Seawater desalination has become an essential necessity on our planet. Desalination technologies, such as reverse osmosis, are nowadays a world reference regarding water production dealing with the ever-increasing water demand for different uses: urban, industrial, tourism or agricultural.
However, far from being a completely optimised technology, the technical and scientific communities dedicate their effort to improve the process towards perfection, from different points of interest: energy, environment and economy.
There are two main challenges at the top of the list: on the one hand, reducing energy dependence and CO2 emissions generated by desalination plants and, on the other hand, converting the main waste of this process (brine) into a valuable source of several by-products (Mavukkandy, Chabib, Mustafa, Al Ghaferi & AlMarzooqi, 2019). This article is aimed at examining this second challenge.
Brine environmental impact
It is important to remark that brine represents 0.5-0.6 m3 of each m3 (i.e. 50 – 60 %) of seawater treated in reverse osmosis desalination plants. The vast majority of the brine produced, which basically is composed of the same elements than seawater at double concentration plus certain chemical additives, is being discharged into the sea. The main discharge points of brine into the sea worldwide are located in the Middle East, Mediterranean, Indian Ocean, South Pacific and central North Atlantic. Only a marginal volume of brine is currently being used directly in specific applications, such as microalgae biotechnology (ITC, 2020).
Numerous studies have been published in relation to environmental impact on flora and fauna caused by brine discharges to coastal waters. Although it is not within the scope of this article to delve into the analysis of those studies or the technologies to minimize the adverse environmental impacts of the discharge, such as the design of Venturi diffusers (ITC, 2010), the need to turn brine from a waste to a resource is absolutely unanimous. Spain, especially the Canary Islands and the Mediterranean coast, has always been at the forefront in the analysis of brine environmental impacts, as well as its potential.
During this past decade, political statements, the industry sector and social actors have all introduced the notion of Circular economy as a vital concept for our society. The desalination industry is particularly focused on the potential of brine, due to its chemical composition. Contrary to popular belief, seawater is much more than just water and sodium chloride. In fact, in absolute terms, certain elements can be found in higher volumes in the ocean rather than mineral reserves on land; such as magnesium (Loganathan, Naidu & Vigneswaran, 2017), which was classified as one of the Critical raw materials by the European Commission on 2017; as a result of both its importance for the economy of Europe and its import dependence. Lithium is another clear example (Yang, Zhang, Ding, He & Zhou, 2018), which demand has increased radically during the past years, because of its use in Li-ion batteries.
Multiple minerals and metals can be extracted from brine. In addition to the ones mentioned in the previous paragraph, calcium carbonates and sulphates used in the construction industry can be obtained by chemical precipitation (Ramasamy, 2019). Acids and bases, such as hydrochloric acid and sodium hydroxide, can also be produced from brine by electrochemical technologies like bipolar membrane electro dialysis (Kumar, Phillips, Thiel, Schöder & Lienhard V, 2019). Similarly, another valuable product, sodium hypochlorite, has also been obtained through an electrochemical process (Malvi Technologies LLC, 2017). Furthermore, less expected elements like uranium (Wiechert et al., 2018), cesium and rubidium (Chen et al., 2020) could also be extracted from the brine by adsorption and ion exchange processes.
Can brine reduce CO2 emissions?
If these reasons were not enough to understand brine as a resource instead of a waste, brine has also been used to capture CO2 (Mustafa, Mourad, Al-Marzouqi & El-Naas, 2020), helping to reduce CO2 emissions which are directly linked to climate change, additionally producing valuable by-products, such as sodium bicarbonate, among others. Moreover, electrical energy can be generated combining brine with low conductivity waters as a result of their osmotic pressure gradient, by the application of technologies such as pressure retarded osmosis (PRO) (Schunke, Hernández Herrera, Padhye & Berry, 2020) or reverse electro dialysis (RED) (Mei & Tang, 2018). Consequently, planning the construction of large desalination plants located next to wastewater treatment plants, to be able to reduce the associated energy consumption, could become a worthwhile model for the future.
Definitely, a good deal of different alternatives without any foreseeable future yet. Despite all those benefits, SWRO brine reuse at large scale is not a reality yet other than laboratory or pilot plant scale. In many cases, specific advances are required concerning performance of selective membranes or process optimisation for emergent technologies, such as membrane distillation and crystallization (MD/MCr) (Das, Dutta & Singh, 2020), high pressure reverse osmosis (HPRO) (Davenport, Deshmukh, Werber & Elimelech, 2018) or electro dialysis with bipolar membrane (BMSED) (Chen et al., 2018), among others. Finding economic feasibility to these processes will mainly depend on resources and time dedicated to research & development from both industry and research groups to overcome those obstacles.
Technologies and processes stated above target specific elements from brine although brine discharge is not completely avoided. Zero liquid discharge (ZLD) (Charisiadis, 2018) takes special relevance when brine is meant to be fully exploited. This concept involves a combination of different processes targeted at the eradication of any liquid discharge, maximising water production and generating as many by-products as possible. Thermal processes, such as evaporation, are implemented to extract different salts depending on their solubility. They have been the dominant choice. However, in order to optimise those operations, lately there has been a shift from thermal processes to membrane technologies (Wallace, 2019). Some companies are starting to offer new solutions adapted to any specific type of feed water in terms of salinity, temperature, pre-treatment and even subjected to local demand of the extracted products. Economic feasibility of the specific process will vary depending on those characteristics.
Regardless of applying selective extraction of targeted compounds or a ZLD route, energy consumption tends to be the main variable to be tackled in order to develop a feasible process. Therefore, it seems logical to think about renewable energies as the perfect partner to minimise carbon footprint of brine management processes.
Brine: from waste to resource
DESAL+ LIVING LAB acts as an R&D&I platform located in the Canary Islands (Spain). It consists of a coordinated group of public and private entities with existing plants and R&D&I infrastructure that cooperates in applied research on desalination and water-energy nexus, including SWRO brine management. After many years of research and development in this field, the question about whether it would be technically and economically feasible to reuse brine as a valuable resource has completely changed. Now the questions are all about when this inflexion point will occur or which technology or combination of them will definitely break the existing barriers and achieve the transformation of brine from a waste to a precious resource.