This replenishes the minerals that the water lost throughout the intensive filtration process. This is particularly essential if the desalinated water is to be used in agricultural lands since it has been found that desalinated water that had not gone through remineralization could result in poor crops. 

6. Post-treatment

While the process of reverse osmosis has effectively removed many particles from saltwater, the resulting water still needs to be purified further such that it is safe for human consumption. This may be done by using UV rays or chlorine during the post-treatment stage. 

7. Distribution

Once the water has been desalinated and cleaned after the post-treatment stage, the purified water is then distributed to the consumers. 

Reverse osmosis is a pretty cool process. It removes contaminants in water, both organic and inorganic. Such systems use relatively low amounts of chemicals and are also low-maintenance. And yes, while it may sound surprising, did you know that this process isn’t new? It has been done by people even in ancient times through ancient techniques. This means that if ever you run out of the water and you live by the beach, you can just grab a pail of saltwater and do the process yourself to have some potable water! What’s even better is that these days, reverse osmosis can be done not just in plants, but in your own home, too, using a much smaller reverse osmosis system. 

It is expected that more uses of reverse osmosis will be discovered in the future, as this technology becomes cheaper, more energy-efficient, and more accessible. We’re so lucky that science and technology have indeed gone so far. 

Andrea Vaso

When Andrea is not writing, you’re most likely going to find her drinking coffee, reading a novel, or watching medical dramas. She holds an undergraduate degree in biology and as of this moment, she is attending medical school.