**2.2 Chemical and analysis**

All chemicals and reagents used for the experiments were analytical grades and supplied by Merck (Darmstadt, Germany). The pH meter, HANA 211(Romania) model glass-electrode was employed to measure pH values of the aqueous phase. The initial pH of the working solution was adjusted by addition of diluted HNO3 or 0.1M NaOH solutions. Dinitrosalicylic acid [3, 5(NO2)2C6H2-2OH-COONa.H2O] (DNS) method was developed to detect and measure substrate consumption using colorimetric method. Before analysis, liquid samples were filtered by a 0.45 μm syringe membrane (Sartorius Minisart).

Scan Electron Microscope (SEM): The anode electrode before and at the end of the experiment was examined by a Scanning Electronic Microscope (SEM) (Phillips XL30, Holland). Finally, images of the samples were taken under SEM at magnifications of 5000. SEM images were used to demonstrate the physical characteristics of the electrode surface and to examine the growth of yeast on the anode surface.

### **2.3 MFC**

Different kinds of MFCs were made up to investigation of mass transfer area on performance of MFC. All MFCs fabricated from Plexiglas material were used as MFCs in laboratory scale. The volume of each chamber (anode and cathode chambers) was 800 ml with a working volume of 615 ml. The sample port was provided for the anode chamber, wire point input and inlet port. The selected electrodes in MFC were graphite plates, size of 40×70×1.2mm. Proton exchange membrane (PEM; NAFION 117, Sigma–Aldrich) was used to separate two compartments. Proton exchange membrane, nafion, was subjected to a course of pretreatment to take off any impurities that was boiling for 1h in 3% H2O2, washed with deionized water, 0.5 M H2SO4, and finally washed with deionized water. In order to maintain membrane for good conductivity, the anode and cathode compartments were filled with deionized water

Effect of Mass Transfer on Performance of Microbial Fuel Cell 237

where P is generated power and E measured cell voltage; Rext denotes external resistance and I indicates produced current. The online recorded produced current and power were normalized by the surface area of the used membrane. Analog digital data acquisition was fabricated to record data point in every 4 min. Measurements were carried out at variable resistances which were imposed to the MFC. The current in the MFC was automatically calculated and recorded dividing the obtained voltage by the specified resistance. Then, the system provides power calculation by multiplication of voltage and current. The provisions were provided for online observation of polarization curve showing the variation of power density and MFC voltage with respect to current. The online system had the ability to operate automatically or manually. While it operates in auto-mode, the assembled relays were able to regulate automatically the resistances. Voltage of MFC was amplified and then data were transmitted to a microcontroller by an accurate analog to digital converter. The microcontroller was also able to send the primary data to a computer by serial connection. In addition, a special function of MATLAB software (7.4, 2007a) was used to store and display synchronically the obtained data. The power, current and voltage were

Columbic efficiency can be calculated by division of total coulombs obtained from the cell

Total coulombs are obtained by integrating the current variation over time (*Cp*), where CT is the theoretical amount of coulombs that can be produced from carbon source, calculated as

 *CT= (FbSV.M-1)* (4) For continuous flow through the system, CE can be calculated on the basis of generated

In equation (4), *F* is Faraday's constant , b the number of moles of electrons produced per mole of substrate (24 mol of electrons were produced in glucose oxidation in anaerobic anode chamber), *S* the substrate concentration, *q* flow rate of substrate and *M* the molecular weight of

In batch mode, polarization curves were obtained at steady state condition by setting an adjustable resistance in data logger. When the MFC was operated in continuous mode, the concentration of glucose in the feed tank solution was kept constant at 30 g.l-1. Several hydraulic retention times (HRT) were examined in continuous operation. The HRT was measured from the volume of medium and the inward flow rate to the anode compartment

Beside the polarity curve, cyclic Voltammeter (IVUM soft, Ivium Technology, Netherland) was also used to analyze for testing oxidation and reduction of organic materials. The potential range of -400 mV to 1000 mV was applied. The working electrode and sense

used substrate (*M*= 180.155 g.mol-1) (Allen and Bennetto, 1993; Oh and Logan, 2006).

and theoretical amount of coulombs that can be produced from glucose (Equation 3):

automatically recorded by the computer connected to the system.

current at steady state conditions as follows (Logan et al., 2006):

follows:

of MFC.

**2.4.2 Cyclic Voltammetry (CV)** 

*I=(E/Rext)* (2)

*CE= (Cp/CT)×100* (3)

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when the MFC was not in use. Neutral red and potassium permanganate were also supplied by Merck Company (Darmstadt, Germany) as mediators and oxidizer agent in continues mode, respectively. The schematic diagram, photographic images and auxiliary equipments of the fabricated MFC cell in batch and continuous systems are shown in Fig. 2. In continuous operation, the MFC was continuously fed with the prepared media in an up-flow mode using an adjustable peristaltic pump (THOMAS, Germany).

Fig. 2. Schematic diagram of cubic two chamber MFC in batch (a) and continues (b) mode
