A microbial desalination cell (MDC) is a biological electrochemical system that implements the use of electro-active bacteria to power desalination of water in situ, resourcing the natural anode and cathode gradient of the electro-active bacteria and thus creating an internal supercapacitor. Available water supply has become a worldwide endemic as only .3% of the earth's water supply is usable for human consumption, while over 99% is sequestered by oceans, glaciers, brackish waters, and biomass. Current applications in electrocoagulation, such as microbial desalination cells, are able to desalinate and sterilize formerly unavailable water to render it suitable for safe water supply. Microbial desalination cells stem from microbial fuel cells, deviating by no longer requiring the use of a mediator and instead relying on the charged components of the internal sludge to power the desalination process. Microbial desalination cells therefore do not require additional bacteria to mediate the catabolism of the substrate during biofilm oxidation on the anodic side of the capacitor. MDCs and other bio-electrical systems are favored over reverse osmosis, nanofiltration and other desalination systems due to lower costs, energy and environmental impacts associated with bio-electrical systems.
MDC Structure
An MDC is constructed similarly to a microbial fuel cell by including two
chambers with two electrodes, an anode and a cathode, in addition to both a third chamber separated by an anion exchange membrane (AEM) and cation exchange membrane (CEM), and a peripheral, external circuit that is responsible for aerobic and anaerobic processes at each respective electrode. Organic matter from the sludge proliferates in the anode chamber and creates a biofilm that generates an electric current. The biofilm thus begins to oxidize the pollutants in the sludge by strictly adhering to the anode, freeing both electrons and protons from the bio-sludge, creating a current of atoms that are collected by the electrodes through circuit transportation. Electrical current is produced by the potential difference generated between the anode and cathode due to the aerobic nature of the cathode chamber.
Applications
Seawater desalination
MDCs are utilized in seawater desalination by primarily acting as a precursor treatment for electrodialysis (ED) due to the inefficiency in salinity removal due to biofouling and membrane scaling by the complex ion composition. Studies show that efficacy of MDC systems diminish over 5000 hours due to membrane scaling such as calcium and potassium accumulation, increasing ohmic resistance and reducing ion exchange through the membrane. However, by utilizing MDCs as a precursor treatment for electrodialysis, results show that system time is reduced by 25% and energy expenditure decreases by 45.3%. Reduction in external resistance increases desalination efficiency to as high as 74%, as demonstrated in upflow microbial desalination cells (UMDC), but increased membrane scaling on the ion exchange membranes by calcium and magnesium accumulation, resulting in a higher internal ohmic resistance and decrease in overall desalination of seawater. With the application of an osmotic MFC (OsMFC) in conjunction with the UMDC as an initial pretreatment of biosolid removal and desalination, 85% of oxygen demand and approximately 97% of salts was reduced after secondary treatment. Subsequent treatment by traditional BES systems such as electrodialysis can function as a more effective system for desalination, provisioning energy demands by the output energy obtained from the MDC pretreatment.
Brackish water desalination
As MDCs contain low electrical conductivity in the desalination chamber and additional energy is not applied to the system, electron conductive-resins are applied to improve conductivity, decrease internal resistance and increase the desalination process of brackish waters. Brackish waters are low in salinity with a high amount of total dissolved solids, which results in difficulties in maintaining strong electrical currents due to increased internal resistance in the cell. MDCs also experience problems with the saturation of ions in the anode chamber which can be combatted by utilizing a microbial capacitive desalination cell (MCDC). MCDCs are analogous to MDCs with the exception of modification to the cation membrane by the addition of activate carbon cloth, permitting the free exchange of protons across both chambers of the cell and increasing the efficiency of desalination.