Biological Seawater Desalination

*Enrique O. Martínez*

#### **Abstract**

More than two billion people worldwide lack access to safe, clean drinking water, and this number is likely to increase due to population growth and rapidly diminishing freshwater supplies. Current seawater desalination methods can effectively provide freshwater and meet the growing demand for this resource. However, they are becoming increasingly controversial owing to their adverse environmental impacts, including high energy consumption and generation of desalination brine. For millions of years, various species of organisms such as plants, microalgae, and bacteria have adapted to environments with variable salinity and have developed mechanisms to eliminate excess intracellular NaCl. This has encouraged scientists to study the possibility of using biological processes for seawater desalination. Biodesalination is an emerging technology for the selective removal of Na + and Cl − from salt water by various halophile organisms, such as macrophytes, microalgae, and cyanobacteria, with very low energy consumption. Microbial desalination cells that allow simultaneous desalination of water in conjunction with wastewater treatment are also included in this category. The direct use of living organisms such as halophile plants, microalgae, and bacteria to desalinate water appears to be a promising field. However, the development and practical applicability of these technologies depend on the living organisms selected for desalinating seawater.

**Keywords:** seawater, biodesalination, mangroves, microalgae, cyanobacteria, microbial desalination cells

### **1. Introduction**

Globally, increasing freshwater scarcity problems have become more evident owing to continuous population growth, unequal distribution of water resources, mismanagement, and climate change [1, 2]. One approach to address this situation is to use abundant sources, such as seawater and brackish groundwater. However, desalination of these waters occurs at the expense of several environmental impacts due to high energy consumption, mostly fossil fuels, and brine discharge [3–5].

In response to this challenge, biodesalination has emerged as an emerging technology to remove salt from seawater, brine, and wastewater with very low energy consumption [6–8]. This process is an emerging technology for the selective removal of Na + and Cl − from salt water by various halophilic organisms, such as macrophytes, microalgae, and cyanobacteria [9, 10]. Also included in this category are bioelectrochemical systems (BES) that allow simultaneous desalination of water in conjunction with wastewater treatment [11]. Another line of research in the field of biodesalination involves biomimicry and bioinspiration. Within this area, biomimetic

membranes based on aquaporins (ABM) have commercial applications [12–18]. Bioinspired and biomass-based solar seawater evaporators (BSSE) are also included in this field of research [19–23].

#### **2. Mangroves and desalination**

Halophyte plants, which can survive and thrive under saline conditions, have long been used to restore salt-affected soils. This remediation technology, known as phytodesalination, takes advantage of the capacity of halophytes to absorb large amounts of sodium ions (Na+ ) from affected ecosystems and their elimination through accumulation and translocation to different parts of plants [24–26]. A variant of the application of this technology has been the use of halophyte plants in constructed wetlands (CW) for the treatment of wastewater and thus be able to reuse the treated water for irrigation without affecting the soil [27, 28].

Although phytodesalination appears to be a green and promising technology, its application in desalinating water presents technical difficulties [12]. However, a group of halophytes, mangroves have particular adaptations to live in aquatic environments, so they can be used in desalination processes [29].

To survive in saline or brackish environments, mangroves strictly control the absorption of water and ions in their roots, analogous to the reverse osmosis (RO) process, allowing the xylem sap to be almost free of salt [30, 31]. Detailed knowledge of the salt exclusion mechanisms of mangroves has served for the design of filter membranes based on mangrove roots [32]and for artificial trees capable of filtering salt from seawater [33, 34].

Regarding the practical experiences of seawater desalination with halophytes, reed beds with *Avicennia marina* and *Rhizophora mucronata* have been reported [35–38]. This particular phytotechnology is inspired by constructed wetlands (CW) used in wastewater treatment, but in this case, it consists of establishing a flow of seawater through the roots of mangroves to remove NaCl, as shown in **Figure 1**. Another approach uses mangrove plants inside a greenhouse as part of a biothermal desalination process to produce freshwater through transpiration and condensation [35, 37].

#### **3. The use of microalgae and cyanobacteria to desalinate water**

Microalgae constitute a heterogeneous group of photosynthetic organisms that can grow in either saline or freshwater and in different climates [36]. These characteristics have led to their direct use as novel and sustainable biological techniques for seawater desalination [12, 39–42]. In fact, it has been found that some common genera of freshwater microalgae, many of which have been widely used in wastewater treatment, such as *Chlorella*, *Scenedesmus* and *Desmodesmus* among others, can survive and grow in water with high NaCl levels [6, 15, 43].

Cyanobacteria, also known as blue-green algae, are prokaryotic (without intracellular structures) photosynthetic organisms with a long evolutionary history [44]. As with microalgae, it has been found that cyanobacteria can develop in environments with high concentrations of NaCl and have the potential to be used in biological desalination processes [45]. Within this group, studies have been conducted on species of the genera *Spirulina*, *Synechococcus*, and *Phormidium*, which are found in hypersaline environments [9, 46–48].

#### **Figure 1.**

*Conceptual design of constructed wetlands using mangroves as a desalination system. Raw seawater enters the constructed wetland and loses salt as it circulates through the roots of mangroves. The resulting brackish water can be treated using conventional processes such as reverse osmosis.*

### **3.1 Biodesalination mechanisms in microalgae and cyanobacteria**

Microalgae and cyanobacteria remove salts from water via bioadsorption and bioabsorption mechanisms [49, 50]. In this sense, it has been shown that microalgae exposed to salt stress respond by increasing biomass and lipid synthesis, which can be converted into biofuels and other high-value products [6, 51].

#### **Figure 2.**

*Schematic representation of microalgae desalination technology integrated with seawater reverse osmosis (SWRO). In this system, pretreated seawater is fed into an algal bioreactor to remove salts from the water. The algae were then removed from the desalinated water, which was then subjected to conventional processes, such as reverse osmosis. The biomass generated in the bioreactor was processed to eliminate salt and recover lipids and other products generated by microalgae.*

#### **3.2 Desalination experiences with microalgae and cyanobacteria**

A promising application of microalgae is the treatment of produced water (PW), saline wastewater, or the pretreatment of saline water with high nutrient and metal contents [6, 41, 52].

Integration of desalination with algae or cyanobacteria using reverse osmosis (RO) membranes has also been proposed [6]. The idea is to use microalgae or cyanobacteria as a prior coarse desalination step, and then leave the final purification step to reverse osmosis to obtain fresh water with less energy expenditure (**Figure 2**).

However, the use of photosynthetic organisms to desalinate seawater has not advanced beyond the laboratory scale because of unresolved technical difficulties, most notably the separation of biomass from water [53]. Flocculation and sedimentation are the most economical separation technologies that take advantage of the production of extracellular polymeric substances (EPS) [53, 54]. Another approach for the efficient use of microalgae and cyanobacteria in seawater desalination is to immobilize them in natural or synthetic polymer matrices in the form of beads before introducing them into a photobioreactor, which increases the efficiency of desalination and the survival of these microorganisms [55–57].

#### **4. Microbial desalination cells**

Microbial desalination cells (MDC) are a variant of bioelectrochemical systems (BES) or microbial electrochemical systems (MES) [7, 58] designed to dilute saline water by electrodialysis simultaneously with the removal of organic matter from wastewater streams [11, 55, 59]. The basic design of an MDC usually consists of three chambers: an anodic chamber inoculated with microorganisms, desalination chamber, and cathodic chamber. An anion exchange membrane (AEM) separates the anode chamber from the intermediate desalination chamber and a cathodic exchange membrane (CEM) separates the intermediate chamber from the cathode chamber (**Figure 3**).

In principle, the desalination mechanism of MDCs depends on the electricity produced by microorganisms that decompose the organic matter of wastewater in the anode chamber and transfer the electrons produced to the anode. The electrons transferred at the anode surface travel through an external wire and an external resistor to reduce the oxidized species (electron acceptors) at the cathode. Because of the electric potential difference in the MDC, the anions (Cl− ) and cations (Na<sup>+</sup> ) in the desalination chamber move towards the anode through the AEM and towards the cathode through the CEM, respectively, as in an electrodialysis (ED) process [8, 11, 60–62]. These processes treat wastewater, produce electricity, and remove total dissolved solids (TDS) from salt water.

The microorganisms involved in the generation of electricity in bioelectrochemical systems (BES), known as exoelectrogens, are mostly Proteobacteria of the genera *Pseudomonas*, *Shewanella*, and *Geobacter*, although eukaryotes such as yeasts have also been tested [61, 63–67]. However, if natural wastewater is used, biofilms composed of consortia of various species of bacteria develop on the anode [63, 67].

#### **4.1 MDC configurations**

The first microbial desalination cell design was based on the prototype proposed by Cao et al. [11], which used potassium ferricyanide as an oxidant. However, owing

#### **Figure 3.**

*Schematic representation of a microbial desalination cell. The degradation of dissolved matter in the wastewater occurs in the anode chamber. The electrons produced by exoelectrogenic bacteria travel to the cathode chamber, where the circuit is closed. Simultaneously, seawater circulates between the two electrode chambers separated by ion-exchange membranes. As water circulates, ions are lost, which are directed to the electrode chambers owing to the potential difference between them.*

to their toxicity and cost, it is preferable to use air cathodes, but these also have limitations and require expensive catalysts [62, 68–70]. Therefore, a more economical and sustainable alternative was sought, such as cathodes built by living organisms capable of producing oxygen through photosynthesis, known as biocathodes [71–74]. Some of these MDCs with biocathodes have given rise to more complex designs known as photosynthetic microbial desalination cells (PMDCs) [75–79]. From these relatively simple models, more complex experimental configurations have been developed, the descriptions of which can be found in excellent reviews on MDC that study the desalination process in depth [62, 65, 80, 81].

Different microbial desalination cell configurations exhibit different performance and applications [7, 60, 61, 65, 80, 82]. Laboratory-scale studies have shown that stacked MDC (SMDC), osmotic MDC (OsMDC), osmotic fuel cell coupled MDC (OsMFC-MDC), microbial electrolysis desalination and chemical production cells (MEDCC), upflow MDC (UMDC), upflow stacked MDC (USMDC) and microbial reverse electrodialysis cells (MREC) are most appropriate for integration as pretreatment in larger scale reverse osmosis processes [59, 83]. The general characteristics of the most commonly used microbial desalination cells are presented in **Table 1**. More recent developments such as supercapacitive MDC (SC-MDC), quadruple MDC (QMDC), two-chamber tubular MDC (TTMDC), microbial salinity cell (MSC), MDC with integrated osmosis membranes (MDC-FO) and flow electrode MDC (FE-MDC) look promising [84–89].

Another design with perspectives is the photosynthetic MDC (PMDC), which can operate under different conditions according to the purpose pursued, maximizing desalination, wastewater treatment, energy production, or product synthesis [54, 72, 83–85]. There is another variant, the microbial salinity cell (MSC), which is


#### **Table 1.**

*Summary of the most common microbial desalination cell configurations.*

not a proper MDC [86]. In an MSC, desalination does not occur in the intermediate chamber but in the anode chamber simultaneously with the reduction in chemical oxygen demand (COD). In MSC systems, recirculation is applied to minimize pH imbalance, which is a major bottleneck in MDC systems.
