Introduction

The recent issues associated with climate change affect the natural water regeneration cycle. Water is an increasingly scarce resource, with approximately 20% of the world's population living in areas without sufficient water, and another 10% approaching this situation. Of all the fresh water in the world, 69% is found at the poles and on the highest mountain peaks, in a solid state. Another 30% resides in soil moisture or in deep aquifers. Only 1% of the world's fresh water drains through hydrographic basins in the form of streams or rivers and is deposited in lakes, lagoons, and other surface bodies of water, and it can also end up in aquifers (^{Fritzmann, Löwenberg, Wintgens, & Melin, 2007}). Therefore, the search for options for sustainable water supplies is required. The desalination of seawater is an important alternative to this problem.

The methods used to desalt water have advantages and disadvantages, which depend on various factors associated with each process. Studies have focused on generating cost databases (operating and capital) for different desalination methods (^{Ettouney, El-Dessouky, Faibish, & Gowin, 2002}). In addition, the costs of desalinating seawater and brackish water are different because of the TDS (total dissolved solids), which include minerals, salts, metals, cations, and anions dissolved in water. More saline water (higher TDS) portends higher desalination operating costs (^{Karagiannis & Soldatos, 2008}; ^{Zotalis, Dialynas, Nikolaos, & Angelakis, 2014}). Some studies have correlated data and generated mathematical models on costs and capital for desalination methods (^{Wittholz, O’Neill, Colby, & Lewis, 2008}).

Regarding the environmental factors involved in desalination, there are complexities in the pre- and post-treatments of membrane processes (^{Fritzmann et al., 2007}). Thermal processes, on the other hand, do not require delicate treatment (^{Gude, 2015}). Finally, the costs associated with water transportation to and from the desalination plant should be considered (^{Zhou, 2005}).

In terms of MCDM techniques, authors have compared the effectiveness of AHP and PROMETHEE for the best energy supply option (^{Georgiou, Mohammed, & Rozakis, 2015}). The comparison considers supply alternatives and environmental, economic, social, and technical criteria.

In terms of research, one of the first studies using MCDM evaluated the desalination of brackish water in Jordan (^{Mohsen & Al-Jayyousi, 1999}) by employing alternative technologies such as the multiple distillation effect (MED), multiple stage instantaneous distillation (MSF), reverse osmosis (RO), electrodialysis (ED), steam compression distillation (CV), and the AHP method. Other studies addressed the selection of the best seawater desalination plant using AHP (^{Hajeeh & AI-Othman, 2005}) and considering MED, MSF, CV, and RO technologies and alternative plant construction. Research has also been undertaken on the use of diffuse AHP with three desalination alternatives (MED, MSR, and RO) to supply fresh water to the state of Kuwait (^{Hajeeh, 2006}). Another study integrated two stages and utilized diffuse AHP and TOPSIS for the desalination of brackish water (^{Ghassemi & Danesh, 2013}) and for obtaining drinking water using electrodialysis. Studies have been conducted using MCDM tools for sustainable cities (^{Si, Marjanovic-Halburd, Nasiri, & Bell, 2016}) and solar energy for desalination (^{Shatat, Worall, & Riffat, 2013}).

Regardless of the various works employing MCDM technology, to our knowledge there are no available studies on the desalination of seawater for its possible use in public consumption and agroindustry in developing countries. Therefore, this work evaluates the desalination of seawater with the following multi-criteria tools and technological alternatives: MSF, MED, CV, ED, RO, and NF.

Methodology

In this section, the basic characteristics of the desalination and MCDM methods used in the present study are mentioned.

Results

The parameters shown in Table 1a are the criteria (C_j) and their environmental, economic, and technical sub-criteria (SC_j). The technological alternatives are shown in Table 1b. Desalination is analyzed by means of AHP, ELECTRE, and TOPSIS. The results of each method are mentioned below.

Discussion

When comparing the technologies, ED had the lowest ranking and was in last place. Although it is one of the best alternatives for desalinating brackish water, its performance for desalinating seawater is lower due to the high concentration of salts. It has low energy consumption when treating waters with salt contents lower than 3 500 ppm of TDS, but seawater can have up to 30 000 ppm.

The thermal technologies then follow: MSF came in fifth place, CV was in fourth place, and finally, MED was in third place. These have some advantages over membrane methods but are not more important, according to the evaluation. They are characterized as providing slightly warmer water with lower salt concentrations than OR or NF. In addition, they emit chemical compounds into the environment (anti-fouling), which generates harmful long-term effects in the discharge area (^{Gude, 2015}). The last advantages in sub-criterion 3 refer to the quality or purity of the fresh water obtained from those processes. Thermal technologies have an advantage of desalting water and leaving it with minimum concentrations of salts (TDS less than 50 ppm). These are appropriate technologies to obtain water with minimum saline content, although some regulations allow up to 1 500 ppm of TDS. They have higher energy consumption than other options, which makes the process more expensive, especially in developing countries where fuels are expensive or inaccessible, such as in the Middle East.

Lastly, RO is in first place, and NF is in second place. RO performs better in preventing the selective passage of ions and salts through the membrane, generating water with lower TDS concentrations, which is a positive outcome. To achieve that, however, a higher pressure gradient is required, that is, a higher energy consumption. Recent studies consider it possible to desalinate water using NF, obtaining a quality similar to that obtained by RO but with a lower energy consumption (^{Adham, Cheng, Vuong, & Wattier, 2003}). Despite that result, reverse osmosis in sub-criterion 8 has a greater weight than most of the other sub-criteria because it is a one-stage process, unlike NF which employs two stages. NF is a more complex process to operate and control in comparison with RO. In addition, the water recovery capacity and the quality of the salt concentration of the final water are exceeded. Therefore, RO is the most viable option for desalinating seawater for possible agro-industrial purposes, and even domestic consumption.

Finally, the hierarchy or ranking is the same for AHP and TOPSIS. In ELECTRE, a slight difference is noted, and the three thermal processes are in third place (MED, MSF and CV). ELECTRE presents a more imprecise algorithm than TOPSIS and AHP, which use ranking to produce scores. The ELECTRE method only produces a hierarchy. The scores consist of the differences in the dominances between the rows and columns. It is observed that both TOPSIS and AHP, along with the MSF, MED, and CV technologies, present very similar scores. It can be said that among the methods, the ELECTRE method yields less precise results and is more complex to operate.

As for AHP and TOPSIS, both methods use the same decision matrix and the same ranking. They reflect very good results and have high coherence among them.

Conclusions

The use of MCDM delivered results that are currently consistent with reality, and membrane desalination processes are used and developed mainly by RO. The advantages of NF are recent and are expected to improve over time.

All the MCDM methods used resulted in similar rankings. The best option for the hierarchical order of the methods (AHP, ELECTRE, and TOPSIS) is RO, NF, MED, CV, MSF, and ED.

The results are highly dependent on economic criteria because energy consumption is relevant and directly related to the operating costs of the processes.

The results are concordant with reality. For example, in Latin American countries, membranes show a clear superiority over thermal methods for the desalination of seawater, due to the costs associated with each of the processes.

In the future, a similar study would use renewable and applied energies in specific agricultural sectors located near sources of seawater.