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Treating seawater into domestic water is highly efficient, time-saving and cost-effective with RO filtration technology according to the most standard process. Let’s refer the following article now!

In the face of the current situation of freshwater and clean water sources, which are becoming increasingly scarce today, besides treating river water into domestic water, treating seawater into domestic water is also gradually becoming popular. So, how to treat seawater so that it can be used in daily life? Check out the article below for more information!

(In this article, the main stages are listed for easy reference step by step because the topic of seawater treatment is very wide).

1. Overview of seawater treatment into domestic water

– About two-thirds of the planet’s surface is covered with water, but only 2.5% of this water is fresh water and 0.3% is suitable for human consumption.

– Salinity is the total mass of dissolved salts (TDS or parts per thousand – ‰ refer to www.coastalwiki.org/wiki/Salinity).

– According to the actual salinity scale, typical “standard” seawater has a salinity of 35‰ or 3.5%, in many places it may be higher or lower. In Vietnam, the salinity is lower, about 25-35‰ or 2.5-3.5% because the seas connect with many freshwater rivers and there is a big seasonal variation.

proportion-of-water-source-on-earth

Proportion of water source on Earth

sea-water-standard-containing-35-percent-or-35-percent-salts-tds-and-other-major-ingredients

Sea water (standard) containing 3.5% or 35‰ salts (TDS) and other major ingredients

Currently, desalination accounts for about 1% of the world’s drinking water.

– The Middle East region has seawater purification plants accounting for about 50% of global filtration capacity. Kuwait is the country with the highest percentage of desalination, and Arabia has the world’s largest Al Khafji solar-powered seawater purification plant.

– Large RO water purification projects, the cost range from 0.4 – 0.6 USD/m3 because using solar energy on site.

– The higher salinity, the higher energy cost.

– Desalinated water lacks minerals, especially iodine, which is essential for health.

– Desalination systems can adversely affect the marine environment such as containing chemicals from anti-scalent for RO membranes, CIP cleaning work discharged directly into seawater.

– Affecting small species, fish eggs, etc., so the ditch collecting sea water in to desalination needs the lowest speed (0.1m/s) for fish to escape, naturally gravity filtering with sand with its backwash to allow the fish eggs and small organisms returning to ocean.

– The discharge from RO system has high salinity (2-3 times of original) so it should be diluted with seawater to return to sea and seabed so as not harmful to benthic organisms

– Pipelines from the sea to the ground are less expensive, but can be corroded or degraded, contaminating nearby aquifers. Therefore, an amount of tree cover is needed to limit metal ions from entering the aquifer

2. Processes of treating seawater into domestic water

There are some technologies to separate salt from seawater into Non-salt water. Depending on nature and combination of projects, there may be political implications for national projects.

– Transporting fresh water from other places by large ships or fixed pipelines (For example, Singapore buys water from Malaysia).

– Recycle and re-use fresh water from wastewater to limit additional seawater filtration (Singapore, Israel…).

– Using heat for distillation, including:

+ Solar energy.

+ Utilize and exchange heat of chimneys of thermal power plants.

+ Heat of nuclear reactor: Provide up to 120,000m3 of clean water/day. However, the project has ended. (Source: https://en.wikipedia.org/wiki/BN-350_reactor).

– Mechanical filtration with specialized reverse osmosis membrane for sea water is SWRO. This is considered the most popular and optimal solution for small to large capacites.

3. De-salt with specialized membrane SWRO and BWRO

Because seawater can contain many impurities and high salinity, the filtration will go through some steps, of which 2 important steps are:

– Filtering sea water (salt) -> Brackish water.

– Filtering brackish water -> Domestic water.

The process of desalination into domestic water

The process of desalination into domestic water

Basic description of the 6-step process to treat seawater into domestic water.

3.1 Step 1: Garbage filtering

Sea water contains about 3.5% salt, it also contains algae, microorganisms, even industrial waste, plastic bottles… that need to be filtered on the channel leading to water from the sea to receiving lakes

3.2 Step 2: Sterilization

Chlorine use is most common from on-site chlorine generators using either available salts in seawater or a higher chloride from rejected water of SWRO system.

The main purpose of this step is to kill bacteria and algae present in seawater, to avoid clogging to the rear filtration as well as clogged to membranes. At the same time, the oxidation of the a part organic matter to increase filtration efficiency in the next steps

3.3 Step 3: Sedimentation

To remove suspended substances from water at the highest rate, depending on the concentration and nature of the suspended matter, size of receiving tanks, it is possible to add sedimentation aids such as PAC, Iron alum…

3.4 Step 4: Filtering

It is possible to use multi-layer sand of different sizes to increase the efficiency of seawater treatment into domestic water:

– Sand filter for particles more than 10 micrometers in size.

– Fast filtration by pressured sand tower or slow filtration by sand bed

– Fast, high-efficiency filtration can also be applied by stacking filter discs in assembled filtering disc unit

3.5 Step 5: UF and Nano filtration

With a UF filter with a filter size of 0.01 – 0.1 micrometers or NF filter 1.0 – 10.0 nanometers to reduce the load on the RO membrane.

size-and-filtration-capacity-of-uf-membrane-compared-to-nf-membrane

Size and filtration capacity of UF membrane compared to NF membrane

Accordingly, UF and Nano membranes have removed viruses, bacteria and suspended substances. However, UF only allows single and multivalent ions to pass through the membrane. Nano only allows monovalent ions through the membrane to continue to the RO membrane. These monovalent ions are also the main components of seawater. The investment of Nano membrane is higher but the maintenance cost is lower.
Please refer more information at article “What is UF filter and compare with Nano membrane”
https://pre.uce.com.vn/vi/mang-uf-la-gi-so-sanh-mang-uf-va-mang-nano/

3.6 Step 6: RO membrane filtration and adding back its minerals

This is the most important step in the process of treating seawater into non-salted water. Use SWRO reverse osmosis membrane to reduce salt from seawer into brackish water.
Next, filter it again with a BWRO brackish water filter to remove remaining salts from brackish water into salt-free water or almost pure water
If the salt-free water is used for drinking, it is needed to replenish the necessary minerals for body such as Calcium, Magnesium, Potassium, Sodium, Phosphorus, Fluorine, HCO3– … This can be mixed from UF filtered water or independently and automatically dosing for these each component

Note: Some projects may combine Nano/RO filtration instead of RO/RO.

4. Application of RO water after desalination

RO water of desalination projects are widely used in many different fields, including:

– As water supply stations for daily life for residents and cities in coastal areas, having no or lack of fresh water to use. These projects often extremely cost of investment and distribution under the quality management of the State.

– To be continued treating for ultra pure water for high-pressure boilers of thermal power plants (Ion exchange resins: Cationic resin -> Degassing -> Anionic resin -> Mixed resin -> Ultra pure water).

– Suppling water to coastal resorts, then the wastewater is reused to re-supply cooling towers, swimming pools, watering golf courses, aquariums and landscapes.

Above are informaiton related to desalination into domestic water. Hopefully that it will be less or more useful information to customer who are in concern. Currently, for further information about this topic or other water treatment, please contact us directly – UCE Co., Ltd for the fastest support.