Microbial desalination cells (MDCs), a recent technological discovery, allow for simultaneous wastewater treatment and desalination of saline water with concurrent electricity production. The premise for MDC performance is based on the principles that bioelectrochemical (BES) systems convert wastewaters into treated effluents accompanied by electricity production and the ionic species migration (i.e. protons) within the system facilitates desalination. One major drawback with microbial desalination cells (MDCs) technology is its unsustainable cathode chamber where expensive catalysts and toxic chemicals are employed for electricity generation. Introducing biological cathodes may enhance the system performance in an environmentally-sustainable manner. This study describes the use of autothrophic microorganism such as algae and Anammox bacteria as sustainable biocatalyst/biocathode in MDCs. Their great potential for high valuable biomass production combined with wastewater treatment presents these systems as a viable option to replace expensive/unsustainable catalysts for oxygen production in MDCs. Since alga is a photosynthetic microorganism, the availability of light as well as the electron-donating anodic process may have significant effects on the biocathode performance. A series of experiments evaluating these effects proved that algae perform better under natural light/dark cycles and that higher COD concentrations do not necessarily improve the power density. Furthermore, three different process configurations of photosynthetic MDCs (using Chlorella vulgaris) were evaluated for their performance and energy generation potentials. Static (fed-batch, SPMDC), continuous flow (CFPMDC) and a photobioreactor MDC (PBMDC, resembling lagoon type PMDCs) were developed to study the impact of process design on wastewater treatment, electricity generation, nutrient removal, and biomass production and the results indicate that PMDCs can be configured with the aim of maximizing the energy recovery through either biomass production or bioelectricity production. In addition, the microbial community analysis of seven different samples from different parts of the anode chamber, disclosed considerable spatial diversity in microbial communities which is a critical factor in sustaining the operation of MDCs. This study provides the first proof of concept that anammox mechanism can be beneficial in enhancing the sustainability of microbial desalination cells to provide simultaneous removal of ammonium from wastewater and contribute in energy generation.

Partial nitritation combined with the anaerobic ammonium oxidation (Anammox) process offers a way of replacing the conventional nitrogen removal process of nitrification-denitrification, lowering the need for oxygen and chemical input, as well as reducing the production of sludge. However, as a by-product of the biochemical reaction driven by anammox bacteria, it produces nitrate-nitrogen (NO3- - N) (16-26% nitrogen removed), which is problematic. Microbial desalination cells (MDCs) are a promising technology capable of converting biodegradable organics into electricity (by electroactive bacteria), providing for simultaneous desalination, and wastewater treatment. Despite being a promising technology, MDCs have limitations. The first-proof of-concept of MDC was demonstrated using acetate as the organic source, expensive platinum as a catalyst, and ferricyanide as an electron acceptor in the cathode that makes MDC costly, environmentally unfriendly, and unsustainable. This research investigated the integration of the anammox and nitration processes in MDCs as a long-term biocatalyst/biocathode for sustainable and energy-efficient nitrogen removal and electricity generation. A series of experiments were designed and performed to evaluate the performance of the anammox process as a biocatalyst in MDCs. The results concluded that the anammox process can be used as a biocatalyst to accept electrons in MDCs producing 444 mW/m3 of power density and 84% of ammonium nitrogen removal. Furthermore, the concept of using a one-stage nitritation anammox process as a biocathode in MDC was evaluated and produced a maximum power output of 1007 mW/m3. Two configurations of anammox MDCs (anaerobic-anammox cathode MDC (AnAmmoxMDC) and nitritation-anammox cathode MDC (NiAmoxMDC) were compared with an air cathode MDC (CMDC), operated in fed-batch mode. The NiAmoxMDC showed better performance in terms of power production and nitrogen removal. The co-existence of aerobic ammonium oxidizing bacteria (AOB) and anammox bacteria in the same biocathode of single-stage NiAmoxMDC concluded the resource-efficient wastewater treatment. Furthermore, two-stage nitritation anammox as a biocathode in MDC was evaluated and proved to be energy-efficient bioelectrochemical wastewater treatment by producing 1500 mW/m3 (300 mW/m2) of maximum power output. This research provides the first proof of concept that nitritation-anammox biocathode can provide a sustainable and energy-efficient nitrogen removal along with desalination and bioelectricity generation.

Global increases in water demand and decreases in both the quantity and quality of fresh water resources have served as the major driving forces to develop sustainable use of water resources. One viable alternative is to explore non-traditional (impaired quality) water sources such as wastewater and seawater. The current paradigm for wastewater treatment is based on technologies that are energy intensive and fail to recover the potential resources (water and energy) in wastewater. Also, conventional desalination technologies like reverse osmosis (RO) are energy intensive. Therefore, there is a need for the development of sustainable wastewater treatment and desalination technologies for practical applications. Processes based on microbial electrochemical technologies (METs) such as microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and microbial desalination cells (MDCs) hold promise for the treatment of wastewater with recovery of the inherent energy, and MDCs could be used for both desalination of seawater and energy recovery. METs use anaerobic bacteria, referred to as exoelectrogens, that are capable of transferring electrons exogenously to convert soluble organic matter present in the wastewater directly into an electrical current to produce electrical power (MFC and MDC) or biogas (MEC). In my dissertation, I investigated the three types of METs mentioned above to: 1) have a better insight on the effect of 4 oxygen intrusion on the microbial community structure and performance of air-cathode MFCs; 2) improve the desalination efficiency of air-cathode MDCs using ion exchange resins (IXRs); and 3) enrich for extremophilic exoelectrogens from the Red Sea brine pool using MECs. The findings from these studies can shape further research aimed at developing more efficient air-cathode MFCs for practical applications, a more efficient integrated IXRMDC configuration that can be used as a pre-treatment to RO, and exploring extreme environments as a source of extremophilic exoelectrogens for niche-specific applications of METs.