Microbial desalination cells for fresh water productiontowards the implementation of sustainable desalination technology

Abstract

The discovery of the ability of some bacteria to exchange electrons with electrically conductive materials has given rise to a new field of study called electromicrobiology. Due to the capacity and versatility of these microorganisms, a platform of technologies called Microbial Electrochemical Technologies (MET) has been developed, with the potential to offer solutions to the challenge of limited resources such as energy and water. Although the birth of electromicrobiology aspired to recover the chemical energy present in organic contaminants, two decades of research have made possible the appearance of multiple applications, including water desalination. Microbial Desalination Cell (MDC) technology is a sustainable and energyself-sufficient bioelectrochemical technology that treats wastewater, produces power, and desalinates water simultaneously in the same device without external power input. The desalination process in the MDC device is driven by the energy provided by electroactive microorganisms through the degradation of organic matter contained in the wastewater. However, implementing MDC technology on a real scale depends on overcoming its current limitations. This requires a systematic study of these devices in the laboratory to be able to further study microbial processes, technological improvements, etc. Currently, the desalination of seawater and the reuse of treated water have been proposed to alleviate the problems associated with the scarcity of water resources worldwide. Reverse osmosis (RO) is the most widely implemented desalination technology, but new approaches are still needed to decrease its high energy consumption (3-4 kWh m-3). In this regard, Microbial Desalination (MDC) technology has been proposed as a pretreatment step for RO technology to reduce desalination energy from the RO process and increase potable water production using wastewater as a renewable energy source. With this objective, the MIDES project (the research project in which this thesis has been framed) has developed the world's largest demonstration plant of MDC technology. The memory of this thesis consists of 9 chapters, 5 of which are experimental. The main results presented served as a reference and a starting point for the subsequent scale-up of the technology within the MIDES-H2020 project, where this thesis is contextualized (Research Framework). Chapter 1 (Introduction) provides an overview of the current situation of desalination technologies and the origin, rationale and state-of-the-art of METs. As a final part of this chapter, the MDC systems (mechanism, the main factors that worsen their performance) are detailed in-depth together with their state of the art. In addition, the current challenges that exist and can be overcome for implementing this technology on a real scale are presented. The objectives of this thesis, collected in Chapter 2, have systematically investigated the MDC bioelectrochemical device on a laboratory scale (described in Chapter 3) to deepen and understand the electrochemical behavior of this type of system and thus promote its use, development, design, and optimization to produce drinking water with low energy cost. One of the current challenges of this technology is the choice of the cathode reaction used in the MDC system to carry out desalination. For this reason, the first experimental chapter (Chapter 4) deals with the comparison of the operation of two MDC systems under a different cathode strategy (air cathode versus the use of liquid catholyte) for the desalination of synthetic water (brackish water and seawater). Once the performance of these systems without cathodic limitation was known, the study of the electrochemical behavior of the MDC system was carried out on a laboratory scale for the desalination of synthetic brackish water under the cathodic strategy of greater efficiency in desalination (Chapter 5). In this study, the main parameters of the MDC systems (desalinated water production and generated energy) were under different values of external resistance. These treatment and production parameters are also affected by many other factors, such as the initial concentration of the saline stream (Chapter 6), which can vary depending on the scenario where this technology can be studied/implemented. After the operation of the MDC system was studied in non-real conditions (synthetic waters), desalination was carried out operating with real waters, both residual (urban, industrial) and saline (brackish, seawater) to validate on a laboratory scale the system under real conditions (Chapter 7). One of the crucial economic and performance factors, not only in MDC systems but also in bioelectrochemical systems in general, is carbonaceous conductive materials. Electroactive microorganisms use these materials forelectron transfer. For this reason, the study of new materials that promise adhesion and electron transfer with electroactive biofilms is of great relevance in these technologies. The study shown in the last experimental chapter (Chapter 8) includes the physicochemical characterization of a commercial carbonaceous material that is subsequently activated through a CO2 and temperature treatment. The influence of the activation of the material against electroactivity by electroactive bacteria that colonize its surface was learned. Finally, a general discussion is presented to contextualize the results collected in the five experimental chapters exposed above, together with some general conclusions and future work that could be achieved in the future for the continuous development of MDC technology (Chapter 9).