level: the increased frequency of the signal warns about a higher contamination level, while a low frequency informs about a low contamination level. Biorecovery In 2010, A. ter Heijne et al. constructed a device capable of producing electricity and reducing Cu2+ ions to copper metal. Microbial electrolysis cells have been demonstrated to produce hydrogen. Wastewater treatment MFCs are used in water treatment to harvest energy utilizing anaerobic digestion. The process can also reduce pathogens. However, it requires temperatures upwards of 30 degrees C and requires an extra step in order to convert biogas to electricity. Spiral spacers may be used to increase electricity generation by creating a helical flow in the MFC. Scaling MFCs is a challenge because of the power output challenges of a larger surface area. Types Mediated Most microbial cells are electrochemically inactive. Electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl viologen, methyl blue, humic acid, and neutral red. Most available mediators are expensive and toxic. Mediator-free Mediator-free microbial fuel cells use electrochemically active bacteria such as Shewanella putrefaciens and Aeromonas hydrophila to transfer electrons directly from the bacterial respiratory enzyme to the electrode. Some bacteria are able to transfer their electron production via the pili on their external membrane. Mediator-free MFCs are less well characterized, such as the strain of bacteria used in the system, type of ion-exchange membrane and system conditions (temperature, pH, etc.) Mediator-free microbial fuel cells can run on wastewater and derive energy directly from certain plants and O2. This configuration is known as a plant microbial fuel cell. Possible plants include reed sweetgrass, cordgrass, rice, tomatoes, lupines and algae. Given that the power is obtained using living plants (in situ-energy production), this variant can provide ecological advantages. Microbial electrolysis One variation of the mediator-less MFC is the microbial electrolysis cell (MEC). While MFCs produce electric current by the bacterial decomposition of organic compounds in water, MECs partially reverse the process to generate hydrogen or methane by applying a voltage to bacteria. This supplements the voltage generated by the microbial decomposition of organics, leading to the electrolysis of water or methane production. A complete reversal of the MFC principle is found in microbial electrosynthesis, in which carbon dioxide is reduced by bacteria using an external electric current to form multi-carbon organic compounds. Soil-based Soil-based microbial fuel cells adhere to the basic MFC principles, whereby soil