**1. Introduction**

"Sustainable development is the one which satisfies the present needs without threatening the capacity of future generations to meet their own needs". This definition by Brundtland [1] clearly shows the idea of limits imposed on technology and "social organization due to the capacity of the environment to satisfy present and future needs", as Jonker and Harmsen mention [2].

Population growth has increased dramatically throughout the world in the last two centuries. Global population is expected to go from the present 7.7 billion inhabitants to some 10 billion people by the year 2050. One of the biggest challenges to face in the near future is how to guarantee drinking water supply all over the world. This is an important challenge for humanity, since drinking water availability will have to increase significantly.

Apart from this important population growth, another effect that has been observed in the past few decades is the rural exodus to the big cities. This migration is causing demographic imbalance, depopulating certain areas and causing big population increases in others. In the driest countries, the most densely populated areas are actually on coastal regions due to very well-known social and economic reasons.

Siegel [3] says that according to the American government, 40 out of 50 states and in 60% of the USA surface, there will shortly be an alarming difference between the water available and the increasing demand for this resource. He also explains how Israel, with 60% desert land, can be an example for all other countries, not only because of solving their water problem, but also for having sufficient to provide Palestinians and Jordanians with the resource. Even Iran depends on a similar water system and China knows enough about the Israeli water system to be able to manage its own water needs.

The situation in Cape Town, South Africa, as described by Bates Ramírez [4], should serve as an example, since a critical water situation arose when four million inhabitants ran out of water supply. This was the first time that such a large number of people were ever put at such a serious risk of lack of water. The dangerous situation in South Africa was a wakeup call for other cities in similar circumstances, such as Mexico City, Sao Paulo, and Cairo, who all face water shortages.

As the global population grows and climate change increases temperatures, water will become even more scarce.

Oceans have roughly 97.5% of the planet's water, and the 2.5% remaining water deposit is divided up in glaciers, ice, phreatic layers, rivers, lakes, and atmosphere.

According to the Madrid Complutense University [5], polar ice caps and glaciers hold 69.3% of fresh water, groundwater, 30.3%, lakes, 0.26% and rivers, just 0.006%. The remaining fresh water is found in living beings (0.003%) on the planet, including the atmosphere. Therefore, of the total freshwater reserve on the planet, we only have a volume of 127,679,000 cubic hectometers in rivers and lakes.

In AQUASTAT [6], three types of water withdrawal are distinguished: agricultural (including irrigation, livestock, and aquaculture), municipal (including domestic), and industrial water withdrawal. A fourth type of anthropogenic water use is the water that evaporates from artificial lakes or reservoirs associated with dams. It is worth highlighting the consensus on the use of water by humans, allocating 12% for domestic use, 19% for industry, and 69% for agricultural use. These numbers, however, are strongly biased by a few countries which have very high-water withdrawals. Table 1 shows the water withdrawal ratios by continent.


**Table 1.** Water withdrawal percentages by continent.

The above shows not only the scarcity of the water resources and the difficulty to guarantee water supply, but also that there are not many water supply alternatives. Water desalination is particularly suitable for cities situated near the coast or with brackish water.

According to data published by the International Desalination Association (IDA) and Global Water Intelligence (GWI) in the Water Security Handbook 2019–2020 [7], over 17,000 desalination plants have been contracted, reaching a total of 107 Hm3/day of cumulative installed desalination capacity in 2019. Desalination is operational in 174 countries. There are more than 300 million people around the world who rely on desalinated water for some or all their daily needs, with 146 Hm3/day of cumulative installed reuse capacity in 2019.

The various desalination technologies are differentiated by cost, product quality, and energy consumed. Most plants desalinate through a thermal process or using membranes.

Thermal desalination methods use heat to evaporate saltwater and they condense it again, now without salt. They basically imitate the natural water cycle of evaporation and rainfall.

Urrutia [8] explained that thermal processes that have been used since the appearance of desalination in the 1950s; they are mainly used in oil exporting countries today.

Torres Corral [9] pointed out that from these beginnings until the 1980s, desalination was mainly done by distillation processes, building dual water and electric power plants, as long as the market for the power plant was viable.

In the following section we will show how the cost of the selected desalination technology is the most significant cost in the final price of desalinated water. Torres justified how the various oil crises have significantly affected applied technology. The first crisis in 1973 led to optimizing the process. The second crisis in 1979 caused a shift to the use of the reverse osmosis process, by developing membranes which removed over 99% of salt, with mechanical resistance capable of withstanding 70 Kg/cm2 to overcome the osmotic pressure.

Reverse osmosis is based on the natural osmosis which occurs in cell membranes in living organisms, in which water diffusion moves from an area with low concentration of solutes to another with a higher concentration. The system used to desalinate is the opposite (hence the term "reverse")—the saltwater propelled to break the osmotic pressure goes through a semi-permeable membrane, which retains water with higher saline concentration (brine) and allows water for human consumption to pass.

According to Stover [10], in the 1990s and 2000s, the innovation in the desalination industry focused on reducing energy consumption, improving the performance and reliability of the reverse osmosis membranes and the innovation of energy recovery devices. It is also worth highlighting the improvement in the processes, such as the use of a second layer of reverse osmosis for the retained water on the first stage (brine), increasing fresh water compared to raw water and decreasing residual brine. With these measures, not only did the energy used for desalination fall by half, but it was also possible to build fairly big reverse osmosis desalination plants.

Scott [11] showed that the fact that desalination costs have decreased in recent years is due to the progressive incorporation of membrane processes by those countries where energy is expensive, thus being able to replace thermal processes.

However, even in these oil exporting countries, the tendency is changing due to oil prices, as Ibáñez Mengual [12] stated, since the evaporating processes are associated with a thermal power plant. When there is an imbalance between supply and demand for electric energy, this is reflected in desalinated water production decreasing. This explains why at the time of writing desalination projects in the Middle East used 50% evaporation technology and 50% reverse osmosis technology, with a tendency to increase this latter technology.

Another relevant question to consider is the cost. There are several factors that contribute to costs: the type of technology used, the type of water to desalinate, the quality of the water that is demanded, the cost of energy, etc. Usually, the cost is divided into three blocks: investment costs, fixed operating costs and variable operating costs.

Voutchkov [13] estimated a cost share for the membrane desalination technology: approximately 35% costs are for energy; the recovery part roughly 30%, then personnel (5%), taxes (8.5%), and industrial profit (6%). The remaining percentage to reach 100% would be made up by membrane replacement, chemical products, maintenance, and other costs. It is clear from these figures how important the cost of energy is. Voutchkov reported the worldwide evolution of energy consumption, which has gone from 22 kWh/m<sup>3</sup> in the early 1970s to a consumption of the order of 2.8 kWh/m3 (pure desalination) nowadays. It was during the 1990s when the greatest technological advances occurred. The impact of these advances can be seen on the production price of the cubic meter of desalinated water, which in the 1980s was around more than \$2/m3, and currently it is hovering around \$0.60/m<sup>3</sup> of desalinated water.

However, desalination is not free from controversy and criticism related to the impact in the environment caused by the desalination plants. Latteman and Höpner [14] warned that the Persian Gulf has always had intense desalination activity, but that other regional centers were prominently emerging, such as the Mediterranean Sea, the Red Sea, the coastal waters of California, China, and Australia. Kämpf and Clarke [15] claimed in 2012 that the brine discharge was non-compliant with the Environmental Impact Assessment (EIA). The monitoring process in South Australia was flawed, and in 2015 the current license was modified based on the results of an independent review of the monitoring performed for the desalination plant operations. The Environment Protection Authority (EPA) has set strict compliance limits and monitoring requirements for the environmental license for the plant. Fuentes-Bargues [16] published a study on the environmental impact assessment process in Spain for seawater desalination projects with 12 years' worth of data, identifying brine discharge as the main impact. However, Shemer and Semiat [17] defended the use of reverse osmosis desalination and claimed that brine discharge has minimal impacts. Recently, Saracco [18] warned of the risk of brine contamination and pointed out how substantial damage to the marine ecosystem can be observed in the Persian Gulf area, that requires corrective action. Most of the cited authors agree that the main effect of seawater desalination plants is the discharge of the brine and its impact on the marine environment. They also agree that the solution is for the environmental authorities of each country to implement environmental impact studies that establish strict compliance limits and monitoring requirements to verify that the measures adopted are adequate.

The desalination process has required technology development these past decades at all the stages of the technology to reduce cost and negative impact and still meet the needs of society. This is more pressing where there are not enough freshwater resources to supplement quality desalinated water.

Finally, it is worth mentioning that to achieve more efficiency in the process, more complex and expensive technologies have been developed, resulting in increasing the size of the desalination plants to decrease operation and maintenance costs. Thus, in 2018, each of the ten biggest desalination plants in the world produced more than half a million cubic meters a day, with the largest of them reaching a million cubic meters daily. The following factors have to be taken into account before justifying the decision to implement a project: technical complexity, efficiency, size, and cost, forcing a search for the best-suited method of procurement to develop the project, and achieving its objectives.

The objective of the present paper is to study the requirements that new desalination plants need to meet to be compatible with sustainability requirements. Four areas are considered:

