Seawater Desalination Plant

1. Water Desalination Methods

2. Water Desalination Equipment

3. Which Desalination Method Is More Effective

On modern sea vessels, the main source of obtaining fresh water is seawater desalination. It is used for drinking, technical needs and other purposes.

Recently, the most widespread method for desalination of seawater on ships has been the reverse osmosis method, based on passing seawater through a semipermeable membrane under pressure from 20 to 60 bar (depending on the salinity of seawater). Desalinated water is passed through the reverse osmosis membrane. The seawater concentrate that does not pass through is drained back into the sea. This method is highly energy efficient and compact, which is very important in the limited conditions of a sea vessel.

Let's consider other methods of seawater desalination in comparison with reverse osmosis.

1. Water Desalination Methods

Chemical. One of the fastest desalination options. For this, silver or barium salts are added. The initial salt content is no more than 2000-3000 mg/l. They are added strictly according to the calculations with a special device - a dispenser. The consumption of chemicals is approximately 3 to 5% of the resulting fresh water. When reagents enter the water, a chemical reaction and sedimentation occur. The treated part enters either the tank or the next filtration stage, the sediment is drained into the drain. A sorption filter is used for additional purification of water. After treatment with reagents, decay products may remain, which must also be eliminated. Water treated with reagents is used for cooling, make-up or technical purposes. Chemicals are selected based on the composition of the water, but no more than 5% of the total volume. The advantages of the method are the rapid treatment of water. The disadvantages are that such water is not recommended for use as drinking water. Also, with a high concentration of salts, a large volume of reagents will be required.

Fig. 1. Wagner-300 MK desalination plant on board the ship Viktor Buinitsky

Distillation. The method is implemented by heating the liquid and collecting condensate. For this, equipment is installed - distillers. There is an option in which boiling does not occur, to obtain condensate, the pressure and temperature change. Salt water is heated in special chambers. In the evaporation chamber, the pressure is lower than during processing. And due to the temperature difference, the flow that enters the evaporation stage turns into steam. Its formation also requires the heat of the evaporated water. When the liquid gives off heat, it cools down. The processes are called adiabatic (without boiling) and isothermal (with boiling). The advantage of the method is its safety and the ability to desalinate a large volume of water. Disadvantages in the consumption of electricity to heat the liquid.

Ion substitution. This method uses synthetic resin, since its consumption is lower than that of natural resin. The water flow passes through the resin and impurities are attached to the granules, and they give off safe elements. The filter is regenerated by washing the resin in the solution. The advantage of the method is in combining filtration and desalination, if several columns with different loading are used. The disadvantages include frequent regeneration at high concentrations of salts.

Reverse osmosis. One of the varieties of ultrafiltration technology is used. A stream of water under pressure passes through the membrane. Salt molecules are larger than the porous surface of the filter, so they remain in the concentrate (polluted water). The advantage of the method is the ability to desalinate water of any degree of salinity. Such treatment allows you to get drinking water, since reagents are not used in the filtration process. There is no need to change the temperature of the water for processing. The disadvantages include the service life of the membrane. At high concentrations of impurities, it must be washed more often.

Electrodialysis. The method is based on desalination using membranes and resin with a positive and negative charge. Suitable for processing slightly salted water. When current is applied, an electric field is created and dissolved salts are directed to the electrodes, passing through the membranes. The advantage of the method is almost complete desalination of water. Water temperature is not important for filtration. The membranes used are resistant to sea water. Low energy consumption. Possibility to remove a certain type of salts from water.

Fig. 2 Seawater desalination plant on board a ship

Freezing. The process is borrowed from natural desalination. Fresh water freezes faster than salts. Liquid butane is used to imitate natural freezing. But this method is not very popular, as it allows purifying a small volume of liquid. And the installations used for crystallization are large.

2. Water desalination equipment

Naturally, the unit is selected based on the desalination method. The water treatment system on ships may include:

• Preliminary filters. They clean from mechanical impurities. There are disk, cartridge, mesh and sorbent-based filters.
• Desalinators. Actually, the filters themselves for desalination.
• Disinfection devices.
• Filters for final processing.

Desalinators based on membranes and ion exchange are most often installed on ships. Distillers are also relevant. The most effective desalination method is a combination of salt removal with ionic resin and subsequent treatment with reverse osmosis. Such water meets drinking standards.

Each desalination system, regardless of the chosen method, has its own technical requirements. This can be expressed in the ambient temperature, maintaining a certain pressure, water temperature and impurity concentration. If the requirements are not taken into account, the equipment will not work efficiently. For example, membranes can only desalinate cold water and they require maintaining a certain pressure range. Except for the electrodialysis method, in this case the water temperature does not matter. Ionic resin requires mandatory washing with a special concentrate, which must be prepared in advance. As for cleaning the filtration elements, this is a separate category. During desalination, any filter is filled with impurities. These are not only salts, but also other contaminants that are present in sea water. To maintain the quality of desalination, it is necessary to remove these elements from the filtration modules. To avoid manual cleaning, automation is used. Almost all modern desalination units have this function as standard.

3. Which is desalination method more effective?

Each method has its advantages and disadvantages. Therefore, the choice depends on the requirements and productivity. But still, some methods are the most important. Freezing and chemical methods are more outdated and are rarely used. Due to high operating costs. Chemicals cannot be used to desalinate drinking water, and crystallization requires the installation of large-sized equipment. Ionic resins are also not very popular. It's all about resin regeneration. It requires washing with a special salt concentrate. To do this, it is necessary to install additional equipment to feed the solution to the filter medium. And then the disposed water must be purified again, because it is very salty. And discharging such water on a permanent basis will significantly increase the level of salt content in seawater.

The most common methods are distillation, both with and without boiling, and reverse osmosis. Such installations do not require significant operating costs. Modern membranes for desalination of seawater are more protected than for fresh water. This is necessary to protect against the aggressive effects of the elements. Sea water not only has increased hardness and salt microelements, but also other impurities. Therefore, the elements are made of durable polymer materials. Of course, with a high salt concentration, the membranes need to be cleaned more often, and their service life is not particularly long. But they make it possible to obtain absolutely clean fresh water, without additional filtration modules.

As for distillers, in this case there are several options. When using an external power source to bring water to a boil, a lot of energy is spent. Therefore, the cost of maintaining such equipment will not be cheap. But at the same time, the method does not use chemicals and allows you to qualitatively desalinate water of any degree of salinity without reducing productivity. If you choose a method based on the difference in temperature and pressure without boiling, then energy costs are reduced. Because a slightly different approach is used to generate heat.

Fig.3 Distiller for desalination

Therefore, when talking about the efficiency of seawater desalination, reverse osmosis systems will be a priority. They do not use chemicals, which means that purified water can be used as drinking water, the operating costs are minimal and the systems have sufficient productivity. Modern desalination plants based on reverse osmosis are distinguished by their compactness. In the case of yachts, a minimum configuration is used and such a system takes up little space. As for large vessels, in this case the configuration is expanded due to the addition of additional modules.