The process of filtering saltwater to produce freshwater is called desalination. You can filter the saltwater with reverse osmosis and thermal distillation. Reverse osmosis (using a membrane to separate the salt from the water) is the best and most popular desalination method. Another excellent method of desalination is thermal distillation (using heat sources to separate the salt from the water).

These are the two main methods of desalination. Depending on the amount of water you want to separate from the salt, reverse osmosis can be the more cost-effective of the two because thermal distillation requires more power.

Let’s get to know more about these methods of desalination, starting with the factors that determine the most suitable desalination method.

Factors Determining the Most Suitable Desalination Method

How do I know which of the methods to adopt? A couple of factors can point out the desalination method most suitable for you. They include:

The Volume of Energy Needed for the Process

There are household thermal distillation applications that provide drinking water. On the other side, reverse osmosis uses lesser power and works well on an industrial scale.

The Targeted Output of Fresh Water Production

The amount of water you want to produce should determine the process of filtration. For example, reverse osmosis produces more freshwater than thermal distillation.

Water Quality

This also determines the preferred desalination process. For example, do you use fresh water for industrial manufacturing or as drinking water? Reverse osmosis is widely regarded as the best method of desalinating seawater but it also produces a lot of wastewater.

Pre-Filtration

The necessity of pre-treating water with certain chemicals before the actual filtration also points out which desalination method is more suitable for you.

The Feed Water

The concentration of salt differs among various sources of seawater. Seawater has the highest concentration. Brackish water has a lower concentration. Specific desalination methods are more suitable to filter brackish water than seawater or ocean water.

Filter Salt Water With the Reverse Osmosis Desalination System

Just as osmosis is a natural process, reverse osmosis is also (only that the latter is in the opposite order). Reverse osmosis is simply the generation of force or pressure by a pump to push seawater through a partially permeable membrane. This means that not everything can go past this membrane.

For example, seawater flows through the membrane. While the water permeates through the membrane to the other side, the salt is trapped on the opposite side. The membrane’s pores are nano-sized (of the smallest sizes possible), which is designed to be bigger than water but smaller than salt. This is why it can permit water and trap salt simultaneously.

Aside from salt or sodium, the miniature size of the pores of the membrane also traps other agents of contamination present in seawater.

Being established and invented in the mid-twentieth century, it has become the foremost desalination method in the world today. The RO desalination plant provides more drinking water than other desalination plants.

What Are Other Contaminants Filtered Out During This Desalination Process?

The other contaminants include:

  • Salmonella (a bacteria)
  • Novo virus (a virus)
  • Giardia (protozoa)
  • Metal and mineral ions
  • Nitrates among others

Factors Influencing the Effectiveness of RO Desalination System

The following factors determine how effective this method is in filtering or desalinating seawater:

  • The external pressure used to push water towards the membrane must be greater than the pressure involved in achieving osmosis. Inadequate water pressure will result in a failed filtration process.
  • The state of the water (hard or soft) also plays a crucial role. Softened water is filtered more quickly than hard water.
  • The size of the membrane’s pores must be able to let water permeate but keep sodium and other contaminants at bay. A faulty membrane will make the filtration process ineffective.

What Does Make Reverse Osmosis Very Effective and Popular?

It requires low energy.

Modern RO systems use an energy recovery device which means just half of the original input power is used because the rest comes from the power recovery.

It incorporates another water filter.

Modern RO systems are integrated with activated carbon filters or coconut shell activated carbon filters, which help with pre-treatment and post-treatment of the water, making it odorless and tasteless. This further enhances the quality and wholesomeness of the desalinated water.

Without this water undergoing pre-filtration and post-filtration, the raw fresh water will have an unpleasant odor and taste. Pre-treatment also prevents seawater from corroding distribution pipes and storage tanks.

Its decontaminating Ability

RO systems filter out a host of contaminants in saltwater, significantly lessening the contamination level of the drinking water.

Filter Salt Water Using Thermal Distillation or Desalination

This process involves using heat sources (direct or indirect) to evaporate and then condense sea water. The resultant condensation of vapor back into liquid is usually free of salt. Therefore, it has household and industrial applications.

Types of Thermal Distillation or Desalination

Distillation by Boiling

Filter salt water by using a cooker, drinking container (preferably metal), and a metal container.

We have outlined the steps to this process below:

  1. Switch on your cooker (electric or gas stove).
  2. Place your empty metal cooking container on the cooker and your metal cup in its middle. Ensure that the cup is short enough, so it doesn’t hinder the lid of the container from closing correctly.
  3. Gradually fill the container with sea water and keep at it till the container is ¾ full. Do not fill it to the brim or till it starts flowing into the cup. You should stop pouring before the saltwater reaches the top of the glass.
  4. Once the water has reached its desired level, turn the container’s lid upside down and seal the container. Ensure that the lid is directly facing the cup so that when the vapor condenses, it drops directly into the cup.
  5. Ensure your lid is not faulty. Otherwise, a lot of vapor will evaporate without condensing back into freshwater.
  6. To protect the cup from breaking or melting while easily shifting from its position in the pot (if it’s a glass or plastic cup), use low heat to let the saltwater boil slowly.
  7. Using high heat may cause the water to boil in a volatile manner causing salt water to spill into the cup, which will require you to start the process all over again and most likely with a fresh cup.
  8. This process usually takes some minutes. Monitor the condensation process as the seawater vaporizes and condenses on the cover of the lid. The condensed water slowly trickles down to the lowest point, which is the point of the lid that drops the water into the cup.
  9. After you have completed vapourization and condensing of the steam, gently take the cup out of the pot. Note that the cup will be very hot, so you should use a hand covering to protect your hand from being scalded. Do not remove in a hurry so that the remaining salt water will not contaminate your distilled freshwater.
  10. Leave the cup to cool off. Now, you can use it as drinking water or for any other purpose.

Distillation by Solar Energy

Unlike boiling, this process uses the natural heat from the sun. Therefore, it takes a lot of hours to filter the salt from the water and works best when there is sufficient sunlight.

Let’s see the steps to this process below:

  1. Pour salt water into any suitable container and place outside, directly exposing it to the sun. Just like the boiling process, you shouldn’t fill the container to the brim. Leave some allowance to prevent it from spilling into where you’ll collect your freshwater.
  2. Ensure you use a container that isn’t leaking as that will jeopardize the collection of vapor and condensation into freshwater.
  3. Position a container or bowl of smaller size gently in the middle of the larger bowl. Make sure the mouth of this bowl is at a good distance from the salt water below.
  4. To guarantee stability, wedge the bowl between some sand to keep it still. You can also weigh the smaller bowl with something heavy to prevent it from moving around in the larger container.
  5. Use a watertight plastic wrapping that you haven’t used before to seal the mouth of the larger container. We recommend that you use a new wrap or seal to ensure there’s no tear on it. A torn wrap means the vapor will go into the air instead of condensing on the wrap.
  6. Ensure your wrap is produced by a quality brand, and the covering on the bowl shouldn’t be too tight or loose – just the right fit.
  7. Then gently position a rock or something heavy in the middle of the plastic wrap or covering. This causes that spot to sag a bit, giving the wrap a point of gravity that will enable vapor condensed on the wrap to slide down to the center and drop into the smaller bowl.
  8. Once again, ensure that the bowl is directly exposed to the sun and that the smaller bowl and rock weighing down the plastic wrap is well-positioned.
  9. Once the smaller bowl is done collecting the freshwater, you can gently take it out. It can serve as drinking water, and it’s free from salt.