Desalination

Desalination plant Shevchenko BN350 on the coast of the Caspian Sea.

Desalination is a process by which salt is removed from sea or brackish water. Desalination plants, also known as desalination plants (see terminology note), are industrial facilities for desalination, generally from the sea or salt lakes, to obtain drinking water. Seawater has dissolved mineral salts. Due to the presence of these salts, sea water is brackish and not drinkable for humans, and ingesting it in large quantities can cause death. 97.5% of the water that exists on our planet is salty and only less than 1% is suitable for human consumption. Purifying sea water is one of the possible solutions to the scarcity of drinking water. Through the desalination of sea water, fresh water suitable for supply and irrigation is obtained. Seawater desalination plants have produced drinking water for many years, but the process was very expensive and until relatively recently they have only been used in extreme conditions. There is currently a production of more than 99 million cubic meters of desalinated water per day throughout the world, which means the supply of more than 100 million people.

Desalination plants also have drawbacks. In the salt extraction process, saline residues and polluting substances are produced that can harm flora and fauna. In addition, they represent a high cost of electricity consumption. In order to avoid this, studies are currently being carried out to build more competitive, less polluting desalination plants that use renewable energy sources.

Note on terminology

The DRAE defines desalination as a more precise term than desalination, since desalination is defined more generically, as the process of removing salt to any product, not just salt water. However, desalination seems to be quite common in Spain. Desalination —and not desalination— is part of the name Spanish Association for Desalination and Reuse. The term has also been used at the National Autonomous University of Mexico. Desalination in addition to being a more precise term, is widely used in South America and among the Spanish-speaking technical community globally.

Methods

The following methods can be used:

• Thermal processes
  • Multi-stage distillation (MSF)
  • Multieffect Distillation (MED)
  • Mechanical steam pressure (MVC)
  • Solar distillation
• Membrane technologies
  • Electrodialysis (ED)
  • Micro, nano and ultra filtration
  • Reverse osmosis (RO)

Thermal processes

Multi-Stage Distillation (MSF)

This method evaporates seawater by applying a heat source and then condenses it. Repeat the operation several times, in some cases adding elements to the process that help capture any substance present in the impure water that you want to extract. The heat source is applied in each of the phases.

Multi-Effect Distillation (MED)

This method is similar to the previous one, but the primary heat source applies only to the first stage. For the next stage, the heat of the steam generated in the previous stage is used.

Mechanical Vapor Compression (MVR)

It is an energy recovery process where energy is added to low-pressure vapor (usually water vapor) by compressing it. The result is a smaller volume of steam at a higher temperature and pressure, which can be used to do useful work. Typically, the compressed steam can be used to heat the mother liquor to produce the steam at low pressure.

Solar distillation

This method uses a closed space exposed to sunlight inside which the water evaporates and then condenses on contact with the colder surface that separates it from the outside. The drops are carried down a slope until they meet at a margin of space. Its production is 1-4 l/day/m².

Membrane Technologies

Electrodialysis (ED)

Micro, nano and ultra filtration

Reverse Osmosis (RO) Desalination

Components of the design of a reverse osmosis desalination plant.

Reverse osmosis (RO) is a process in which fresh water is obtained from salt water. Natural osmosis is a phenomenon that consists in that, if there is a semipermeable membrane separating two solutions with the same solvent, the solvent passes through it, but not the dissolved salts, from the side where the concentration of salts is lower towards the highest, until on both sides of the membrane the solutions have the same concentration. This process is carried out without input of external energy, and is generated by what is called osmotic pressure.

Reverse osmosis consists of passing the solvent (in this case water) through the semipermeable membrane from the side where the most concentrated solution is (seawater, with dissolved salts), to the opposite side, without passing the salts In this case, energy is required, in the form of pressure, which will be slightly higher than the osmotic pressure that would push the low concentration solvent towards the high concentration side. The pressure necessary to achieve reverse osmosis depends on the amount of dissolved salts and the degree of desalination that you want to obtain. The use of energy in the process results in an increase in entropy.

The sea is a virtually unlimited source of salt water. A reverse osmosis plant needs to process a volume of seawater up to three times greater than the total amount of desalinated water that will be obtained in the end. For this reason, the design of the wells or collection system must consider this factor for its capacity.

The use of a graphene sheet with 1.8 nm pores to replace membranes in the reverse osmosis process for water desalination is in the research phase (TRL2). According to current research, much higher efficiencies would be obtained than with current membranes, and lower energy requirements would be required. In the current state, the drawback is the cost of graphene membranes, but it is hoped that in the future these costs can be reduced.

Production process

Generally, a large tank or raft is used that is filled by gravity to sea level, after coarse filtering. The water from the pond is transported by feed pumps to the desalination system. A supplement of chemical products arrives at the entrance of the feed pumps by means of dosing pumps. This is how the water is prepared to pass four types of filters that retain particles larger than four microns. The main step in water production is the separation of H₂O from the mixture of salts and minerals present in seawater. This step is carried out in the reverse osmosis stage, ensuring that the salts do not cross the membranes of the RO modules. Previously, diatom and microalgae particles must not reach the membranes and for this there are three previous sand filtration steps before the last microfiltration step using synthetic fiber cartridges. Filtration success also depends on the proper introduction of coagulants. According to the filtration quality, the membrane change cycle is generated between 2 and 5 years. Chemical dispersants introduced prior to microfiltration prevent mineral precipitation within the membranes.

As all aspects of the process are automated, the job of the operators is supervision and maintenance.

High pressure regulation and energy recovery

The rejected brine is 55% of the raw water (although it depends on the desalination technology used). While 45% of the water obtained leaves at atmospheric pressure, a regulated back pressure must be ensured in the reject flow. This reject flow always contains something like 55% (100% - % gained) of the pressure energy provided by the pumps and recovery of this energy is highly desirable for higher performance. A portion of the recovered energy may return to the same desalination and recovery cycle more than once.

While the plant is in production mode the outlet pressure is controlled by a regulating valve. 'Pressure Exchanger' converters are used and with them in the pressure exchange up to 95% of the energy of the reject flow can be recovered directly by means of pumping using positive displacement. That energy recovery pump increases the flow of more raw water at the inlet of the membranes. The plant uses the 'Pressure Exchanger' near each group of reverse osmosis element tubes.

Quality of produced water

The osmotic water or the permeate from the reverse osmosis modules must be conditioned to meet certain high quality characteristics, since the water produced has an acidic pH and a low carbonate content, which makes it a highly corrosive product. This requires its preparation before its distribution and consumption. The pH is adjusted with calcium carbonate to a value of 7.7. Additionally, if required by municipal regulations for the use of drinking water, sodium fluoride and hypochlorite are also added.

Desalination plants in Spain

Spain is the fifth country in number of desalination plants in the world with a total of 900 plants that have a capacity of 1.45 million cubic meters per day.

Due to their function, desalination plants must be installed near a water source, specifically, the sea. They are dedicated to desalinating seawater, at a distance of between a few tens of meters to 3 kilometers. The further you are from the coast, the greater the pressure necessary to capture the water and, therefore, the energy consumption will be higher, which will make the whole process more expensive.

The first desalination plant in Spain was built in 1965 in Lanzarote with evaporation technology, through solar energy, which is hardly used today, and which has been replaced by reverse osmosis. In Las Palmas de Gran Canaria, the first desalination plant in Spain was built using the reverse osmosis method in 1971.^{[citation required]} It is currently managed by Emalsa.^{[citation required]} Among those of more recent construction, the one in El Prat de Llobregat stands out in the highly populated metropolitan region of Barcelona, affected by intermittent droughts and with relatively contaminated surface water from the Llobregat basin.

Environmental impact

Desalination of seawater is becoming inevitable for the growing demand for fresh water. However, desalination is a process that consumes a lot of energy and has a negative impact on the environment. The discharge of residues from desalination processes is considered a major challenge. Environmental problems can be collected in the following points:

Marine ecosystems

The discharge of concentrated brine makes life difficult for marine ecosystems. Marine life is also influenced by the intake of seawater for the desalination plant. When a large amount of water is withdrawn from the sea, marine organisms and algae are sucked into the intake, causing a disturbance in the ecosystem.

Pollution

Concentrated brine is not only concentrated in salt, but also contains chemicals such as pre- and post-treatment antiscaling agents. In addition, the brine discharged from thermal distillation plants comes out at a relatively high temperature, which means thermal pollution that influences marine life in such a way that only some marine plants or animals can withstand the high temperature near the outlet of the plants. thermal distillation plants.

Energy

High energy consumption is considered the most influential factor inhibiting the growth of seawater desalination. Currently, most desalination processes are powered by energy obtained from fossil fuels that results in gas emissions that pollute the environment. In the case of solar energy-based desalination processes, it is considered a promising method to alleviate the environmental impact of water desalination and also provide a sustainable source of drinking water.

Get Out

Many methods are currently used for brine removal. The brine can be discharged into the sea or river, into solar ponds, or injected into deep saline aquifers. Discharge into the sea or ocean is the least expensive method. When the brine is discharged into the sea, it tends to sink to the bottom. Typically, the brine discharge is diluted with seawater to reduce its salinity before it is discharged into the sea. The brine is discharged to great depths of seawater that normally has a strong current. This reduces the detrimental effects of the brine on marine life. Brine discharges to a solar pond or injection into a deep saline aquifer is a more expensive method. These solar ponds and saline aquifers are often located far from the desalination plant, requiring a long pipeline to transport. This method has drawbacks because it can increase salt in the soil and increase groundwater salinity if linear is not used below the solar pond. Using a solar pond for brine disposal carries the risk of contaminating groundwater.

History of desalination

The history of desalination stretches back thousands of years. The ancient Greek philosopher Aristotle (384-322 BC) already described in his Metrological work that "salt water, when it turns into steam, becomes sweet and the steam does not form salty water again when it condenses& #3. 4; laying the foundations of thermal desalination. In this same work he also described that & # 34; a fine wax vessel will contain drinkable water after being submerged for a sufficient time in seawater, this having acted as a filter for salt & # 34; thus laying the foundations of membrane-based desalination. Based on Aristotle's observations, in the 15th century Leonardo da Vinci proposed adapting an alembic to a wood-burning stove to "produce large quantities of potable water from seawater at very little cost".

The first patents for thermal desalination devices appeared in the 17th century. William Walcot proposed a patent in 1675 on "the art of making corrupt water useful for use and sea water sweet and fresh in large quantities by very cheap and simple methods". A few years later, in 1683, Robert Fitzgerald (Robert Boyle's nephew) patented a similar invention that caused a dispute between the two: "a way of turning salt or brackish water into fresh water, suitable for drinking, boiling, washing or other ordinary use by means of a certain device or devices not previously used in our domains". Neither of the two patents was used due to technical problems in its large-scale extension.

The first invention with large-scale application dates back to 1852, when Alphonse René Le Mire of Normandy filed a patent on "the production of fresh water from seawater". Thanks to the recent popularization of steam-powered ships, De Normandy's invention was a commercial success that led him to sell more than two thousand of his inventions and found a company to produce them. De Normandy's device can also be considered the first to be successfully used on land on a large scale. In 1861, three of his desalination apparatus were used by the United States Union Army for months in Key West, Florida, where they provided more than 1,000 L of potable water per day.

The first modern desalination plant was built in 1961 by the Dow Chemical Company in Freeport, Texas. The plant was based on a multi-effect distillation system with 17 energetically integrated effects (stages) with a single heat input point. The production capacity was 3800 m³ / day (1 MGD) of potable water from nearby seawater. The presence of the desalination plant in Freeport considerably increased the security of access to water for the local population and industry.

The first desalination plant based on the reverse osmosis process was built in 1965 in Coalinga, California. It was a pilot plant with the capacity to produce 19 m³ / day of drinking water from the brackish water available in the locality wells (salinity 2500 mg / L) and used primitive acetate membranes cells devised by Sidney Loeb at the University of California, Los Angeles (UCLA). The objective of the pilot plant was to see if it was possible to avoid the consumption of fresh water transported by train or truck from other locations. The primitive desalination available at the time was finally discarded due to its high cost and a canal was built to transport fresh water continuously to the city.

The latest major milestone in the history of desalination is the construction of the first seawater desalination plant based on reverse osmosis technology. The plant was built in Jeddah (Saudi Arabia) in 1976 and used Red Sea water (salinity 40,000 mg/L) to produce 12,000 m³/day of drinking water. The semi-permeable membranes used at the plant were much more sophisticated than those used at Coalinga and were based on a composite material made up of a thin layer of spirally wound polyamide very similar to those used today. Its membranes allowed it to work between 60 and 70 bar of pressure (depending on the degree of contamination) with an average recovery of 30% in drinking water.