Electricity production from municipal solid waste using microbial fuel cells

Abstract

The organic content of municipal solid waste has long been an attractive source of renewable energy, mainly as a solid fuel in waste-to-energy plants. This study focuses on the potential to use microbial fuel cells to convert municipal solid waste organics into energy using various operational conditions. The results showed that two-chamber microbial fuel cells with carbon felt and carbon felt allocation had a higher maximal power density (20.12 and 30.47 mW m^-2 for 1.5 and 4 L, respectively) than those of other electrode plate allocations. Most two-chamber microbial fuel cells (1.5 and 4 L) had a higher maximal power density than single-chamber ones with corresponding electrode plate allocations. Municipal solid waste with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as an electron acceptor improved the maximal power density to 1817.88 mW m^-2 (~0.49% coulomb efficiency, from 0.05–0.49%). The maximal power density from experiments using individual 1.5 and 4 L two-chamber microbial fuel cells, and serial and parallel connections of 1.5 and 4 L two-chamber microbial fuel cells, was found to be in the order of individual 4 L (30.47 mW m^-2) > serial connection of 1.5 and 4 L (27.75) > individual 1.5 L (20.12) > parallel connection of 1.5 and 4 L (17.04) two-chamber microbial fuel cells . The power density using municipal solid waste microbial fuel cells was compared with information in the literature and discussed.

Keywords

Municipal solid waste microbial fuel cells power density alkali hydrolysis pre-treatment carbon felt

Introduction

Biomass energy is contained in wood fibre (lignocelluloses, semi-lignocelluloses and lignin), corn, sweet beet, algae and organic waste, etc., that might be converted to methane, hydrogen, biodiesel, ethanol and electricity. Of these, organic waste, such as municipal solid waste (MSW), municipal sewage sludge (bio-solids), food waste and agricultural waste, are cited to have the most potential for extraction of bioenergy (Lee et al., 2014; Lohri et al., 2015). The organics in MSW are mostly processed by thermal treatment, composting and anaerobic digestion, yielding heat for energy production and a solid residue often used as a soil amendment to improve agricultural yields. Also, organics decomposing in MSW landfills produce methane, which is often collected and used to produce energy. Residues from thermal treatment can be used as aggregate, soil amendment or solidification after careful pre-treatment. Compost and anaerobic digestate from composting and anaerobic digestion can be further used for soil amendment in agriculture and forestry.

Recently, microbial fuel cells (MFCs) have been investigated for the production of electricity (Oliveira et al., 2013;Poggi-Varaldo et al., 2014). Current developments in organic bioenergy have focused on biofuels, such as biodiesel from algae, hydrogen from microbial electrolysis cells and electricity from MFCs. The current and power density (PD) can be affected by operational conditions, such as pH, temperature, substrate concentration, organic loading rate, hydraulic retention time (HRT), micro-organisms’ activity, parallel or serial connection and static magnetic field (Akman et al., 2013; Jadhav and Ghangrekar, 2009; Jafary et al., 2013; Li et al., 2011b). In order to enhance the PD from MFCs, various and nano-engineered electrode materials, electrode architectures and cost-effective electrodes have been considered and increasingly investigated (Gadhamshetty and Koratkar, 2012; Kumar et al., 2013; Lefebvre et al., 2013). On the other hand, separators’ (membranes) characteristics, such as resistance, diffusion and mass transfer of oxygen, proton and substrate, biofouling, pH splitting and innovative membranes, have also been intensively studied to improve the electricity generation of MFCs (Choi et al., 2012; Leong et al., 2013; Li et al., 2011a). Apart from the studies of the electrode plate and membrane, substrate pre-treatment has also been found effective in increasing the soluble chemical oxygen demand (COD), therefore enhancing the MFCs’ performance (Lee et al., 2014; More and Ghangrekar, 2010; Xiao et al., 2013; Yusoff et al., 2013).

Recently, most MFCs have been investigated using liquid substrate (Cavdar et al., 2011; Kumar et al., 2013; Sekoai and Kana, 2014; Zhang et al., 2012a) rather than solid waste, such as sludge, food waste, cattle manure, cellulose, algal biomass, straw and vegetable waste (Ge et al., 2013; Gregoire and Becker, 2012; Hassan et al., 2012; Inoue et al., 2013; Jia et al., 2013; Jiang et al., 2009; Lee and Nirmalakhandan, 2011; Mohan et al., 2010; Rashid et al., 2013; Rikame et al., 2012; Wang et al., 2012a, 2012b, 2013; Zhang et al., 2012b). MFCs have not yet been investigated by using a solid MSW substrate (total solid (TS) 6%) directly instead of using substrates of MSW leachate and/or other organic liquids or wastes.

This study aimed to investigate the effect of MSW with various pre-treatment and operational conditions, with O₂ and K₃Fe(CN)₆ as electron acceptors, on the power production of MSW MFCs with potentially cost-effective electrode plates.

Materials and methods (methodology)

MFCs

MFCs is a potentially promising technology in converting biomass to electricity while leaving the anaerobic digestate to be used as soil amendment, etc. Generally, two kinds of MFCs containing one or two chambers are used. The anodic chamber of the two-chamber MFC is anaerobic and micro-organisms decompose the organic substrate to produce electrons and protons (hydrogen ions, H⁺). Hydrogen ions go through a proton exchange membrane (PEM) to the cathodic chamber. The cathodic chamber is exposed to a solution containing electron acceptors, such as oxygen, K₃Fe(CN)₆, K₂Cr₂O₇, KMnO₄ and K₂S₂O₈, which react with e^- from the anodic chamber via copper wire and H⁺ from the anodic chamber via the PEM to complete the half reaction. In this study, MSW was used as the substrate and oxygen and K₃Fe(CN)₆ were used as electron acceptors. MFCs have the advantage of treating wastewater or bio-waste to produce electricity while converting the bio-waste to soil amendments. However, the electricity production efficiency or potentially unfriendly electron acceptors may be disadvantages in the wastewater or bio-waste treatment.

The reaction equations of the anodic and cathodic chambers in the MSW (C_38.3H₆₀O_25.63N) MFCs are as follows.

Anodic chamber using MSW as substrate:

C_{38.3} H_{60} O_{25.63} N + 50.97 H_{2} O \to 38.3 C O_{2} + N H_{3} + 158.94 H^{+} + 158.94 e^{-}

(1)

Cathodic chamber via O₂ and K₃Fe(CN)₆ as electron acceptor:

39.735 O_{2} + 158.94 H^{+} + 158.94 e^{-} \to 79.47 H_{2} O

(2)

158.94 F e {(C N)}_{6}^{3 -} + 158.94 e^{-} \to 158.94 F e {(C N)}_{6}^{4 -}

(3)

Conventional fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of electro-chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several types of fuel cells currently under development, each with its own advantages, limitations and potential applications, and can be seen in the following: polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and reversible fuel cells. The advantages and disadvantages of MFCs and conventional fuel cells can be seen in Supplementary Table S-1, available online.

MSW substrate

The synthetic organic fraction of MSW (OFMSW) was used as the substrate of the MFCs in this study. The physical and chemical properties of the synthetic OFMSW were typical of the MSW described in the following. Physically, the OFMSW was composed of newspaper (18 g, 30%), office paper (18 g, 30%), cardboard (21 g, 35%) and potato (3 g, 5%). The size of the OFMSW was cut to less than 0.1 cm and blended with 940 mL (g) distilled water to obtain a TS of ~6% (volatile solid (VS) ~4%). Accordingly, a large amount of OFMSW was prepared with this ratio for the whole experiment. Chemically, the basic characteristics of carbon, hydrogen, oxygen, nitrogen, sulphur and chlorine of the OFMSW were measured using an elemental analyser (Heraeus varioIII-NCH). Carbon, hydrogen, oxygen and nitrogen were measured to be about 46%, 6%, 41% and 1.4% (C_38.3H₆₀O_25.63N), respectively. The elemental analysis of the synthetic OFMSW was compared with the Taichung and Taiwan MSW and the basic characteristics of MSW as described by Lo et al. (2012). They all have a similar C/N ratio. Along with the original MSW, pre-treatment of MSW by alkali hydrolysis was also used for performance evaluation of the MSW MFCs. To the MSW (around 10 L, solid MSW blended with distilled water) a suitable amount of 6 N NaOH was slowly added for the alkali hydrolysis pre-treatment and blended completely with a stirrer till reaching pH 13. After standing for 24 h, a suitable amount of 1 N H₂SO₄ was added and stirred slowly for the recovery of the pH to neutral 7. This process is able to increase the soluble COD from the MSW to between ~5000 and ~20,000 mg L^-1, which is suitable for the microbial degradation and therefore enhances the MFCs performance.

Anaerobic sludge seeding

Anaerobic sludge seeding was obtained from the sludge anaerobic digester of Fu-Tien waste water treatment plant (WWTP) located at Taichung, central Taiwan. After the anaerobic sludge (24 L) was taken from the WWTP, it was left to settle for about 4 h and the settled sludge (about 12 L, VS ~3%) was distributed between six plastic reactors (each reactor, the total volume 5 L). To each reactor containing 2 L anaerobic sludge, 100 mL MSW was added daily for acclimation of the MSW. When a total volume of 4 L was achieved and stable biogas production was observed, a sludge retention time (SRT) of 20 days was operated. Each reactor had two ports, one for MSW input and digestate output for parameters’ analysis and the other for biogas product collection using the water replacement method. The basic characteristics of the sludge can be found in Lo et al. (2012). The acclimated MSW was used in the anodic chamber of the MSW MFCs in the experiment.

Experimental

Digestate output and MSW input at MFCs

First, the anodic chamber was opened and the digestate was stirred thoroughly. Digestate (75 or 200 mL) was taken out for filtrate and digestate analysis, such as measurement of pH, oxidation reduction potential (ORP), electrical conductivity (EC), COD, TS and VS for performance evaluation. After the digestate was taken out for analysis, 75 or 200 mL MSW substrate was introduced into the MFCs, and the anodic chamber was stirred to ensure homogeneity. Finally, the lid of the MFCs was sealed and the experiment was started. Supplementary Figure S-1, available online, shows the schematic of the digestate output and MSW substrate input at the MFCs.

Effect of various electrode plate allocations on MSW MFCs

Single- and two-chamber MFCs (1.5 and 4 L) were used in this experiment. The MFCs containing working volumes of 1.5 L (10 × 10 × 18 cm) and 4 L (15 × 15 × 25 cm) were acrylic reactors. For the 1.5 and 4 L single-chamber MFCs, ambient air (oxygen) exposure was used as the electron acceptor, while for the 1.5 and 4 L two-chamber MFCs (anodic and cathodic chambers with two identical 1.5 or 4 L working volume reactors), ambient air was injected into distilled water at the cathodic chamber as the electron acceptor.

Electrodes of carbon felt, stainless steel, carbon paper and carbon plate were used in five different modes of anode/cathode allocation: carbon felt/carbon felt, carbon felt/stainless steel, carbon felt/carbon paper, carbon felt/carbon plate and carbon plate/carbon plate. The carbon content of the carbon felt was around 91.83%. The size of the carbon felt, carbon plate and stainless steel was 6^width × 8^height × 0.5^thickness cm, while that of carbon paper was 6^width × 8^height × 0.018^thickness cm. They were installed at the centre between the anodic and cathodic H-type chambers. The PEM (6 × 6 × 0.0183 cm) used in this experiment was DuPontTM Nafion® 117. It was composed of a perfluorosulfonic acid (PFSA) polymer (perfluorosulfonic acid/PTFE copolymer in acid (H⁺) form, non-reinforced film). All this information can be seen in Supplementary Figure S-2, available online.

Acclimated MSW of 1.5 and 4 L was added into the anodic chamber of the MSW MFCs after the SRT of 20 days. A PEM was installed at the midpoint between the two chambers for the two-chamber MSW MFCs or exposed to ambient air for the single-chamber MSW MFCs, respectively. Ambient air was injected into the anodic chamber (filled with distilled water) as the O₂ electron acceptor for the two-chamber MSW MFCs, while it was exposed to the PEM as the O₂ electron acceptor for the single-chamber MSW MFCs, respectively. The anodic and cathodic electrode plates (five allocations as mentioned above) were connected to copper wire by an iron clip and connected with an external resistance of 1000 Ω and multi-meter (ABM 817A). Voltage (V) and current (I) could be measured by the multi-meter, which was also connected to the computer by a software installation. All the 1.5 and 4 L single- and two-chamber MSW MFCs were analysed over 2 weeks for each operational condition and were maintained in a homeostatic oven of 35 °C suitable for the anaerobic process.

The cathodic chamber of the two-chamber MFCs was filled with distilled water and was injected with ambient air at a rate of 4 L min^-1 by an air pump (K-8000) as the electron acceptor.

The electrical power (P) from the MSW MFCs can be calculated as follows:

P = I \times V

(4)

where P is power (Watt, W, joule s^-1), I is current (Ampere, A, coulomb s^-1), V is voltage (V, joule coulomb^-1). We varied the electrical resistance from 3000 kΩ to 1 Ω step by step with a resistance adjuster, then the corresponding I and V could be obtained (on the 7th day and 14th day, measured twice). Further, we calculated the P by multiplying I with V and normalised to the projected electrode area, then the PD (mW m^-2) was obtained. Once V (mV), current density (mA m^-2) and PD were obtained, the polarisation curve could then be plotted (current density as the x-axis versus V and PD as two y-axes) and the performance of the MFCs with the five different electrode plate allocations could be evaluated.

Effect of MSW with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor

The two-chamber MSW MFCs (1.5 L) with a carbon felt/carbon felt allocation were used. The pH, ORP and EC at the anodic chamber and the voltage of the two-chamber MFCs were examined. We measured these data with four different operational conditions, that is, O₂ and 0.2 M K₃Fe(CN)₆ used as electron acceptors at the cathodic chamber and MSW with and without alkali hydrolysis pre-treatment at the anodic chamber (carbon felt/carbon felt + MSW + O₂ as the electron acceptor: Scenario 1; carbon felt/carbon felt + MSW + 0.2 M K₃Fe(CN)₆ as the electron acceptor: Scenario 2; carbon felt/carbon felt + MSW with alkali hydrolysis pre-treatment + O₂ as the electron acceptor: Scenario 3; carbon felt/carbon felt + MSW with alkali hydrolysis pre-treatment + 0.2 M K₃Fe(CN)₆ as the electron acceptor: Scenario 4). The pre-treatment of the MSW with alkali hydrolysis used for the performance evaluation of the MSW MFCs was intended to increase the soluble COD of the MSW from ~5000 to ~20,000 mg L^-1, to increase the microbial degradation and enhance the MFCs performance. The cathodic chamber of the two-chamber MFCs was (a) exposed to water and injected with air at 4 L min^-1 by an air pump (K-8000) as the electron acceptor, and (b)exposed to 0.2 M K₃Fe(CN)₆ as the electron acceptor for performance comparison.

Effects of ultrasound, bottom ash and magnetic field on the PD of MFCs

Two-chamber MSW MFCs (1.5 L) with the carbon felt/carbon felt allocation and O₂ as the electron acceptor were used in this experiment with various MSW treatments: (a) MSW with arbitrary ultrasound pre-treatment (60 Hz, 50 W, 24 h); (b) MSW with the addition of bottom ash (0.2 g g^-1 MSW) as described at Lo et al. (2012); and (c) MSW with arbitrary magnetic field exposure (400 mG, magnetic pellet put inside the anodic chamber, measured by Gauss meter, TES-1390). The PD results were compared and discussed.

Effects of serial and parallel connection on the PD of MFCs

Two-chamber MSW MFCs (1.5 and 4 L) with the carbon felt/carbon felt allocation and O₂ as the electron acceptor were used again in the next experiment, using serial and parallel connections of MSW MFCs with 1.5 and 4 L two-chamber MFCs for comparison. The results of the PD performance were compared and discussed.

Analysis of variance (ANOVA) of PD and voltage of various MFCs

ANOVA analysis of voltage and PD by polarisation curves for 1.5 and 4 L single- and two-chamber MFCs with various operational conditions was examined with SPSS software. The results showed the significance of the various operational conditions on the electricity production of the MSW MFCs. A p-value of less than 0.05 was considered to be significant.

Results

Effects of various electrode plate allocations on PD and voltage of MSW MFCs

The results of the effects of the various anode/cathode allocations on the PD and voltage performance by polarisation curves are shown in Figure 1(a)–(d) and Figure 2(a)–(d), respectively.

The voltage trend over 2 weeks (336 h) can be found in Supplementary Figure S-3, available online. For the two-chamber MSW MFCs, the maximal PD (mW m^-2) was found to be in the order of 20.12 (carbon felt/carbon felt) > 7.92 (carbon plate/carbon plate) > 7.59 (carbon felt/carbon plate) > 2.63 (carbon felt/stainless steel) > 2.27 (carbon felt/carbon paper) for the 1.5 L model; while the ranking for the 4 L model was 30.47 (carbon felt/carbon felt) > 10.84 (carbon felt/carbon plate) > 7.03 (carbon felt/carbon paper) > 3.40 (carbon plate/carbon plate) > 0.21 (carbon felt/stainless steel).

For the single-chamber MSW MFCs, the maximal PD (mW m^-2) was found to be in the order of 3.90 (carbon felt/carbon plate) > 1.91 (carbon plate/carbon plate) > 0.33 (carbon felt/stainless steel) > 0.13 (carbon felt/carbon paper) > 0.009 (carbon felt/carbon felt) for the 1.5 L model; while for the 4 L model the ranking was 1.06 (carbon felt/carbon plate) > 0.42 (carbon felt/stainless steel) > 0.32 (carbon plate/carbon plate) > 0.03 (carbon felt/carbon felt) > 0.007 (carbon felt/carbon paper). The carbon felt/carbon felt allocation had a higher maximal PD than the other electrode plate allocations in the two-chamber MSW MFCs, and the 4 L two-chamber MSW MFCs were shown to have a higher maximal PD (30.47 mW m^-2) than that (20.12 mW m^-2) of the 1.5 L ones with the carbon felt/carbon felt allocation. On the other hand, the 1.5 and 4 L single-chamber MSW MFCs (Figure 1(a) and (c)) were shown to have a higher maximal PD with carbon felt/carbon plate compared with the other electrode plate allocations. In general, the higher PD was also accompanied with a higher current density, as can be seen in Figure 2(a)–(d). A comparatively higher voltage trend was shown to have a comparatively higher maximal PD, as can be seen in Supplementary Figure S-3 and Figure 1(a)–(d).

MSW MFCs with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor

The pH, ORP and EC at the anodic chamber and the voltage of the 1.5 L two-chamber MFCs with the carbon felt/carbon felt allocation were examined. We measured these data under four various operational conditions, that is, O₂ and 0.2 M K₃Fe(CN)₆ used as electron acceptors at the cathodic chamber, and MSW pre-treatment with and without alkali hydrolysis at the anodic chamber. The results showed that the pH values (Supplementary Figure S-4(a), available online) varied from 5.93 to 7.75, which is suitable for the anaerobic digestion process. The ORPs (Supplementary Figure S-4(b)) were found to be negative and higher in Scenarios 3 and 4, with values of −410 ~–316.20 mV, than those of Scenarios 1 and 2, with −300.2 ~–172.8 mV. The MSW with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor made the ORPs more negative, which could be beneficial for the anaerobic digestion process and the increase of PD. ECs (Supplementary Figure S-4(c)) remained between 1.35 and 3.57 mS cm^-1 in Scenarios 1 and 2, and were measured to be 8.44–37.0 mS cm^-1 in Scenarios 3 and 4. Scenarios 3 and 4 were thus found to have higher ECs than Scenarios 1 and 2. This was attributed to the release of Na ions from the MSW with alkali hydrolysis pre-treatment in Scenarios 3 and 4, which contributed to the EC levels. Scenarios 1, 3 and 4 had similar voltages (Supplementary Figure S-4(d)) of 0.657–0.862 V and had higher voltages than Scenario 2 (0.344–0.705 V) up to about 230 h. After 230 h, the voltages of Scenarios 3 and 4 (~0.9 V) were found to be higher than those of Scenarios 1 and 2 (~0.68 V). Higher pHs, ECs and Vs and more negative ORPs were found in Scenarios 3 and 4 with the MSW pre-treated with alkali hydrolysis and K₃Fe(CN)₆ used as the electron acceptor.

Current density versus voltage can be seen in Figure 3(a) and current density versus PD can be found in Figure 3(b). The MSW with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor enhanced the current density, as can be seen in Figure 3(a), and likewise enhanced the PD, as can be seen in Figure 3(b). The maximal PD (mW m^-2) was measured to be in the order of 1817.88 (Scenario 4) > 192.74 (Scenario 3) > 49.35 (Scenario 2) > 20.12 (Scenario 1). The PD of Scenarios 2, 3 and 4 was about 2.45, 9.58 and 90.35 times that of Scenario 1, respectively. Table 1 indicates the PD of MSW MFCs with the various operational conditions used in this study and compares the values with those reported in the literature. Supplementary Table S-2 (available online) shows the VS removal, electricity production potential and PD for the tested MFCs. Scenario 4 also showed the highest electricity potential of ~0.49% (measured coulomb/theoretical coulomb, from 0.05%–0.49%) compared with the other tested conditions. The electricity production, calculated by means of VS removal, showed that the MSW MFCs appeared to have a rather low transformation efficiency.

Table 1.

PD of MSW MFCs compared with those from the literature.

MFC	Substrate/pre-treatment	Electrode plate allocation (anode/cathode)	Electron acceptors	Maximal PD, mW m^-2, m² = electrode projected area	Maximal PD, mW m^-3, m³ = working volume	References
Single- and two-chamber (1.5/4 L)	MSW	Carbon felt/carbon felt	H₂O + Air	0.009/0.029 and 20.12/30.47	0.029/0.035 and 64.38/36.56	This study
Single- and two-chamber (1.5/4 L)	MSW	Carbon felt/stainless steel	H₂O + Air	0.327/0.419 and 2.63/0.206	1.046/0.503 and 8.416/0.247	This study
Single- and two-chamber (1.5/4 L)	MSW	Carbon felt/carbon paper	H₂O + Air	0.126/0.007 and 2.27/7.03	0.403/0.008 and 7.264/8.436	This study
Single- and two-chamber (1.5/4 L)	MSW	Carbon felt/carbon plate	H₂O + Air	3.900/1.060 and 7.59/10.84	12.48/1.272 and 24.29/13.01	This study
Single- and two-chamber (1.5/4 L)	MSW	Carbon plate/carbon plate	H₂O + Air	1.910/0.329 and 7.92/3.40	6.112/0.395 and 25.34/4.08	This study
Two-chamber (1.5 L)	MSW	Carbon felt/carbon felt	0.2 M Fe(CN)₆⁻³	49.35	157.92	This study
Two-chamber (1.5 L)	MSW with alkali hydrolysis pre-treatment	Carbon felt/carbon felt	H₂O + Air	192.74	616.77	This study
Two-chamber (1.5 L)	MSW with alkali hydrolysis pre-treatment	Carbon felt/carbon felt	0.2 M Fe(CN)₆⁻³	1817.88	5817.22	This study
Two-chamber (1.5 L)	MSW with ultrasound pre-treatment (50 W, 24 h)	Carbon felt/carbon felt	H₂O + Air	9.3	29.76	This study
Two-chamber (1.5 L)	MSW with bottom ash addition (0.2 g g^-1 MSW)	Carbon felt/carbon felt	H₂O + Air	19.16	61.31	This study
Two-chamber (1.5 L)	MSW exposure to magnetic field (400 mG)	Carbon felt/carbon felt	H₂O + Air	12.04	38.53	This study
Two-chamber (1.5 and 4 L) with serial connection	MSW	Carbon felt/carbon felt	H₂O + Air	27.75	88.80	This study
Two-chamber (1.5 and 4 L) with parallel connection	MSW	Carbon felt/carbon felt	H₂O + Air	17.04	54.53	This study
Two-chamber	Fermented food waste	Carbon felt/carbon paper coated with platinum		15		Lee et al., 2014
Two-chamber	Leachate of fermented MSW	Carbon felt/carbon cloth coated with platinum		5.9		Lee et al., 2014
Single-chamber (30 mL)	Sodium acetate, etc.	Carbon paper/graphite felt, carbon paper/carbon paper, carbon paper/stainless steel mesh	H₂O + Air	109.5, 32.7, 3.1		Zhang et al., 2012a
Single-chamber (430 mL)	Vegetable waste	Graphite plate/graphite plate	Air	57.38		Mohan et al., 2010
Tubular air cathode					230	Gregoire and Becker, 2012
Two-chamber		Graphite fibre brush/graphite granules	Fe(CN)₆⁻³		23,000–41,000	Zhang et al., 2012
Single-chamber		Graphite fibre brush/carbon cloth	Air		12,000–19,000	Zhang et al., 2012
Two-chamber		Graphite fibre brush/graphite granules	Bio-cathode		44,000–81,000	Zhang et al., 2012
Three-chamber	Sewage sludge				13,200	Zhang et al., 2012
Two-chamber	Food waste leachate		Potassium permanganate		15,140	Rikame et al., 2012
Two-chamber	Sewage sludge		Fe(CN)₆⁻³		8500	Jiang et al., 2009
Cassette-electrode	Cattle manure slurry		Air	765	16,300	Inoue et al., 2013
Single-chamber (22 mL volume liquid media)	Food waste	Graphite brush/carbon cloth	Air	~556	~18,000	Jia et al., 2013
Single, batch and SCME/single, batch and TBE	Cattle manure	Graphite brush/carbon cloth	Air	37, 93	200, 300	Lee and Nirmalakhandan, 2011
Two-chamber (100 mL)	Algal biomass and activated sludge	Carbon cloth/carbon brush	H₂O + Air	1780		Rashid et al., 2013
Two-chamber	Cellulose	Carbon paper/carbon paper	Fe(CN)₆⁻³	188		Hassan et al., 2012
Two-chamber (118 mL)	Sewage sludge	Carbon felt/carbon felt	Fe(CN)₆⁻³		38,100, 24,000, 11,700	Wang et al., 2013
Two-chamber (1 L)	Straw	Graphite plate/graphite plate	Fe(CN)₆⁻³	0.00309		Wang et al., 2012a
	MSW leachate				5900–8600	Cavdar et al., 2011
Two-chamber (0.7 L)	MSW fermentation effluent	Graphite rod	Aerobic condition	210		Sekoai and Kana, 2014

MSW: municipal solid waste; PD: power density.

Effects of ultrasound, bottom ash and magnetic field on the PD of MFCs

MSW with ultrasound pre-treatment (60 Hz, 50 W, 24 h), MSW with added bottom ash (0.2 g g^-1 MSW) and MSW with magnetic field exposure (400 mG) did not show any increase in the PD and voltages, as indicated in Figure 4(a) and (b). The maximal PD (mW m^-2) was found in the order of 20.12 (control) > 19.16 (added bottom ash) > 12.04 (magnetic field exposure) > 9.3 (ultrasound pre-treatment). These operational conditions seemed not to enhance the PD and a more suitable range of operational conditions may therefore need to be selected to examine the potential improvement of the PD performance of MSW MFCs.

Effects of serial and parallel connection on the PD of MFCs

Figure 5(a) and (b) shows the results of PD and voltages versus current density with serial and parallel connection. The maximal PD (mW m^-2) was found to be in the order of 4 L (30.47) > 1.5 and 4 L serial connection (27.75) > 1.5 L (20.12) > 1.5 and 4 L parallel connection (17.04), as shown in Figure 5(a). The maximal voltage of 4 L, 1.5 and 4 L serial connection, 1.5 L, and 1.5 and 4 L parallel connection was 618, 1433, 691.5 and 579 mV, respectively, as can be seen in Figure 5(b). Serial connection of the MSW MFCs increased the voltage, while parallel connection of the MSW MFCs reduced the PD.

Results of ANOVA analysis

ANOVA analysis of the various electrode plate allocations in the 1.5 and 4 L single- and two-chamber MSW MFCs can be found in Supplementary Table S-3, available online. For voltage, the p-value was found not to be significant (0.159) in the 1.5 L MSW MFCs with carbon felt/carbon felt allocation between the MSW pre-treated with ultrasound, added bottom ash, magnetic field exposure and control. An insignificance of 0.844 was also found between the 1.5 L single- and 1.5 L two-chamber MSW MFCs with carbon felt/stainless steel allocation.

For PD, the p-values of 0.317, 0.123 and 0.294 were found to be insignificant in the 1.5 and 4 L two-chamber MSW MFCs between the 1.5 L MSW MFCs with MSW pre-treated by ultrasound, added bottom ash, magnetic field exposure and control, and between the 1.5 and 4 L serial and parallel connection with carbon felt/carbon felt allocation. A p-value of 0.201 was found between the 1.5 L single- and 4 L single-chamber MSW MFCs with carbon felt/stainless allocation. p-values of 0.245 and 0.132 were found between the 1.5 L single- and 1.5 L two-chamber MSW MFCs and between the 1.5 and 4 L two-chamber MSW MFCs, respectively, with the carbon felt/carbon plate allocation.

All other p-values were found to be lower than 0.05, indicating the significant variation with the various electrode plate allocations used in this study, as shown in Supplementary Table S-3.

Discussion

The PD of MSW MFCs can be affected by pH, temperature, organic loading rate, SRT (or HRT), MFCs type and volume, electrode materials, electron acceptor, PEM, substrate concentration and pre-treatment (Akman et al., 2013; Jadhav and Ghangrekar, 2009; Jafary et al., 2013; Li et al., 2011a). The operation of the MSW MFCs in this study was found to be suitable after an SRT of 20 days, with neutral pH and at about 35 °C, similar to the findings of Jadhav and Ghangrekar (2009).

A higher maximal PD was found with the carbon felt/carbon felt allocation than with other anode/cathode allocations (carbon felt/stainless steel, carbon felt/carbon paper, carbon felt/carbon plate, carbon plate/carbon plate) in the 1.5 and 4 L two-chamber MSW MFCs (Figure 1(b) and (d)). This phenomenon may be attributed to the comparatively higher oxygen provision by air-pump injection. The 4 L two-chamber MSW MFCs with carbon felt/carbon felt allocation had the maximal PD of 30.47 mW m^-2, which is higher than the 20.12 mW m^-2 of the 1.5 L two-chamber MSW MFCs. It is thought that the higher MSW addition (200 mL d^-1) in the 4 L MSW MFCs removes more VSs and produces more electricity than the 1.5 L MSW MFCs (75 mL d^-1). When normalised to the same projected electrode area, the maximal PD was observed to be higher in the 4 L MSW MFCs than in the 1.5 L MSW MFCs. However, the maximal PD (mW m^-2) was found to be higher in the 1.5 L MSW MFCs (64.38) than in the 4 L MSW MFCs (36.56) when the power (W) was normalised to the working volume of the anodic chamber, as indicated in Table 1. The projected area of the electrode and the working volume are thought to have a potential effect on the maximal PD of the MSW MFCs. In addition, most of the maximal PDs in the 1.5 and 4 L two-chamber MSW MFCs were shown to be higher than those in the 1.5 and 4 L single-chamber MSW MFCs, indicating that a sufficient supply of the electron acceptor (O₂) was a key factor affecting the PD performance of the 1.5 and 4 L single- and two-chamber MSW MFCs (Figure 1).

The PD from the MSW with the alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor was shown to have the highest maximal value of 1817.88 mW m^-2 (Scenario 4), which is close to the 1780 mW m^-2 reported by Rashid et al. (2013) and higher than the others, as shown in Figure 3(b) and Table 1. Measurement precision, accuracy and calculation process of the PD can be found in Supplementary Table S-4, available online. The results of PD (210 mW m^-2) reported by Sekoai and Kana (2014) were close to the values found in this study (192.74 mW m^-2) for Scenario 3. The PD of the MSW MFCs was increased when using MSW with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor owing to the increase of soluble COD from the MSW and the higher reducing capability (electron acceptance) of the K₃Fe(CN)₆. While the commonly used electron acceptors are O₂ and K₃Fe(CN)₆, the other electron acceptors include KMNO₄, K₂S₂O₈, I^-3, Fe⁺³, SO₄^-2, NO₃^-1 and K₂FeO₄. However, environmentally friendly chemicals used as the electron acceptors need to be considered for practical operation. The pre-treatment of MSW by alkali hydrolysis (6 N NaOH, added to pH 13, 24 h) in this study may have enhanced the PD owing to the increase of soluble COD (~5000 to ~20,000 mg L^-1) from the MSW. Other pre-treatments of MSW by suitable acids, heat, sonication, O₃ and microwave may also have the potential to increase the soluble COD, resulting in a potentially higher PD (Lee et al., 2014; More and Ghangrekar, 2010; Xiao et al., 2013; Yusoff et al., 2013).

The pre-treatment of MSW by ultrasound (60 Hz, 50 W, 24 h) in this study (Figure 4(a)) did not increase the maximal PD (9.3 mW m^-2) compared with the control (20.12 mW m^-2). However, suitable ultrasound treatment (40 kHz, 5 min, 120 W) (More and Ghangrekar, 2010) was found to improve the electricity generation. The addition of bottom ash (0.2 g g^-1 VS MSW) co-digested with MSW at the anodic chamber also did not increase the maximal PD (19.16 mW m^-2; Figure 4(a)) and showed a low current density compared with the control, though its addition to anaerobic digesters has been found to improve the anaerobic performance (Lo et al., 2012). Magnetic field exposure (400 mG) appeared not to improve the maximal PD (12.04 mW m^-2; Figure 4(a)) and showed a comparatively low current density (Figure 4(b)) similar to the addition of bottom ash. However, higher magnetic field exposure (100 mT) (Li et al., 2011b) was found to increase the PD of the MFCs. These arbitrary operational conditions seemed not to enhance the PD in this study compared with the control, and a more suitable range of operational conditions may need to be selected to check the potential enhancement of the maximal PD performance of the MSW MFCs.

Serial connection of the 1.5 and 4 L two-chamber MSW MFCs increased the voltage (up to 1433 mV); however, their maximal PD (27.75 mW m^-2) ranged between 20.12 mW m^-2 (individual 1.5 L MFCs) and 30.47 mW m^-2 (individual 4 L MFCs) (Figure 5(a) and (b)). Parallel connection of the MFCs (individual 1.5 and 4 L MFCs) showed lower maximal PD (17.04 mW m^-2) and voltages (579 mV) compared with the individual 1.5 and 4 L MFCs (Figure 5(a) and (b)). The results for serial and parallel connections of the MSW MFCs were similar to those reported by Jafary et al. (2013). Examination of anaerobic digestate at the anodic chamber, including the pH, organic content, K₂O, P₂O₅, seed germination and toxicity characteristics leaching procedure (TCLP), showed it to have the potential for use as fertiliser and in soil amendment (Romero et al., 2013).

Conclusions

The maximal PD was found to be higher in two-chamber MSW MFCs (1.5 and 4 L) with the carbon felt/carbon felt allocation. Most of the two-chamber MFCs (1.5 and 4 L) showed higher maximal PD than the single-chamber MFCs (1.5 and 4 L). K₃Fe(CN)₆ as the electron acceptor and MSW alkali hydrolysis pre-treatment for 1.5 L two-chamber MFCs could increase the maximal PD to 1817.88 mW m^-2 with ~0.49% Coulomb efficiency (from 0.05%–0.49%). The maximal PD (mW m^-2) with serial and parallel connection of two-chamber MFCs was found to be in the order 4 L (30.47) > 1.5 and 4 L serial connection (27.75) > 1.5 L (20.12) > 1.5 and 4 L parallel connection (17.04).

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors are grateful for the fund granted by the National Science Council, Taiwan, ROC [No. NSC 101-2622-E-324-004-CC3].

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Supplementary Material

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Electricity production from municipal solid waste using microbial fuel cells

Abstract

Keywords

Introduction

Materials and methods (methodology)

MFCs

MSW substrate

Anaerobic sludge seeding

Experimental

Digestate output and MSW input at MFCs

Effect of various electrode plate allocations on MSW MFCs

Effect of MSW with alkali hydrolysis pre-treatment and K3Fe(CN)6 as the electron acceptor

Effects of ultrasound, bottom ash and magnetic field on the PD of MFCs

Effects of serial and parallel connection on the PD of MFCs

Analysis of variance (ANOVA) of PD and voltage of various MFCs

Results

Effects of various electrode plate allocations on PD and voltage of MSW MFCs

MSW MFCs with alkali hydrolysis pre-treatment and K3Fe(CN)6 as the electron acceptor

Effects of ultrasound, bottom ash and magnetic field on the PD of MFCs

Effects of serial and parallel connection on the PD of MFCs

Results of ANOVA analysis

Discussion

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References

Supplementary Material

Effect of MSW with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor

MSW MFCs with alkali hydrolysis pre-treatment and K₃Fe(CN)₆ as the electron acceptor