Introduction
A microbial fuel cell (MFC) is a bioreactor that converts chemical energy in the chemical bonds in organic compounds to electrical energy through catalytic reactions of microorganisms under anaerobic conditions. (Allen and Bennetto, 1993; Gil et al., 2003; Moon et al., 2006; Choi et al., 2003). It MFC structure consist of an anaerobic sections of an anode (negative) electrode and a cathode (positive) electrode which is separated by a Proton Exchange Membrane (PEM). It has been recognized for numerous years that it is possible to generate electricity directly by the use of bacteria in the breaking down of organic substrates. There has been a serious interest in MFCs among academic scientists as a method to generate electricity and hydrogen from biomass without the negative net carbon emission into the environment. Applications of MFCs can be used in the breaking down of organic matter at wastewater treatment facilities. They have also been considered in the use as biosensors for biological oxygen demand (BOD) monitoring, electricity generation and Biohydrogen. On the negative side Coulombic efficiency and Power output are considerably affected by the types of microbe in the anodic chamber of the microbial fuel cell, Configuration and operational conditions. Presently, practical applications of MFCs are limited because of its power density level being low of several thousandths mW/m2. Many efforts in improving the performance, reduced construction and operating costs of microbial full cells are continually being analysed. This report presents a review on the recent and past experimental advances in research, with key emphases on the applications of using Microbial Fuel Cells (MFC).
As mentioned before the major applications of Microbial Fuel Cells (MFCs) include Biohydrogen production, Electricity generation, Wastewater treatment and Biosensor technology.
Applications
Microbial Biosensor Technology
A biosensor is an analytical device which integrates a biological recognition element with a physical transducer to generate a measurable signal proportional to the concentration of the analytes (Cunningham et al. 1998). Once the concentration of the organic element is constant, bacteria will produce a continuous electrical current. On the other hand, when there metabolism is affected due to, change in organic element concentration, the substrate consumption level will alter with the electrical current. Thus, at varying substrate concentration, microbial full cells can act as biosensors.
Biochemical oxygen demand (BOD) is a measure of the content of biodegradable organic matter in water. BOD is conventionally determined by evaluating the dissolved oxygen concentration of a water sample incubated at 20°C before and after 5 or 7 days (Greenberg et al. 1992). As a result, testing time is consumed and it also entails expert personnel to acquire reproducible outcomes. Many Alternative practises to gain quicker, precise, and simple BOD test have been investigated by numerous researchers who constructed biosensors based on DO probes and the usage of immobilized microorganisms as a biological recognition element (Liu and Mattiasson 2002). Such biosensors usually give worthy correlation among signal and BOD concentrations, however suffer from poor operating stability and a low measurement range would generated.
Microbial fuel cells containing two(2) compartments with the anode and cathode divided by a Nafion cation exchange membrane have been researched as BOD sensors in a few studies. The water to be evaluated is introduced into the anode compartment where a biofilm on the anode surface transforms organics into an electrical current. An aerated buffer mixture is introduced to the cathode compartment where oxygen is diminished to water on the cathode surface (Chang et al. 2004, Kim et al. 2003a, Kim et al. 2003b, Moon et al. 2004).
Kumlanghan et al. (2007) utilized a comparable microbial fuel cell setup, however rather of improving a bio electrochemically dynamic culture on the anode, a stable anaerobic microbial consortium was sustained in a separate reactor, then was introduced to the microbial fuel cell for every evaluation. The thought was that a new inoculum for every investigation would produce a quicker reaction than a biofilm culture developing on the anode surface. An operationally easier MFC-based was created by Di Lorenzo et al. (2009). As opposed to utilizing an aerated solution fed into the cathode compartment, a cation exchange membrane was squeezed by a gas-diffusion cathode. No catholyte pumping was obliged where oxygen reached the cathode by inactive dissemination from the air. Peixoto et al. (2011) explored a submersible microbial fuel cell (Min and Angelidaki 2008) as BOD sensor. This project evades the pumping of the anolyte; nevertheless, air must be constantly fed to the cathode (Peixoto et al. 2011).
Previous studies also showed that, charge was correlated with biochemical oxygen demand concentrations in a two-chambered structure up to 520 mg/L; nonetheless, a direct relationship could be detected just up to 206 mg/L BOD (Kim et al. 2003a). It was identified that the low coulombic yield at high concentrations was triggered by the poor buffering capability of the wastewater.
Chang et al. (2004) perceived linear correlation concerning current and biochemical oxygen demand (BOD) concentrations up to 100 mg/L. Higher concentrations could be evaluated by model fitting or by dropping the flow rate through the sensor. A low flow rate intensifies the hydraulic residence spell in the sensor which marks in conveying the steady-state BOD concentration down to a linear range. On the other hand, an amplified hydraulic retention time would similarly increase the reaction time of the BOD sensor.
Discussion
In one recent experiment, it saw to use respiratory inhibitors for the improvement of BOD sensors for wastewater that contained high concentrations of nitrate and oxygen. For the experiment the MFCs were fed with Artificial Wastewater (AWW) containing nitrate or azide. It was seen that the signal gradually decreased with increased nitrate concentration. A comparison between AWW with nitrate and without AWW was analysed and showed a current of 3.2mA with 37 mg/l of nitrate and 4.9mA without the influence of the nitrate concentration. It also showed a decrease in current with concentrations of azide. Replacing the anoxic AWW with aerated AWW also showed a decrease in current until additional concentrations of azide was added which increased the current with 2mM of azide. From these results it specified that azide would be used in complex concentrations eliminating the effects of higher redox potential electron acceptors. The use of sodium cyanide showed an increase in current generation with as little as 0.5mM concentration. Similar results were obtained using oligotrophic MFCs fed with an environmental sample i.e river water that contained nitrate and showed similar current increases when azide was added. Therefore, the use of the respiratory inhibitors is recommended for precise BOD measurement of environmental samples comprising nitrate and/or oxygen with a Microbial Fuel Cell BOD sensor.
In another, present research work was done with a very compact MFC structure, known as submersible microbial fuel cell (SMFC). It was developed by Min and Angelidaki (2008) it which the purpose of the study was to familiarize and exam the SMFC configuration as an onsite BOD 5 biosensor. The BOD 5 might be the most used assessment to determine the amount of pollutants of organic matter in water. But on the downfall it is done onsite and its time and also labour consuming. A recent online biosensor to measure BOD was established using the MFC concept (Kim et al. 2003). The traditional configuration was a mediator-less MFC, with two (2) chambers divided by a cation exchange membrane and a continuous flow of wastewater.it was concluded that the configuration was to complex and the setup was not appropriate for onsite applications. (Gil et al. 2003, Kim et al. 2003, Kang et al. 2003, Chang et al 2004, 2005, Moon et al. 2004, 2005). The biosensor was operated at room temperature of (22 ± 2 ºC) during a three (3) week period to biofilm development on the surface of the anode. The cathode chamber was constantly flushed with air (5 mL/min). The biosensor was then submerged in a glass vessel tightly and anaerobically closed with 1 L of domestic wastewater. With different concentrations of diluted wastewater, BOD 5 values of up to 78±8 mg O 2 /L which could be evaluated based on a linear relation. It was also seen that the Temperature and pH values influenced the current densities resulting in the optimum pH of 7. Therefore it was concluded that for the SMFC, no special anode chamber was required because the sensor cab be directly immersed in a wastewater waterway or an anaerobic device.

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