The process of filtering saltwater to produce freshwater is called desalination. You can filter the saltwater with reverse osmosis and thermal distillation.
Reverse osmosis uses a membrane to separate the salt from water. It is the best and most popular desalination method. The thermal distillation method removes salt from the water with heat.
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 understand 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 is best? A couple of factors can point out which of the desalination method is most suitable. They include:
The Volume of Energy Needed for the Process
There are household thermal distillation applications that provide fresh 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.
This also determines the preferred desalination process. 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.
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.
When 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 remove 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 Makes 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 desalinated water.
Without this water undergoing pre-filtration and post-filtration, the raw fresh drinking 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.
Purify Salt Water Using Thermal Distillation
This process involves using heat sources (direct or indirect) to evaporate and then condense sea water. The resultant condensation of fresh water 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
Purify salt water by using a cooker, a cooking pot (preferably metal), and a glass bowl.
We have outlined the steps to this process below:
- Switch on your cooker (electric or gas stove).
- Place your empty metal cooking pot on the cooker and your glass bowl in its middle. Ensure that the bowl is short enough to prevent the pot lid from closing correctly.
- Gradually fill the pot with sea water and keep at it till it is ¾ full. Do not fill it to the brim or till it starts flowing into the bowl. You should stop pouring before the saltwater reaches the top of the glass.
- Once the water has reached its desired level, turn the lid upside down and seal the pot. Ensure that the lid is directly facing the bowl so that when the vapor condenses, it drops directly into the bowl.
- Add ice cubes on the top of the lid to speed up the process.
- Ensure your lid is not faulty. Otherwise, a lot of vapor will evaporate without condensing back into freshwater.
- To protect the pot 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.
- Using high heat may cause the water to boil in a volatile manner causing salt water to spill into the bowl, which will require you to start the process all over again and most likely with a fresh bowl.
- 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 bowl.
- After you have completed vapourization and condensing of the steam, gently take the bowl out of the pot. Note that the bowl 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 water.
- Leave the bowl to cool off. Now, you can use the distilled water as drinking water or for any other purpose.
Distillation by Solar Energy
Unlike boiling, solar desalination uses the natural heat from the sun. Therefore, it takes a lot of hours to filter the salt from the water and works best under direct sunlight.
Let’s see the steps to this process below:
- 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.
- Ensure you use a container that isn’t leaking as that will jeopardize the collection of vapor and condensation into freshwater.
- 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.
- To guarantee stability, wedge the cup between some sand to keep it still. You can also weigh the smaller cup with something heavy to prevent it from moving around in the larger container.
- 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 plastic wrap or seal to ensure there’s no tear on it. A torn wrap means the water vapor will go into the air instead of condensing on the wrap.
- 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.
- 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 water vapor condensed on the wrap to slide down to the center and drop into the smaller bowl.
- 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.
- 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.