Abstract
The fermentative conversion of organic substrate to biohydrogen produces volatile fatty acid (VFA) rich effluents, typically a 40 % acetate and 60 % butyrate mixture. These VFA products can be used as feedstock for microbial fuel cells (MFC), to recover more energy as electricity, or microbial electrolysis cells (MEC), to recover more hydrogen. The effect of pH and temperature on hydrogen production rate in MECs from acetate using continuous flow MEC was evaluated from daily hydrogen production rates and yield per mol substrate (acetate). The highest hydrogen production rate was achieved at 850 mV, pH 5cathode amounting to 200 cm3 L(anode)-1 day-1 and H2 yield 1.1 mol / mol substrate converted to hydrogen. The temperature of 30 ± 1 oC, was found to be best for hydrogen production in the system tested, with the performance of the reactor being reduced at a higher temperature, 42 ± 4 oC and at a lower temperature of 23 ± 2 oC.Experiments on the effect of immobilized electron mediators methylene blue (MB) and neutral red (NR) on the maximum power densities (Pmax) and voltage generation from acetate were conducted. The results showed that the improvement the power generation of a MFC (with MB anode) by the factor of 2 at temperatures of 8 ± 1 oC, 23 ± 2 oC and 33 ± 2.5 oC. The highest peak power density of Pmax (MB) = 11.78 W m-3 (7.5 mA) was achieved for the MFC (MB treated anode), compared to Pmax (control, plain carbon veil) =5.3 W m-3 (5.2 mA) at 35.5oC. Neutral red however inhibited MFC performance at temperatures of 8 ± 1 oC, 23 ± 2 oC and 33 ± 2.5 oC with MFCs (NR) producing highest power density Pmax (NR) = =3.06 W m-3 (3.19 mA) at 35.5 oC.
The effect of different acetate and butyrate concentrations, along with a full substrate switch on MEC performance was assessed. Two MEC cells were operated, one containing a bioanode acclimated to acetate (AC) and another with bioanode acclimated to butyrate (BU), for 20 mmol L-1 substrate. When the substrate concentration was changed from 20 mmol L-1 to 10 mmol L-1 and to 5 mmol L-1, to acetate and butyrate mixtures (10 mmol L-1 and 10 mmol L-1) and then finally changed over from acetate to butyrate and vice versa were evaluated. The highest hydrogen production rate was observed with 20 mmol L-1 acetate amounting to 250 cm3 L(anode)-1 day-1 for the reactor (BU), when the substrate was switched from butyrate to acetate. The optimal concentration for butyrate was 10 mmol L-1 with a hydrogen production rate of 203 cm3 L(anode)-1 day-1 and H2 yield 0.5 mol / mol of substrate destroyed. These results indicate that the hydrogen yield from the acetate and butyrate present in hydrogen fermentation effluent could be used to produce hydrogen in a MEC.
The effect of four different electrode configurations on MEC performance was evaluated. Untreated carbon cloth roll (UCC) anodes; stainless steel mesh and carbon cloth roll anodes (RR); J cloth (artificial cloth made from non conductive fibers of the same as stainless steel cloth) carbon cloth roll (JC) and methylene blue treated cloth roll (MB) anodes were built. The MEC with RR anode performed best 175±5 cm3 L(anode)-1 day-1 and H2 yield 0.67 mol / mol for 20 mmol L-1 acetate. The hydrogen production decreased after several days of operation, biofilm coming off the electrode surface. MEC (UCC) had most stable hydrogen production 165±5 cm3L(anode)-1 day-1 and H2 yield 0.46 mol / mol) whilst (MEC JC and MEC (MB) produced small amounts of hydrogen 20.5±1.5 cm3 L(anode)-1 day-1 and 7.75±0.25 cm3 L(anode)-1 day-1 respectively. A design for a scaled up 19 L MEC reactor was produced from this experimental data.
| Date of Award | 2015 |
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| Original language | English |
Keywords
- Microbial fuel cells