**4.3 Scale up of MDCs and integration with other desalination systems**

Scaling MDCs for use in real-life scenarios has always been challenging. However, no large-scale studies have been conducted to date [59, 62, 96]. The problems encountered in the laboratory have been expanded and multiplied by increasing the scales [100]. Therefore, the practical use of MDCs requires coupling or integration with other desalination processes [8, 90]. A diagram of the integrated system of an MDC with a reverse osmosis process is shown in **Figure 4**.

MDCs have provided good results at the experimental level when integrated with other desalination technologies [80, 83]. If MDCs are used as part of the pretreatment processes, a significant proportion of the energy used in desalination by reverse osmosis, or by other conventional desalination treatments, is reduced [7, 59, 62, 83, 101]. However, MDCs generate electricity that can be used in desalination.

The largest scale experience to date has been the Microbial Desalination for Low Energy Drinking Water (MIDES) project, accomplished by an international consortium of ten companies and research organizations from seven countries: Austria,

### **Figure 4.**

*Schematic representation of microbial desalination cell (MDC) technology integrated with seawater reverse osmosis (SWRO). In this system, pretreated seawater is processed in a wastewater-fed microbial desalination cell bioreactor. The resulting desalinated water is then subjected to conventional processes such as reverse osmosis, while brine and treated effluent are taken for final disposal.*

Germany, Hungary, the Netherlands, Portugal, Spain, and Tunisia [102]. Two pilot plants were built, composed of a stack of 15 MDC pilot units, each with an electrode area of 0.4 m2, housed in a 40-foot container with the rest of the peripheral elements, with nominal desalination rates of 4–11 L·m−2·h−1.

### **5. Conclusions**

The growing demand for fresh water, together with concerns about environmental conservation and the water-energy nexus, has led to a growth in research on sustainable desalination technologies. 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 group of living organisms selected to desalinate seawater.

The use of halophyte plants to remove NaCl from soils can be considered the first practical experience of biodesalination. Nonetheless, the use of phytotechnology to desalinate seawater is a nascent field that has not yet been used in practical applications. In any case, it should be noted that the study of mangroves has served as a basis for the development of innovative bioinspired desalination devices.

Regarding the use of microalgae and cyanobacteria, so far it has been shown to be an economical and effective technology to reduce salinity. However, this technology does not allow complete desalination of seawater, although it can serve as an effective pretreatment for reverse osmosis to improve its energy demand, recovery rate, and, therefore, make more sustainable the treatment. Additionally, microalgae can offer additional benefits, such as bioelectricity, biofuels, elimination of nutrients in water, and other products with high added value, making the desalination process more sustainable.

Finally, microbial desalination cells constitute an innovative technology capable of desalinating salt water along with wastewater treatment. Since their introduction in 2009, they have rapidly evolved in design and use, but there is still little evidence that this technology can work in large-scale plants as the sole wastewater treatment and desalination technology. In this sense, the integration of microbial desalination cells in the pretreatment process of reverse osmosis desalination seems to be the most appropriate use of this technology until their efficiency and durability problems are fully resolved.
