Enhancing Electrogenesis by Pretreatment of Mixed Anaerobic Sludge

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Enhancing Electrogenesis by Pretreatment of Mixed Anaerobic Sludge To Be Used as Inoculum in Microbial Fuel Cells B. R. Tiwari and M. M. Ghangrekar* Department of Civil Engineering, Indian Institute of Technology, Kharagpur−721302, India ABSTRACT: Significant Coulombic loss in microbial fuel cells (MFCs) is attributed to utilization of the substrate and electrode space by nonexoelectrogenic micro-organisms. Selective enrichment of electrogens by inoculum pretreatment could offer solution to this problem. This study evaluates the effect of inoculum pretreatments such as acid treatment (pH levels of 4.0 and 5.3, via the addition of 0.1N H2SO4), heat, aeration, and ultrasonication on the performance of MFCs. Heat, ultrasonication, and acid pretreatments at pH of 5.3 were found to have an incremental effect on the power generation. MFC inoculated with acidpretreated sludge at pH 5.3 produced a power density of 2.186 W m−3, which was ∼7 times greater than that produced by a control MFC inoculated with untreated sludge. MFC inoculated with aeration-pretreated sludge produced a slightly lower power density than control MFC. Acid pretreatment at pH of 4.0 was found unfavorable. Acid pretreatment at pH 5.3 was observed to be the most effective inoculum pretreatment, followed by ultrasonication and heat, for improving the Coulombic efficiency of MFCs. anodic chamber of MFCs.7 Since mixed anaerobic sludge is used as inoculum by many researchers, a significant amount of methane production has been reported in bioelectrochemical cells.8,9 According to Angenent et al., in natural and engineered systems where anaerobic digestion occurs, acetate acts as the main substrate for methane production via acetoclastic methanogenesis, which accounts for 70% of the methane produced and the rest is contributed by hydrogenotropic methanogenesis.10 It is reported that, in MFCs, methane production increases as the substrate concentration increases, which indicates that the organic loading rate is an important factor in methane production.11 The presence of methanogens leads to a decrease in power output of MFC as they compete with electrogens for food and space on the anode.12,13 The sludge, which is added as inoculum, is the primary microbial reserve. If methanogens can be specifically eliminated from the inoculum itself, there are greater chances that a larger part of the substrate utilizing micro-organisms would be electrochemically active. Thus, pretreatment methods for seed sludge those are capable of suppressing the activity of methanogens without affecting the activity of electrogenic micro-organisms and other fermentative micro-organisms must be employed. Physiological differences between methanogenic and electrogenic micro-organisms can be exploited for selecting the pretreatments for enrichment of electrogens in the inoculum. While treating real wastewater, even after using inoculum enriched with electrogenic microorganisms, methanogens may dominate in the anodic chamber of the MFC after long-term operation. Hence, in order to reduce loss of electrons in the anodic chamber of the MFC for a longer time of operation, it is also necessary to suppress the activity of methanogens at the periodic intervals.

1. INTRODUCTION Today’s energy intensive world is mostly dependent on the ephemeral deposits of fossil fuels to meet its ever-growing energy demand. This energy insecurity problem can be eliminated by shifting our focus towards the use of alternate sources of energy. The microbial fuel cell (MFC) is one of the novel ecofriendly technologies for tapping renewable energy from waste in the form of electricity.1,2 MFCs may supersede the traditional technologies, because they provide an additional source of fuel from pollutants, with the advantage of being nonpolluting bioelectrochemical processes to obtain electricity. Under anaerobic conditions, micro-organisms in the anodic chamber carry out oxidation of organic and inorganic matter, thereby, releasing electrons, which pass across an external circuit from anode to cathode.1 Protons move across the internal membrane, which separates anodic and cathodic chambers, toward the cathode.3 The electrons then lead to the reduction of oxygen to produce water. Electron acceptors such Mn(IV), Fe(III), etc. can also be used in the cathodic chamber.1 The sludge collected from septic tank, anaerobic reactors/ digesters, pond sediments, and microbes isolated from soils are generally used as inoculum in the anodic chambers of MFCs. Typical mixed anaerobic sludge harbors a variety of species of micro-organisms, which also includes fermentative microorganisms, sulfate reducers, and methanogens including Methanosarcinaceae and Methanosaetaceae,4,5 along with electricity-producing micro-organisms, which are generally called exoelectrogens or electrogens. A mixed culture of microorganisms enhances the capability of MFCs to accept a wider choice of feedstock, provides simplicity in operation, and makes it easier to control the system.6 Mixed cultures are well-suited for the dynamic and complex wastewater environment containing a wide range of organic molecules and other pollutants. Different micro-organisms, which can compete with the electrogens for the substrate, are capable of surviving in the © XXXX American Chemical Society

Received: December 16, 2014 Revised: April 18, 2015

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DOI: 10.1021/ef5028197 Energy Fuels XXXX, XXX, XXX−XXX

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6.0 L, where tap water was used as the catholyte with continuous aeration, by using an aquarium air pump. The anodic and cathodic chambers were separated by the wall of the pot with an approximate thickness of 10 mm (Figure 1) and an electrode spacing of 20 mm was

Since methanogens are sensitive to adverse environmental conditions, such as changes in temperature, pH, and the presence of oxygen to an extent that are unfavorable for the survival of methanogens; some stress exposure of the inoculum to acidic pH, high temperature, high oxygen concentration, ultrasonication, chemical inhibitors such as 2-bromo-ethanesulfonate (BES), chloroform, can be applied to suppress their activity and population. Exposure to heat can lead to the suppression of methanogens, because of their inability to form protective spores. Heat can be easily applied by boiling the seed sludge for a certain time period. Methanogens can be inhibited by simple aeration, because they are strict anaerobes.14 A decrease in methane production rate has been reported at pH 7.8.15,16 Thus, changing the pH of anaerobic sludge to MFC-WT > MFC-Ar > MFC-A4. The electrochemical reactions at the anode of the MFC having inoculum sludge pretreated with acid at pH 5.3 were fastest; this might be due to the effective growth of electrogenic bacterial biofilm on the anode due to better suppression of methanogens. The D

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can be obtained by suppressing the competitive methanogenic archaea by giving pretreatment to the inoculum sludge. Once the COD removal became stable, CE values were found to be 4.92% ± 0.09%, 2.26% ± 0.18%, 2.03% ± 0.08%, 1.85% ± 0.07%, 1.75% ± 0.09%, and 1.67% ± 0.05% for MFC-A5, MFCU, MFC-H, MFC-Ar, MFC-WT, and MFC-A4, respectively (see Figure 4). The CE values for all the MFCs inoculated with

Figure 4. Coulombic efficiency of MFCs inoculated after giving different pretreatment to the mixed anaerobic sludge.

pretreated sludge, except acid pretreatment (pH 4.0), were higher than the CE values for MFC inoculated with raw sludge, indicating that the substrate utilization by the electrogenic bacteria is higher in these MFCs. Among the pretreatment methods used, acid pretreatment of the sludge at pH 5.3 resulted in 181% higher CE, compared to untreated sludge inoculum. The other pretreatments such as ultrasonication and heating resulted in only 29% and 16% increases in CE, compared to untreated inoculum; however, these treatments were better than aeration and acid pretreatment at pH 4.0. The higher CE values for MFC inoculated with ultrasonication-pretreated sludge than control MFC agree with the previous experiments conducted by More and Ghangrekar.17 However, the CE value, even after the best pretreatment method, remained close to 5%, possibly due to the use of simple configuration of the MFC and noncatalyzed electrode materials. It is possible to improve CE by improving the conductivity of the electrolytes; since, in the present case, lowconductivity tap water was used as the catholyte, it resulted in a higher resistance to charge transfer. Also, there is a future possibility of using a combination of the pretreatments to enhance the CE further. 3.6. Methane Production Potential of Sludge after Different Pretreatments. Methane production potential of the inoculums after giving different pretreatments was measured for four consecutive days. The experiment was conducted in batch mode using a synthetic feed that had an initial COD concentration of 3000 mg/L, consisting of acetate as a carbon source. The methane production is a function of the methanogenic population present in the inoculums after receiving different pretreatments. The acid-pretreated sludge (pH 4.0) recorded the lowest methane production (Table 3) and the raw sludge produced the greatest amount of methane. This clearly denotes that all other pretreatments are effective in

Figure 3. (a) COD removal for each feed cycle, and (b) COD removal in MFC inoculated with differently pretreated anaerobic sludges.

can be inferred that the methanogens fraction was greater in the MFC inoculated with raw sludge without giving any pretreatment. Under stable performance, COD removal efficiencies of 81.3% ± 2.2%, 79.8% ± 1.7%, 79.0% ± 2.3%, 75.3% ± 1.7%, 74.5% ± 3.3%, and 47.8% ± 2.2% were observed for MFC-U, MFC-H, MFC-WT, MFC-A5, MFC-Ar, and MFC-A4, respectively (see Figure 3b). An acidic pH level of 4.0 might have eliminated methanogens as well as electrogens in the seed sludge, resulting in the lowest COD removal efficiency. It is reported that the growth and functioning of methanogenic archaea is seized under acidophilic conditions (pH 5.0 was observed to have a favorable effect on enrichment of electrogens and suppressing methanogens. As compared to COD removal efficiency obtained in MFC-WT, COD removal efficiency obtained in MFC-U and MFC-H is higher; whereas COD removal efficiency obtained in MFC-A5 is slightly lower. 3.5. Effect of Pretreatment of Sludge Inoculums on Coulombic Efficiency. CE is the ratio of relative coulombs resulting from microbial oxidation to overall theoretical coulombs present in the feed. Enrichment of electrogenic bacteria on the anode of the MFCs is important for obtaining higher CE values. In mixed culture inoculums, higher CE values E

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The CE values for MFCs exposed to intermittent aeration were found to change from 2.03 ± 0.08 and 2.26 ± 0.18 to 2.01 ± 0.03 and 2.23 ± 0.06 for MFC-H and MFC-U, respectively. The CE values indicate that intermittent aeration for a short duration has an insignificant effect on the performance of MFC. The obligate anaerobic methanogens, if exposed to pure O2, may attain quicker rates of killing. The microbial consortia present inside the anodic chamber of the MFC is very diverse and consists of microbes having different tolerances to changes in the external environment. Hence, the performance of MFC remained unaffected, even after aeration.

Table 3. Methane Production for Sludge with Different Pretreatments pretreatment without pretreatment heat shock aeration ultrasonication acid (pH 4.0) acid (pH 5.3)

amount of VSS used (g)

methanogenic activity (g CH4 COD/g VSS.day)

0.278

0.300

0.301 0.269 0.270 0.246 0.257

0.110 0.121 0.047 0.033 0.051

4. CONCLUSIONS Pretreatment of mixed anaerobic sludge used as inoculum seems to offer promising solutions for controlling methanogenesis in microbial fuel cells (MFCs). Acid pretreatment at pH of 5.3 is effective in enhancing the power output of the MFC by 7-fold, compared to MFCs inoculated with untreated sludge. Even ultrasonication pretreatment was found to be effective in terms of power generation and chemical oxygen demand (COD) removal, and it can offer a practical solution to enhance the power generation from MFCs. More efforts are required to increase the duration for which their effect lasts. Short-duration intermittent anodic aeration is not very effective for enriching electrogens. In the future, a combination of sludge pretreatments can also be explored for inhibiting methanogenesis in MFCs and enhancing energy harvesting in the form of direct electricity.

inhibiting methanogens. The specific methanogenic activity of acid-pretreated sludge at pH 5.3 was 6-fold lower than that of untreated sludge, indicating the effective suppression of methanogens by this pretreatment method. Reducing the anolyte pH from 7.0 to 4.9 and 5.5 reduced the production of methane in a two-chambered MFC, as reported in a previous study.13 3.7. Effect of Intermittent Aeration on Performance of MFC. To check whether the administration of aeration for a short duration to a MFC, which is already under stable operation, can increase the power generation by further inhibiting the methanogens, intermittent aeration of the anodic chamber was carried out in MFC inoculated with heatpretreated and ultrasonication-pretreated inoculums. Aeration was thought to be a practical solution, because it can be easily administered. Before the application of intermittent aeration, the average Uov value produced by the MFC was 211 ± 5 mV and 245 ± 8 mV and it changed to 213 ± 4 mV and 243 ± 6 mV after intermittent aeration for 2 min in MFC-H and 10 min in MFC-U, respectively. There was no considerable change in the average voltage before and after application of aeration in these MFCs. Short-duration aeration was neither helpful nor harmful for the current-generating electrogenic bacteria. Hence, further research is required to determine a suitable solution for suppressing methanogenesis in working MFC. One of the alternatives reported in the literature is the intermittent exposure of the anode directly to air for 30−45 min, which led to a reduction in methane production indicating inhibition of methanogenesis in a single-chamber MFC.9 BES injections, when applied intermittently in a two-chambered MFC, even at minimal doses of 0.1−0.27 mM, was found to inhibit methanogenesis and the effect lasted for the subsequent feed cycles.13 Chloroform was found to inhibit not only methanogenesis but also H2-dependent homoacetogenesis.36 The addition of such chemical inhibitory substances is also worth exploring for inhibiting methanogenesis in working MFCs to enhance electrogenesis. The difference in COD removal values before and after intermittent aeration was in the range of ±1%, which indicates that the activity of microbial population inside the MFC was not affected by the applied short-duration aeration treatment. Before the application of intermittent aeration, the COD removal efficiency was 79.7% ± 1.7% and 81.3% ± 2.2%; after the application of aeration shocks, the COD removal efficiency was determined to be 78.7% ± 0.8% and 81.4 ± 2.1% for MFCH and MFC-U, respectively. One of the reasons can be the short duration of aeration in the anodic chamber of MFC was not sufficient to stress the methanogens present in the anodic biofilm as the diffusion of oxygen probably was not optimum, and methanogens possibly remained protected within the biofilm matrix, because of the presence of facultative bacteria.32

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-3222-283440. E-mail: [email protected]. in. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The grant received from Department of Science and Technology, Government of India (File No. DST/TSG/ NTS/2010/61) to undertake this work is duly acknowledged.

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REFERENCES

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DOI: 10.1021/ef5028197 Energy Fuels XXXX, XXX, XXX−XXX