- Email: [email protected]
Effects of varying the ratio of cooked to uncooked potato on the microbial fuel cell treatment of common potato waste
Science of the Total Environment 569–570 (2016) 841–849
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Effects of varying the ratio of cooked to uncooked potato on the microbial fuel cell treatment of common potato waste Haixia Du a,⁎, Fusheng Li b a b
Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan River Basin Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Mixing raw with boiled potato enhanced the electricity generation efficiency by MFC. • Mixing boiled potato in the potato feed shortened the startup time for electricity generation. • Optimum mass fraction of boiled potato existed based on columbic efficiency. • Composition of dissolved organic matter changed with time by fluorescence EEM. • Hydrolysis rate parameters were estimated by first-order sequential reaction model.
Schematic diagram illustrating the biological reactions by different bacteria in the anode chamber of MFC for treatment of the potato feed.
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 28 June 2016 Accepted 3 July 2016 Available online 8 July 2016 Editor: Simon Pollard Keywords: Vegetable waste Microbial fuel cell Electricity generation Volatile fatty acids Dissolved organic matter
a b s t r a c t The effect of varying the ratio of cooked to uncooked potato in the performance of microbial fuel cell (MFC) treating common potato waste was investigated. Four MFCs were fed with a ratio of cooked (boiled) to uncooked (i.e. waste) potato of 0, 48.7, 67.3 and 85.6%. Respectively, the columbic efficiency was estimated as 53.5, 70.5, 92.7 and 71.1%, indicating significantly enhanced electricity generation and waste degradation at an initial feedstock mixing ratio of 2/3 cooked to 1/3 uncooked potato. The hydrolysis rate parameter (estimated using a firstorder sequential hydrolysis and degradation model) increased from 0.061 to 0.191 day−1 as cooked potato was added which increased electricity generation efficiency from 24.6 to 278.9 mA/m2/d and shortened the startup time for maximum current density from 25 to 5 days. The potato slurries' chemical oxygen demand (COD) decreased by 86.6, 83.9, 84.1 and 86.3%, respectively, indicating no relationship exists between the fraction of boiled potato and the amount of COD reduction. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author at: Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan. E-mail address: [email protected] (H. Du).
http://dx.doi.org/10.1016/j.scitotenv.2016.07.023 0048-9697/© 2016 Elsevier B.V. All rights reserved.
About 1.3 billion tons of food that accounts for 32% of the total food produced for human consumption across the entire food supply chain is lost and wasted every year in the world (Gustavsson et al., 2011).
842
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
According to the UN Food Agriculture Organization (FAO), the lost and wasted food is mainly consisted of fruits and vegetables (40–50%); cereals (30%); fish (30%); and root crops, oilseeds, meat and dairy (20%). In low-income countries, food waste before consumption (including that lost during harvest, processing, distribution and retail) is predominant; while, in middle and high-income countries, food waste generated during consumption (including the discarded one during cooking and food residues after consumption) is dominating. This difference, together with the differences in the dietary culture, lead to significant differences in the composition of food waste among countries and families. Potato is widely planted, processed and consumed in the world. The world's total potato production in 2013 was about 368 million tons (FAO, 2013) and the wasted potato that possesses higher content of biodegradable organic species than other staple crops (Justin, 2014) constitutes for a considerable proportion of the total food waste generated before and during domestic and industrial consumptions. According to the published information (http://dairyknowledge.in/article/potatowaste), of the total production of the potato industry, around 12–20% is wasted. From the same information source, it is found that, in Europe alone, the produced potato pulp, a major byproduct of the potato starch industry, could reach 1 million tons every year, with only a minimal amount of which being utilized. Waste potato without proper handling is the reason for many pests (including late blight, potato leaf roll virus, bacterial ring rot and nematodes) and noisome smells. Soil deposited with cull potato is reported to be infested with such pests as powdery scab, nematodes and weed seeds. The discarded potato in soil environment can also increase greenhouse gas (GHG) emissions during its decomposition by soil bacteria (http://dairyknowledge.in/article/potatowaste). In most industrialized countries, potato waste is mainly treated together with other burnable municipal wastes through incineration. For many developing countries, however, landfill and direct dumping without any proper treatment are still prevailing. Incineration directly relates to the GHG emissions, and landfill and dumping can worsen soil and water environments. Meanwhile, directly dumping to small urban rivers and garbage collection stations nearing the residential areas is blamed for noisome smells and leachates that affect the living environment of humans in most developing countries. Microbial fuel cell (MFC) is a technology that can convert organic matter into electricity (Nandy et al., 2015). Compared to other sustainable disposal methods, such as composting, anaerobic digestion and carbonization, MFC has the advantage of directly recycling the cleanest energy (electricity) from organic waste with a lower generation level of secondary pollutions to water, soil and atmospheric environment. So far, many studies have been conducted using MFC to treat organic waste (including excess activated sludge, food waste and animal waste) (Jiang et al., 2009; Goud et al., 2011; Zhao et al., 2012). The organic waste used in these studies was mostly in the form of leachate or masticated slurries (Goud et al., 2011; Durruty et al., 2012; Li et al., 2014), with the direct use of its solid form being very limited. For potato waste, since a significant part of it from cultivation, storage and processing is in solid forms, its direct use in MFC is of great application significance. Anaerobic biological decomposition of organic solid generally involves two important processes: liquefaction (hydrolysis/acidification) and degradation. Hydrolysis is the first step that determines the overall performance of involved bioreactors. Its occurring extent can be affected not only by the composition of the organic solid (such as the constituting types and sizes) but also by the operation conditions (such as the mixing rate of raw and cooked fractions, temperatures, etc.). In a recent study (Du and Li, 2015), the authors of the current study compared the performance of MFCs fed with potato cubes of three different sizes. Decreasing the size was found effective in promoting hydrolysis and thus improving the electricity generation efficiency and the removal efficiency. Heat treatment is also reported for being capable of promoting hydrolysis of food waste (Ariunbaatar et al., 2015) and activated sludge
(Pang et al., 2015). Therefore, by addition of cooked potato into the raw one, the faster hydrolysis of the cooked one than the raw one may provide the hydrolysis products available for electrogenic bacteria from the beginning of the MFC operation, and thus shorten the startup time for electricity generation and enhance the overall facility efficiency. Taking into consideration of the reported heat treatment effect and the fact that vegetable waste includes cooked species, it is reasonable to infer that mixed feeding of the raw potato with cooked one may be another possible way to enhance the performance of MFC in treatment of solid potato waste. However, in existing literature, relevant studies can hardly be found. In addition to this, the effects of varying the ratio of the cooked one in the total potato feed have not yet been studied. Too intensified hydrolysis may promote the usage of the released organic constituents from organic solid by coexisting bacterial species besides electrogenic ones, resulting in columbic efficiency reductions. It is thus important to quantitatively investigate the effects of the mass fraction of cooked potato in the total potato feed on the overall performance of MFC. In this study, four two-chamber MFCs were operated in parallel to investigate the effects of increasing the mass fraction of boiled potato in the potato feed on the performance of MFC evaluated on the basis of electricity generation and the extent of the potato removal. For the former, the increasing rate of current density in the initial operation period was estimated for comparisons together with the estimations for columbic efficiency. The electrochemical behavior and electron discharge were also analyzed by cyclic voltammetry (CV) and extended linear sweep voltammetry (LSV). For the latter, in addition to COD and volatile fatty acids (VFAs), the changes in the composition of the organic matter were also analyzed using the fluorescence excitation emission matrix (EEM). Moreover, by simulating the time profiles of COD using a first-order sequential reaction model, the rate parameters of hydrolysis and degradation of the potato feed with different mass fraction of boiled potato were estimated and compared. The results of this systematic study were considered very useful as reference for applying MFC for electricity generation from vegetable waste. 2. Materials and methods 2.1. Solid potato for experiment use White potato (namely May Queen) harvested in Hokkaido and consumed as the major type of potato in Japan was used for this study. Compared to the Yellow potato (slightly waxy, velvety and moist) and Russet Potato (floury, dry, light and fluffy), both are also consumed throughout the world, the White potato is featured with medium starch and being slightly creamy and dense in texture (http://www. potatogoodness.com/all-about-potatoes/potato-types/). The fresh white potato was purchased from a supermarket in Gifu, Japan. A part of the purchased potato was boiled using a boiling pot normally used in families to the well-cooked level for eating by soaking raw potato into water and boiling for 30 min. The content of water and organic matter before and after boiling is shown in Table 1. It could be seen from Table 1 that boiling caused losses of water and organic matter; however, the extent of losses was small: 0.9% and 1.5%, respectively. Both the raw and boiled Table 1 The content of water and organic matter in raw and boiled potato, and the mass fraction of boiled potato in the potato feed to MFCs of this study.
Raw potato Boiled potato
Water content (%)
Organic content (%)a
84.6 83.7
90.7 89.2
Mass fraction of boiled potato (%) Wet weight Dry weight a
0 0
48.7 48.3
67.3 68.7
85.6 86.2
Organic content refers to the weight percentage of organic matter in the total solid.
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
potato was immediately cut into small cubes (length of 5 mm) for direct use in MFCs. To study the effect of increasing the mass fraction of boiled potato on the performance of MFC, the boiled potato cubes were mixed into the raw ones with the mass fraction of the boiled potato in the total potato feed being 0, 48.7, 67.3 and 85.6% (in wet weight), respectively, as shown in Table 1. 2.2. MFC configuration and operation conditions Four two-chamber MFCs assembled by Feng et al. (2010) with the total volume of 250 cm3 and the working volume of 240 cm3 for each chamber were used. For all MFCs, carbon felts, each having a length of 6 cm, width of 4 cm and thickness of 0.5 cm, were used as the anode and cathode. A cation exchange membrane (CEM, Zhejiang Qianqiu Group Co., Ltd. China) was used to separate the two chambers (Feng et al., 2010). The inlet, outlet and a sampling port were designed on both chambers and a port for aeration was designed on the cathode chamber. Titanium wire was used to connect the anode and cathode. The use of the two-chamber MFC system was made because, even if its efficiency is generally lower than single-chamber MFC, it is still used in many studies for investigation of the fundamentals of all involved reactions (Venkata Mohan et al., 2010; Feng et al., 2010); and also because the cathode chamber can be effectively used for simultaneous removal of some substances through either oxidative reactions via aerobic bacteria or reductive reactions via electrons transferred from the anode chamber, such as ammonium and its oxidized product of nitrate (Virdis et al., 2010; Du et al., 2015). Anaerobic bacterial consortia were collected from MFCs operated in our previous study (Du and Li, 2015). Inoculation was conducted using 20 mL of the collected bacterial consortia in 100 mM/L phosphate buffer solution (PBS) containing both trace minerals and vitamins (Du and Li, 2015). After inoculation, the bacterial consortia were cultured using sodium acetate until current density reached stable. Subsequently, the four mixed samples of raw and boiled potato cubes were added to the anode chamber of the MFCs. The added weight of potato for each MFC was 2.5 g in wet, which corresponded to an initial total COD loading of 1830 mg/L. All four MFCs were operated for 52 days with 100 Ω of external resistance under the controlled temperature of 30 °C. Every four to six days, pH of the anodic solution was adjusted to keep it at 7.0–7.1 using 1N-NaOH or 1N-HCl. The cathode chamber was fed with the PBS solution and aeration was conducted continuously with a power air pump (power: 2.5 W, air flow: 2 L/min). The schematic diagram of the experiment setup is shown in Fig. 1. 2.3. Measurements and analysis Voltage was measured every minutes using a digital multimeter and a data acquisition system (midi LOGGER GL200A, Graphtec Corporation, Japan), and the data obtained were plotted using averages of every
843
5 min. Current density (I) was calculated according to I = V/(R × A), where, V is the voltage, R is the external resistance and A is the surface area of the anode. When the current output reached stable, the anode potential was analyzed by changing the external resistance from 50 Ω to 30,000 Ω. The electron discharge pattern was evaluated by studying the CV and LSV using a potentiostat system (EC stat-100, EC Frontier, Japan). All the electrochemical measurements were done by using the anode and cathode as the working and counter electrodes against the saturated Ag/AgCl reference electrode. CV was evaluated over a voltage range of −0.6 to +0.6 V at a scan rate of 10 mV/s and LSV over the same voltage range of −0.6 to +0.6 V. In order to compare the availability of potato for electricity generation, the columbic efficiency was calculated according to: Z Columbic efficiency ¼
8
t 0
I dt
F V ΔCOD
ð1Þ
where, F is the Faraday constant, I is the harvested current, V is the volume of anodic solution, ΔCOD is the removed COD and 8 is the mass of oxygen per electron (Logan, 2007). The total content of organic matter solubilized to the liquid phase as a result of hydrolysis of the potato cubes was quantified using COD measured by the colorimetric method (DR/890 Colorimeter) for all sampled anodic solution after filtration through 0.2 μm membrane filter. In addition to COD, the following volatile fatty acids (VFAs) inside the filtered anodic solution were also quantified using a high performance liquid chromatograph (Shimadzu Co., Japan): citrate, isobutyrate, acetate, propionate, butyrate, valerate and isovalerate. The quantification of these VFAs was made in order to further trace the hydrolysis and degradation behavior of potato. Using the filtered samples, the characteristics of dissolved organic matter in the anodic solution was also analyzed on the basis of fluorescence EEM with a spectrofluorometer (RF-5300, Shimadzu Co., Japan). The excitation and emission wavelength was set within the range of 220–400 nm and 280–480 nm, respectively. 2.4. Estimation of the first-order hydrolysis and degradation rate parameters As shown later from the concentration profiles of COD, the electricity generation by MFC with solid potato is accomplished by the process of potato consumption that includes the combined reactions of hydrolysis and degradation. This process could be described by the following firstorder sequential reaction model: dC ¼ kh C a −kd C dt
ð2Þ
dC a ¼ −kh C a dt
ð3Þ
Fig. 1. The Schematic diagram of the experiment setup.
844
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
Fig. 2. Effect of increasing the mass fraction of boiled potato on the current density of MFCs treating potato cubes (the mass fraction of boiled potato: 0, 48.7, 67.3 and 85.6% in wet weight) (‘↑’ indicates pH adjustment).
where, C is the soluble COD concentration in the anodic solution at time t, Ca is the concentration of the net hydrolysis product of the potato solid, kh is the rate parameter of hydrolysis and kd is the rate parameter of degradation. Given the initial COD for hydrolysis as Ca0, the following solution could be obtained to describe the concentration changes of COD inside the anodic chamber. From the total potato feed to each MFC, Ca0 was determined as 1830 mg/L in this study. C¼
kh C a0 −kh t e −e−kd t kd −kh
ð4Þ
kh and kd were searched by minimizing the differences between the calculated COD by Eq. (4) and the observed COD according to the following definition: C iðobsÞ −C iðcalÞ 1 Xn Er ¼ i¼1 n C iðobsÞ
!2 ð5Þ
where, Ci(obs) and Ci(cal) are observed and calculated COD concentration, respectively, and n is the total number of samples collected for COD measurement of each MFC. 3. Results and discussion 3.1. Electricity generation efficiency 3.1.1. Current density The observed time profiles of current density are shown in Fig. 2. The maximum current density reached about 160, 180, 254 and 243 mA/m2 for all four MFCs with 0, 48.7, 67.3 and 85.6% of boiled potato, respectively. Increasing the mass fraction of the boiled potato was also found effective in shortening the time needed for the current density to
reach its maximum level (referred hereafter as “peak time”). As could be seen from Fig. 2 that, compared to the peak time (about 25 days) needed for the MFC with the raw potato only (0% of boiled potato), the peak time (about 5 days) for the MFC with 85.6% of boiled potato was markedly shortened. The marked shortening of the peak time was due to the improved electricity generation efficiency in the initial stage of the MFC operation. Through linear regression analysis of the number of data that led to the largest correlation coefficient, the increasing rate of current density in the initial stage was estimated. As shown in Table 2, for all four MFCs with 0, 48.7, 67.3 and 85.6% of boiled potato, respectively, the estimated increasing rate values were 24.6, 97.0, 120.7 and 278.9 mA/m2/d, indicating clearly that, by increasing the mass fraction of the boiled potato, the startup time needed for MFC to reach its stable stage for electricity generation could be effectively shortened. The obvious differences in the maximum current density, the peak time and the increasing rate of the initial current density may indicate that, for practical applications, mixed feeding of raw and cooked potato is a likely approach for enhancing the overall performance of MFCs. Even within the period where current density reached higher levels, frequent drops in the current density were also observed. The drops were induced by the falling of pH occurring inside the anodic solution of each MFC and were effectively recovered by pH adjustment. Hydrolysis of organic solid during anaerobic fermentation generally leads to formation and accumulation of volatile fatty acids (VFAs) (Jiang et al., 2009). This was also observed in the study (data to be shown later) and was probably the major reason responsible for the observed pH drops. For all four MFCs, after 30 days, current density revealed a trend of gradual decreases following the consistent consumption of organic species that can be readily used for electricity generation (Venkata et al., 2010). This could be explained by the time profiles of COD and VFAs shown later in Fig. 5. 3.1.2. Anode potential The profiles of the anode potential determined against a saturated Ag/AgCl electrode are depicted in Fig. 3. For all four MFCs, within the examined external resistance range (50–30,000 Ω) the anode potential increased as the external resistance increased. The anode potential of the four MFCs with 0, 48.7, 67.3 and 85.6% of boiled potato recorded at 30,000 Ω reached −154, −337, −365 and −387 mV, respectively, revealing a trend of increases with the increases of the boiled potato. Such a trend was probably due to the increased availability of potato for electrogenic bacterial species. A trend of generally lower anode potential was associated with the MFC fed with the raw potato alone (0% of the boiled potato). This thus suggests that the overall availability of the raw potato for electrogenic bacteria was lower. Moreover, it was also noticed that the anode potential increased rapidly as the external resistance increased within the range below 4000 Ω. This implies that more effective electron discharge may occur below this external resistance (Goud et al., 2011). 3.1.3. CV and LSV The electrochemical behavior of MFCs was further evaluated using CV that measures the potential differences across the interface as well
Table 2 The initial increasing rate of current density and the estimated rate parameters for hydrolysis and degradation of different potato feeds. Parameters
Mass fraction of boiled potato (%) 0
rc kh kd
24.6 0.061 0.068
48.7 a
0.996 0.124b
97.0 0.086 0.056
67.3 a
0.991 0.107b
120.7 0.169 0.053
85.6 a
0.993 0.107b
278.9 0.191 0.079
rc: initial increasing rate of current density (mA/m2/day), kh: first-order hydrolysis rate parameter (day−1), kd: first-order degradation rate parameter (day−1). a Linear correlation coefficient. b The difference between calculated and observed COD concentrations defined by Eq. (5).
0.999a 0.151b
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
845
the mass fraction of the boiled potato on the electrochemical activity and power output of the MFCs. The LSV is used to delineate the efficiency of biocatalysts during operation (Srikanth and Venkata, 2012). For the compared four MFCs, the profiles of LSV were also generated. As shown in Fig. 4b, observable differences existed as the mass fraction of the boiled potato differed. The highest oxidation current (2.4 mA) was found in the MFC with 85.6% of boiled potato, followed by that in the MFC with 67.3% (1.9 mA), 48.7% (1.7 mA) and 0% (0.7 mA) of boiled potato. The gradual decreases of the oxidation current may indicate corresponding decreases in the electron discharge efficiency. Regarding the reduction current, the MFC with 48.7% of boiled potato showed the highest value (− 8.0 mA), followed by the MFCs with 0% (− 5.3 mA), 85.6% (−1.1 mA) and 67.3% (−0.1 mA) of boiled potato.
Fig. 3. Effect of increasing the mass fraction of boiled potato on the anode potential of MFCs treating potato cubes measured by varying the external resistance in 5–30,000 Ω when current density was stable (the mass fraction of boiled potato: 0, 48.7, 67.3 and 85.6% in wet weight).
as the redox of the components involved in the biochemical system (Jayarama, 1986; Nandy et al., 2015). As depicted in Fig. 4a, observable differences appeared as the mass fraction of the boiled potato differed. Among the four MFCs, the one with 85.6% of boiled potato showed the maximum oxidation and reduction current (38.9 mA and −20.1 mA); followed by the MFCs with boiled potato of 67.3% (35.0 mA and − 20.6 mA), 48.7% (33.1 mA and − 12.7 mA) and 0% (4.1 mA and −4.8 mA). The oxidation and reduction of components during the potential sweep causes the appearance of current peaks on the CV (Srikanth and Venkata, 2012). Distinct irreversible peaks were found on the sweeps with 48.7% and 67.3% of boiled potato (the peak potential was 0 V and −0.4 V, respectively). For the MFCs with 0% and 85.6% of boiled potato, irreversible oxidation peaks were also found (0.3 V and 0 V, respectively). All these evidenced the positive effect of increasing
3.1.4. Columbic efficiency Based on the observed current density shown earlier in Fig. 2 and the profiles of COD to be shown later in Fig. 5, the columbic efficiency of the four MFCs was estimated as 53.5, 70.5, 92.7 and 71.1%, with the smallest one being associated with the feed with only raw potato (i.e., 0% of boiled potato) and the largest one with the sample with 67.3% of boiled potato. The values of the latter three (70.5, 92.7 and 71.1%) were higher than those of MFCs treating raw potato with different sizes (Du and Li, 2015), mixed food waste of breakfast, lunch and dinner (Jia et al., 2013) and sweet potato wastewater (Cai et al., 2010), as summarized in Table 3. 3.2. Potato removal efficiency 3.2.1. Concentration changes of COD The time profiles of COD in the liquid phase of the anodic chamber are shown in Fig. 5a. For all four MFCs, COD revealed a similar trend: the concentration increased in the initial period and then gradually decreased. Such a trend is not typical and could be observed in anaerobic treatment of a variety of organic solids, such as activated sludge, food waste and vegetable waste (Ramdani et al., 2012; Grimberg et al.,
Fig. 4. Cyclic voltammograms (a) and extended linear sweep voltammetric profiles (b) of the anode of MFCs treating potato cubes with different mass fraction of boiled potato (0, 48.7, 67.3 and 85.6% in wet weight) when current density was stable.
846
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
Fig. 5. Effect of increasing the mass fraction of boiled potato on the behavior of COD (a) and VFAs (b) in the anodic solution of MFCs treating potato cubes with different mass fraction of boiled potato (0, 48.7, 67.3 and 85.6% in wet weight).
2015; Zuo et al., 2015). The observed COD increases indicated the releasing rate of organic matter from the added potato cubes due to hydrolysis was faster than the rate of degradation; while the followed COD decreases indicated the rate of degradation exceeded the rate of hydrolysis or that the reaction of hydrolysis had already stopped.
3.2.2. Hydrolysis and degradation rate parameters The first-order hydrolysis (kh) and degradation (kd) rate parameters estimated by fitting calculated COD concentrations with observed ones, as displayed in Fig. 5a, are summarized in Table 2. The estimated kh increased in the range of 0.061–0.191 day−1 as the mass fraction of the boiled potato increased from 0% to 85.6%, confirming that mixing boiled potato effectively elevated the overall availability of the potato feed for bacterial use. For the degradation rate parameter kd, the estimated values (0.053–0.079 day−1) differed within a range narrower than that of kh, suggesting the effect of increasing boiled potato on degradation was comparatively smaller than the effect on hydrolysis. It is also interesting to see that, compared to the increasing extent for the stimulated hydrolysis rate parameter of COD (2.9 times as the mass fraction of boiled potato increased from 0% to 85.6%), the increasing extent for the estimated rate of initial current density increases was more significant (11.3 times). This may thus suggest that the intensified hydrolysis could greatly improve the performance of MFCs and may also suggest that the increased hydrolysis products were more effectively used by electrogenic bacterial species than by ordinary anaerobic heterotrophic bacteria. The effective use of the hydrolysis products by electrogenic bacteria contributes to enhanced conversion of potato waste to cleanest energy (electricity), and the extent of adversary effects to water, soil, atmospheric and biospheric environments as well as humans, often accompanied with other disposal methods (such as landfill, composting, anaerobic digestion and carbonization), can be greatly alleviated. At the end of the operation of all four MFCs, the total use/consumption of the potato feed assessed by COD reached 86.6, 83.9, 84.1 and 86.3%, respectively; indicating clearly that increasing the fraction of boiled potato did not affect the final overall use/consumption of the potato fed to the MFCs. The obtained COD reductions were also higher than the documented ones with composite vegetable waste (Venkata et al., 2010) and composite food waste (Goud et al., 2011), as compared in Table 3. For the MFC with 0% of boiled potato, the observed COD reached its peak value after 22 days. For the three MFCs with 48.7, 67.3 and 85.6% of boiled potato, however, the time needed was shortened to 16, 12 and 8 days, respectively. The observed effect was a combined result of the effect of mixing boiled potato on hydrolysis and that on degradation. If the effect on hydrolysis alone was isolated for evaluation, the effect of the mixed feeding of raw potato with boiled one was found more prominent, particularly when the mass fraction of boiled potato increased from 0% to 67.3%, as illustrated by the estimations based on Eq. (3) and the first-order hydrolysis rate parameter kh given in Table 2 that:
Table 3 Comparison of the performance of MFCs of this study with previous ones treating different vegetable and food wastes. Substrate type
Substrate form
Mixed raw and boiled potato
Mass fraction of boiled potato (%) Two-chamber 0 48.7 67.7 85.6 Solid cubes with edge length (mm) Two-chamber 3.0 5.0 7.0 Liquid of masticated vegetable waste Single-chamber after removing pulp
Fresh potato
Composite vegetable waste
Composite food waste Mixed food waste of breakfast, lunch and dinner (in volume ratio of 1:2:2) Sweet potato wastewater
MFC type
Initial COD (mg/dm3)
COD removal (%)
Highest current density (mA/m2)
Columbic efficiency (%)
1830 1830 1830 1830
86.6 83.9 84.1 86.3
160.1 181.0 253.9 243.3
53.5 70.5 92.7 71.1
1830 1830 1830 700–2100
88.0 88.5 91.8 62.9
189.1 178.9 163.3 87.1–160
63.9 58.6 51.5 –
Present study
mg/dm3/day
Filtrate of masticated food waste
Single-chamber 1000–2600
44.3–64.8 211–390
–
Filtrate of masticated food waste
mg/dm3/day Single-chamber 2000–4900
77.2–86.4 –
23.5–27.0
Single-chamber 625–10,000
92.2
–
10.1–28.9
Filtrate of fermented sweet potato wastewater after removing ethanol
Du and Li (2015)
Venkata Mohan et al. (2010) Goud et al. (2011) Jia et al. (2013) Cai et al. (2010)
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
847
Fig. 6. Fluorescence EEMs of dissolved organic matter in the anodic solution of MFCs treating potato cubes with different mass fraction of boiled potato (0, 48.7, 67.3 and 85.6% in wet weight) at (1) the initial starting period (day 0); (2) the period with the highest current density; (3) the end of the MFC operation.
the estimated time lengths for the potato feed with the boiled fraction of 0, 48.7, 67.3 and 85.6% to be hydrolyzed for 50% (i.e., Ca/Ca0 = 0.5) are 11.4, 8.1, 4.1 and 3.6 days; and those for 95% (i.e., Ca/Ca0 = 0.95) are 49.1, 34.8, 17.7 and 15.7 days, respectively. Further increasing the boiled potato fraction from 67.3% to 85.6% also shortened the time needed for hydrolysis to occur; however, the extent of shortening was less significant as compared to the range of increases in the boiled potato fraction from either 0% to 48.7% or 48.7% to 67.3%. The estimated results, together with the observed profiles shown in Fig. 5, clearly indicated that mixing raw potato with boiled one could promote the occurrence of hydrolysis and that increasing the mass fraction of the boiled one could further accelerate the hydrolysis of potato. This result supports previous findings on the effect of heat pretreatment on anaerobic fermentation of composite food waste (Ariunbaatar et al., 2015) and activated sludge
(Pang et al., 2015) that heat treatment increased solubilization and/or hydrolysis of organic solids, leading to increases of soluble COD concentrations and their availability for anaerobic microorganisms. The effect of heat treatment is probably due to its capability to soften the potato cells through solubilization and release of pectin in the middle lamella (Vreugdenhil et al., 2007). 3.2.3. Concentration changes of VFAs Following the occurrence of hydrolysis, VFAs were generated. Among the seven VFAs (citrate, isobutyrate, acetate, propionate, butyrate, valerate and isovalerate) analyzed in this study, four (citrate, isobutyrate, acetate and propionate) were detected, as plotted in Fig. 5b in the logarithmic scale. Citrate, being also a major constituting organic acid in the potato tissue (Laties, 1967), was detected at higher
Table 4 Fluorescence intensity of all detected peaks in the fluorescence EEM of samples collected at the initial starting period, the period with the highest current density, and the end of the MFC operation. Mass fraction of boiled potato (%)
The initial starting period (day 0)
The period with the highest current density
The end of the MFC operation
0 48.7 67.3 85.6 0 48.7 67.3 85.6 0 48.7 67.3 85.6
Fluorescence intensity (QSU)a Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
28.2 16.8 18.3 23.2 64.9 74.4 62.2 55.1 15.4 15.4 16.7 14.4
46.2 29.6 33.9 42.5 103.0 117.4 101.6 93.7 24.3 23.7 23.4 23.4
146.1 117.3 122.5 131.0 492.2 436.2 321.7 245.4 197.9 146.9 168.0 131.0
135.9 98.2 108.5 113.2 422.6 518.8 401.0 292.7 109.0 95.0 107.2 78.0
– – – – 290.2 292.3 265.5 231.8 109.2 101.0 107.0 92.8
a Peak 1 (Ex/Em = 290/380): humic acid type substances; Peak 2 (Ex/Em = 250/369) and Peak 5 (Ex/Em = 280/310): large molecular weight peptides and proteins (microorganism related by-products); Peak 3 (Ex/Em = 220/298): lower molecular weight tyrosine-like aromatic amino acids; Peak 4 (Ex/Em = 220/352): lower molecular weight aromatic proteins and BOD-type substances.
848
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
concentrations (220–340 mg/L) throughout the whole operation. Acetate, propionate and isobutyrate were detected only in some periods of the whole operation, with their concentrations being below 7 mg/L. Acetate is generally considered easier for biological degradation and was only detected in the period where higher current density was recorded as shown earlier in Fig. 2. The time when acetate was detected seemed to be earlier for the MFCs treating mixed raw and boiled potato than the sample of raw potato alone, reflecting the effect of intensified hydrolysis on the generation of acids for bacterial use. Although the net generation and consumption of acetate and propionate were not known, the observed profiles confirmed that their degradation occurred in all four MFCs. 3.2.4. Composition changes evaluated by fluorescence EEM The changes of soluble organic constituents in the anodic solution of the MFCs were also analyzed by three-dimensional fluorescence EEM (Jiang et al., 2010; Xiao et al., 2013). To interpret the changes, the fluorescence spectra of the raw and completely boiled potato, and the market-available standard reagents of the four VFCs detected in this study were also measured. As shown in Fig. 6, at the beginning of the operation (day 0), four peaks (Peak 1–Peak 4) corresponding respectively to the Excitation/Emission wavelengths (Ex/Em) of 290/380 nm, 250/ 369 nm, 220/298 nm and 220/352 nm appeared in all four MFCs. Peak 4 (Ex/Em = 220/352 nm), which is characterized as aromatic proteins (Chen et al., 2003), was also found in the raw and completely boiled potato, as well as in the individual standard solutions of the four marketavailable VFAs (data not shown). When each MFC was operated till the time point when current density reached its peak value, a new peak (Peak 5) (Ex/Em = 280/310 nm) appeared in the anodic solution and the fluorescence intensity of the four peaks existed from the beginning (day 0) obviously increased, as could be seen from Table 4, in which the fluorescence intensity of all detected peaks are summarized. The emerge of this new peak reflected the release of some dissolved organic constituents during hydrolysis of the potato solid and was probably corresponding to the larger molecular compounds of peptides and proteins (byproducts of microorganisms) (Reynolds and Ahmad, 1997; Jiang et al., 2010). The fluorescence intensity of the peaks slightly decreased as the mass fraction of boiled potato increased (Table 4). This probably indicated that some organic constituents were released and lost during boiling of the potato, and that the overall availability of the organic constituents detected by a spectrofluorometer was getting higher in the MFCs fed with boiled potato. The former indication could be supported by the slight decrease of the organic content (by 1.5%) for potato after boiling shown earlier in Table 1. The latter could be explained after referencing to literature reports by the followings: each fluorescence peak was a mixture of different organic constituents, with some being easier for degradation and some being difficult (Chen et al., 2003); and boiling accelerated the release and formation of easily degradable organic constituents and thus led to faster overall concentration decreases due to their degradation by bacteria (Ariunbaatar et al., 2015; Pang et al., 2015). At the operation end of all four MFCs, the fluorescence intensity of all peaks dropped to low values close to those at the beginning of the operation, indicating clearly that the emerged organic species were effectively degraded, particularly for the organic constituents reflected by Peak 1 - Peak 4. The peak at Ex/Em of 220/298 nm (Peak 3) showed the same tendency with the fluorescence spectra of the standard reagent of citrate (data not shown), verifying the observed result for VFAs (Fig. 5b) that citrate remained at the end of the operation. 4. Conclusions Increasing the ratio of boiled potato in the potato feed effectively promoted the occurrence of hydrolysis, elevated the maximum current density and shortened the time needed for the current density to reach the maximum. The estimated columbic efficiency suggested the
existence of an optimum mass fraction of the boiled potato since columbic efficiency decreased from 92.2% to 71.1% as the mass fraction further increased from 67.7% to 85.6%. Comparisons with previous studies indicated that the electricity generation efficiency with potato was generally higher than ordinary mixed food wastes and sweet potato wastewater. The results of this study demonstrated the capability of MFC in realizing simultaneous potato waste reduction and electricity generation, and the significant effects of mixed treatment with cooked potato; and thus suggested that MFC may serve as a more beneficial treatment method than other sustainable treatment options to water, soil, atmospheric and biospheric environments as well as humans. The findings obtained can also serve as important reference for further studies needed to identify the changes of the microbial structure (in both density and species) in the anode chamber, and to clarify the impacts of such important operation factors as the feeding concentrations, temperatures and mixed treatment with other types of organic waste. Verification experiments using larger reactors are also needed before MFC can be adopted for practical applications.
References Ariunbaatar, J., Panico, A., Yeh, D.H., Pirozzi, F., Lens, P.N., Esposito, G., 2015. Enhanced mesophilic anaerobic digestion of food waste by thermal pretreatment: substrate versus digestate heating. Waste Manag. 46, 176–181. Cai, X.B., Yang, Y., Sun, Y.P., Zhang, L., Xiao, Y., Zhao, H., 2010. Electricity generation from sweet potato fuel ethanol wastewater using microbial fuel cell technology. Huan jing ke xue = Huanjing kexue/[bian ji, Zhongguo ke xue yuan huan jing ke xue wei yuan hui" Huan jing ke xue" bian ji wei yuan hui.] 31, 2512–2517. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K., 2003. Fluorescence excitation-emission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 37, 5701–5710. Durruty, I., Bonanni, P.S., Gonzalez, J.F., Busalmen, J.P., 2012. Evaluation of potato-processing wastewater treatment in a microbial fuel cell. Bioresour. Technol. 105, 81–87. Du, H.X., Li, F.S., 2015. Size effects of potato waste on its treatment by microbial fuel cell. Environ. Technol. 37, 1305–1313. Du, H.X., Li, F.S., Yu, Z.J., Feng, C.H., Li, W.H., 2015. Nitrification and denitrification in twochamber microbial fuel cells for treatment of wastewater containing high concentrations of ammonia nitrogen. Environ. Technol. 37, 1232–1239. FAO, 2013. Food Wastage Footprint: Impacts on Natural Resources. FAO, Rome (ISBN 97892-5-107752-8). Feng, C.H., Ma, L., Li, F.B., Mai, H.J., Lang, X.M., Fan, S.S., 2010. A polypyrrole/anthraquinone-2,6-disulphonic disodium salt (PPy/AQDS)-modified anode to improve performance of microbial fuel cells. Biosens. Bioelectron. 25, 1516–1520. Goud, R.K., Babu, P.S., Venkata, M.S., 2011. Canteen based composite food waste as potential anodic fuel for bioelectricity generation in single chambered microbial fuel cell (MFC): bio-electrochemical evaluation under increasing substrate loading condition. Int. J. Hydrog. Energy 36, 6210–6218. Grimberg, S.J., Hilderbrandt, D., Kinnunen, M., Rogers, S., 2015. Anaerobic digestion of food waste through the operation of a mesophilic two-phase pilot scale digester-assessment of variable loadings on system performance. Bioresour. Technol. 178, 226–229. Gustavsson, J., Cederberg, C., Sonesson, U., Otterdijk, R.V., Meybeck, A., 2011. Global Food Losses and Food Waste: Extent Causes and Prevention. United Nations, Food and Agriculture Organization, Rome, Italy. Jayarama, R.S., 1986. Studies on Electrode Processes by Cyclic Voltammetry, Chronoamperometry, Chronopotentionmetry and Controlled Potential Coulometry. Sri Venkateswara University, Tirupati. Jia, J., Tang, Y., Liu, B.F., Wu, D., Ren, N.Q., Xing, D.F., 2013. Electricity generation from food wastes and microbial community structure in microbial fuel cells. Bioresour. Technol. 144, 94–99. Jiang, J., Zhao, Q., Wei, L., Wang, K., 2010. Extracellular biological organic matters in microbial fuel cell using sewage sludge as fuel. Water Res. 44, 2163–2170. Jiang, J., Zhao, Q., Zhang, J., Zhang, G., Lee, D.J., 2009. Electricity generation from bio-treatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 100, 5808–5812. Justin, C.C., 2014. Potatoes, milk, and the old world population boom. J. Dev. Econ. 110, 123–138. Laties, G.G., 1967. The inhibition of citrate, isocitrate and α-ketoglutarate oxidation in aged potato slices by γ hydroxyl α-ketoglutarate. Phytochemistry 6, 181–185. Li, Z., Haynes, R., Sato, E., Shields, M.S., Fujita, Y., Sato, C., 2014. Microbial community analysis of a single chamber microbial fuel cell using potato wastewater. Water Environ. Res. 86, 324–330. Logan, B.E., 2007. Microbial Fuel Cells. Jonh Wiley and Sons, Inc., New Jersey. Nandy, A., Kumar, V., Mondal, S., Dutta, K., Salah, M., Kundu, P.P., 2015. Performance evaluation of microbial fuel cells: effect of varying electrode configuration and presence of a membrane electrode assembly. New Biotechnol. 32, 272–281. Pang, L.N., Ni, J.R., Tang, X.Y., Chen, Q., 2015. Short-cut waste activated sludge fermentation and application of fermentation liquid to improve heterotrophic aerobic nitrogen removal by Agrobacterium sp. LAD9. Chem. Eng. J. 259, 911–917.
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849 Ramdani, A., Dold, P., Gadbois, A., Deleris, S., Houweling, D., Comeau, Y., 2012. Biodegradation of the endogenous residue of activated sludge in a membrane bioreactor with continuous or on-off aeration. Water Res. 46, 2837–2850. Reynolds, D.M., Ahmad, S.R., 1997. Rapid and direct determination of wastewater BOD values using a fluorescence technique. Water Res. 31, 2012–2018. Srikanth, S., Venkata, M.S., 2012. Change in electrogenic activity of the microbial fuel cell (MFC) with the function of biocathode microenvironment as terminal electron accepting condition: influence on overpotentials and bio-electro kinetics. Bioresour. Technol. 119, 241–251. Venkata Mohan, S., Mohanakrishna, G., Sarma, P.N., 2010. Composite vegetable waste as renewable resource for bioelectricity generation through non-catalyzed open-air cathode microbial fuel cell. Bioresour. Technol. 101, 970–976. Virdis, B., Rabaey, K., Rozendal, R.A., Yuan, Z.G., Keller, J., 2010. Simultaneous nitrification, denitrification and carbon removal in microbial fuel cells. Water Res. 44 (9), 2970–2980.
849
Vreugdenhil, D., Bradshaw, J., Gebhardt, C., Govers, F., MacKerron, D.K.L., Taylor, M.A., Ross, H.A., 2007. Chapter 24 Potato flavour and texture. Potato Biology and Biotechnology: Advances and Perspectives, p. 535 (Netherlands). Xiao, B.Y., Yang, F., Liu, J.X., 2013. Evaluation of electricity production from alkaline pretreated sludge using two-chamber microbial fuel cell. J. Hazard. Mater. 254-255, 57–63. Zhao, G., Ma, F., Li, W., Chua, H., Chang, C.C., Zhang, X.J., 2012. Electricity generation from cattle dung using microbial fuel cell technology during anaerobic acidogenesis and the development of microbial populations. Waste Manag. 32, 1651–1658. Zuo, Z., Wu, S., Qi, X., Dong, R., 2015. Performance enhancement of leaf vegetable waste in two-stage anaerobic systems under high organic loading rate: role of recirculation and hydraulic retention time. Appl. Energy 147, 279–286.
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Effects of varying the ratio of cooked to uncooked potato on the microbial fuel cell treatment of common potato waste Haixia Du a,⁎, Fusheng Li b a b
Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan River Basin Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Mixing raw with boiled potato enhanced the electricity generation efficiency by MFC. • Mixing boiled potato in the potato feed shortened the startup time for electricity generation. • Optimum mass fraction of boiled potato existed based on columbic efficiency. • Composition of dissolved organic matter changed with time by fluorescence EEM. • Hydrolysis rate parameters were estimated by first-order sequential reaction model.
Schematic diagram illustrating the biological reactions by different bacteria in the anode chamber of MFC for treatment of the potato feed.
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 28 June 2016 Accepted 3 July 2016 Available online 8 July 2016 Editor: Simon Pollard Keywords: Vegetable waste Microbial fuel cell Electricity generation Volatile fatty acids Dissolved organic matter
a b s t r a c t The effect of varying the ratio of cooked to uncooked potato in the performance of microbial fuel cell (MFC) treating common potato waste was investigated. Four MFCs were fed with a ratio of cooked (boiled) to uncooked (i.e. waste) potato of 0, 48.7, 67.3 and 85.6%. Respectively, the columbic efficiency was estimated as 53.5, 70.5, 92.7 and 71.1%, indicating significantly enhanced electricity generation and waste degradation at an initial feedstock mixing ratio of 2/3 cooked to 1/3 uncooked potato. The hydrolysis rate parameter (estimated using a firstorder sequential hydrolysis and degradation model) increased from 0.061 to 0.191 day−1 as cooked potato was added which increased electricity generation efficiency from 24.6 to 278.9 mA/m2/d and shortened the startup time for maximum current density from 25 to 5 days. The potato slurries' chemical oxygen demand (COD) decreased by 86.6, 83.9, 84.1 and 86.3%, respectively, indicating no relationship exists between the fraction of boiled potato and the amount of COD reduction. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author at: Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan. E-mail address: [email protected] (H. Du).
http://dx.doi.org/10.1016/j.scitotenv.2016.07.023 0048-9697/© 2016 Elsevier B.V. All rights reserved.
About 1.3 billion tons of food that accounts for 32% of the total food produced for human consumption across the entire food supply chain is lost and wasted every year in the world (Gustavsson et al., 2011).
842
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
According to the UN Food Agriculture Organization (FAO), the lost and wasted food is mainly consisted of fruits and vegetables (40–50%); cereals (30%); fish (30%); and root crops, oilseeds, meat and dairy (20%). In low-income countries, food waste before consumption (including that lost during harvest, processing, distribution and retail) is predominant; while, in middle and high-income countries, food waste generated during consumption (including the discarded one during cooking and food residues after consumption) is dominating. This difference, together with the differences in the dietary culture, lead to significant differences in the composition of food waste among countries and families. Potato is widely planted, processed and consumed in the world. The world's total potato production in 2013 was about 368 million tons (FAO, 2013) and the wasted potato that possesses higher content of biodegradable organic species than other staple crops (Justin, 2014) constitutes for a considerable proportion of the total food waste generated before and during domestic and industrial consumptions. According to the published information (http://dairyknowledge.in/article/potatowaste), of the total production of the potato industry, around 12–20% is wasted. From the same information source, it is found that, in Europe alone, the produced potato pulp, a major byproduct of the potato starch industry, could reach 1 million tons every year, with only a minimal amount of which being utilized. Waste potato without proper handling is the reason for many pests (including late blight, potato leaf roll virus, bacterial ring rot and nematodes) and noisome smells. Soil deposited with cull potato is reported to be infested with such pests as powdery scab, nematodes and weed seeds. The discarded potato in soil environment can also increase greenhouse gas (GHG) emissions during its decomposition by soil bacteria (http://dairyknowledge.in/article/potatowaste). In most industrialized countries, potato waste is mainly treated together with other burnable municipal wastes through incineration. For many developing countries, however, landfill and direct dumping without any proper treatment are still prevailing. Incineration directly relates to the GHG emissions, and landfill and dumping can worsen soil and water environments. Meanwhile, directly dumping to small urban rivers and garbage collection stations nearing the residential areas is blamed for noisome smells and leachates that affect the living environment of humans in most developing countries. Microbial fuel cell (MFC) is a technology that can convert organic matter into electricity (Nandy et al., 2015). Compared to other sustainable disposal methods, such as composting, anaerobic digestion and carbonization, MFC has the advantage of directly recycling the cleanest energy (electricity) from organic waste with a lower generation level of secondary pollutions to water, soil and atmospheric environment. So far, many studies have been conducted using MFC to treat organic waste (including excess activated sludge, food waste and animal waste) (Jiang et al., 2009; Goud et al., 2011; Zhao et al., 2012). The organic waste used in these studies was mostly in the form of leachate or masticated slurries (Goud et al., 2011; Durruty et al., 2012; Li et al., 2014), with the direct use of its solid form being very limited. For potato waste, since a significant part of it from cultivation, storage and processing is in solid forms, its direct use in MFC is of great application significance. Anaerobic biological decomposition of organic solid generally involves two important processes: liquefaction (hydrolysis/acidification) and degradation. Hydrolysis is the first step that determines the overall performance of involved bioreactors. Its occurring extent can be affected not only by the composition of the organic solid (such as the constituting types and sizes) but also by the operation conditions (such as the mixing rate of raw and cooked fractions, temperatures, etc.). In a recent study (Du and Li, 2015), the authors of the current study compared the performance of MFCs fed with potato cubes of three different sizes. Decreasing the size was found effective in promoting hydrolysis and thus improving the electricity generation efficiency and the removal efficiency. Heat treatment is also reported for being capable of promoting hydrolysis of food waste (Ariunbaatar et al., 2015) and activated sludge
(Pang et al., 2015). Therefore, by addition of cooked potato into the raw one, the faster hydrolysis of the cooked one than the raw one may provide the hydrolysis products available for electrogenic bacteria from the beginning of the MFC operation, and thus shorten the startup time for electricity generation and enhance the overall facility efficiency. Taking into consideration of the reported heat treatment effect and the fact that vegetable waste includes cooked species, it is reasonable to infer that mixed feeding of the raw potato with cooked one may be another possible way to enhance the performance of MFC in treatment of solid potato waste. However, in existing literature, relevant studies can hardly be found. In addition to this, the effects of varying the ratio of the cooked one in the total potato feed have not yet been studied. Too intensified hydrolysis may promote the usage of the released organic constituents from organic solid by coexisting bacterial species besides electrogenic ones, resulting in columbic efficiency reductions. It is thus important to quantitatively investigate the effects of the mass fraction of cooked potato in the total potato feed on the overall performance of MFC. In this study, four two-chamber MFCs were operated in parallel to investigate the effects of increasing the mass fraction of boiled potato in the potato feed on the performance of MFC evaluated on the basis of electricity generation and the extent of the potato removal. For the former, the increasing rate of current density in the initial operation period was estimated for comparisons together with the estimations for columbic efficiency. The electrochemical behavior and electron discharge were also analyzed by cyclic voltammetry (CV) and extended linear sweep voltammetry (LSV). For the latter, in addition to COD and volatile fatty acids (VFAs), the changes in the composition of the organic matter were also analyzed using the fluorescence excitation emission matrix (EEM). Moreover, by simulating the time profiles of COD using a first-order sequential reaction model, the rate parameters of hydrolysis and degradation of the potato feed with different mass fraction of boiled potato were estimated and compared. The results of this systematic study were considered very useful as reference for applying MFC for electricity generation from vegetable waste. 2. Materials and methods 2.1. Solid potato for experiment use White potato (namely May Queen) harvested in Hokkaido and consumed as the major type of potato in Japan was used for this study. Compared to the Yellow potato (slightly waxy, velvety and moist) and Russet Potato (floury, dry, light and fluffy), both are also consumed throughout the world, the White potato is featured with medium starch and being slightly creamy and dense in texture (http://www. potatogoodness.com/all-about-potatoes/potato-types/). The fresh white potato was purchased from a supermarket in Gifu, Japan. A part of the purchased potato was boiled using a boiling pot normally used in families to the well-cooked level for eating by soaking raw potato into water and boiling for 30 min. The content of water and organic matter before and after boiling is shown in Table 1. It could be seen from Table 1 that boiling caused losses of water and organic matter; however, the extent of losses was small: 0.9% and 1.5%, respectively. Both the raw and boiled Table 1 The content of water and organic matter in raw and boiled potato, and the mass fraction of boiled potato in the potato feed to MFCs of this study.
Raw potato Boiled potato
Water content (%)
Organic content (%)a
84.6 83.7
90.7 89.2
Mass fraction of boiled potato (%) Wet weight Dry weight a
0 0
48.7 48.3
67.3 68.7
85.6 86.2
Organic content refers to the weight percentage of organic matter in the total solid.
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
potato was immediately cut into small cubes (length of 5 mm) for direct use in MFCs. To study the effect of increasing the mass fraction of boiled potato on the performance of MFC, the boiled potato cubes were mixed into the raw ones with the mass fraction of the boiled potato in the total potato feed being 0, 48.7, 67.3 and 85.6% (in wet weight), respectively, as shown in Table 1. 2.2. MFC configuration and operation conditions Four two-chamber MFCs assembled by Feng et al. (2010) with the total volume of 250 cm3 and the working volume of 240 cm3 for each chamber were used. For all MFCs, carbon felts, each having a length of 6 cm, width of 4 cm and thickness of 0.5 cm, were used as the anode and cathode. A cation exchange membrane (CEM, Zhejiang Qianqiu Group Co., Ltd. China) was used to separate the two chambers (Feng et al., 2010). The inlet, outlet and a sampling port were designed on both chambers and a port for aeration was designed on the cathode chamber. Titanium wire was used to connect the anode and cathode. The use of the two-chamber MFC system was made because, even if its efficiency is generally lower than single-chamber MFC, it is still used in many studies for investigation of the fundamentals of all involved reactions (Venkata Mohan et al., 2010; Feng et al., 2010); and also because the cathode chamber can be effectively used for simultaneous removal of some substances through either oxidative reactions via aerobic bacteria or reductive reactions via electrons transferred from the anode chamber, such as ammonium and its oxidized product of nitrate (Virdis et al., 2010; Du et al., 2015). Anaerobic bacterial consortia were collected from MFCs operated in our previous study (Du and Li, 2015). Inoculation was conducted using 20 mL of the collected bacterial consortia in 100 mM/L phosphate buffer solution (PBS) containing both trace minerals and vitamins (Du and Li, 2015). After inoculation, the bacterial consortia were cultured using sodium acetate until current density reached stable. Subsequently, the four mixed samples of raw and boiled potato cubes were added to the anode chamber of the MFCs. The added weight of potato for each MFC was 2.5 g in wet, which corresponded to an initial total COD loading of 1830 mg/L. All four MFCs were operated for 52 days with 100 Ω of external resistance under the controlled temperature of 30 °C. Every four to six days, pH of the anodic solution was adjusted to keep it at 7.0–7.1 using 1N-NaOH or 1N-HCl. The cathode chamber was fed with the PBS solution and aeration was conducted continuously with a power air pump (power: 2.5 W, air flow: 2 L/min). The schematic diagram of the experiment setup is shown in Fig. 1. 2.3. Measurements and analysis Voltage was measured every minutes using a digital multimeter and a data acquisition system (midi LOGGER GL200A, Graphtec Corporation, Japan), and the data obtained were plotted using averages of every
843
5 min. Current density (I) was calculated according to I = V/(R × A), where, V is the voltage, R is the external resistance and A is the surface area of the anode. When the current output reached stable, the anode potential was analyzed by changing the external resistance from 50 Ω to 30,000 Ω. The electron discharge pattern was evaluated by studying the CV and LSV using a potentiostat system (EC stat-100, EC Frontier, Japan). All the electrochemical measurements were done by using the anode and cathode as the working and counter electrodes against the saturated Ag/AgCl reference electrode. CV was evaluated over a voltage range of −0.6 to +0.6 V at a scan rate of 10 mV/s and LSV over the same voltage range of −0.6 to +0.6 V. In order to compare the availability of potato for electricity generation, the columbic efficiency was calculated according to: Z Columbic efficiency ¼
8
t 0
I dt
F V ΔCOD
ð1Þ
where, F is the Faraday constant, I is the harvested current, V is the volume of anodic solution, ΔCOD is the removed COD and 8 is the mass of oxygen per electron (Logan, 2007). The total content of organic matter solubilized to the liquid phase as a result of hydrolysis of the potato cubes was quantified using COD measured by the colorimetric method (DR/890 Colorimeter) for all sampled anodic solution after filtration through 0.2 μm membrane filter. In addition to COD, the following volatile fatty acids (VFAs) inside the filtered anodic solution were also quantified using a high performance liquid chromatograph (Shimadzu Co., Japan): citrate, isobutyrate, acetate, propionate, butyrate, valerate and isovalerate. The quantification of these VFAs was made in order to further trace the hydrolysis and degradation behavior of potato. Using the filtered samples, the characteristics of dissolved organic matter in the anodic solution was also analyzed on the basis of fluorescence EEM with a spectrofluorometer (RF-5300, Shimadzu Co., Japan). The excitation and emission wavelength was set within the range of 220–400 nm and 280–480 nm, respectively. 2.4. Estimation of the first-order hydrolysis and degradation rate parameters As shown later from the concentration profiles of COD, the electricity generation by MFC with solid potato is accomplished by the process of potato consumption that includes the combined reactions of hydrolysis and degradation. This process could be described by the following firstorder sequential reaction model: dC ¼ kh C a −kd C dt
ð2Þ
dC a ¼ −kh C a dt
ð3Þ
Fig. 1. The Schematic diagram of the experiment setup.
844
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
Fig. 2. Effect of increasing the mass fraction of boiled potato on the current density of MFCs treating potato cubes (the mass fraction of boiled potato: 0, 48.7, 67.3 and 85.6% in wet weight) (‘↑’ indicates pH adjustment).
where, C is the soluble COD concentration in the anodic solution at time t, Ca is the concentration of the net hydrolysis product of the potato solid, kh is the rate parameter of hydrolysis and kd is the rate parameter of degradation. Given the initial COD for hydrolysis as Ca0, the following solution could be obtained to describe the concentration changes of COD inside the anodic chamber. From the total potato feed to each MFC, Ca0 was determined as 1830 mg/L in this study. C¼
kh C a0 −kh t e −e−kd t kd −kh
ð4Þ
kh and kd were searched by minimizing the differences between the calculated COD by Eq. (4) and the observed COD according to the following definition: C iðobsÞ −C iðcalÞ 1 Xn Er ¼ i¼1 n C iðobsÞ
!2 ð5Þ
where, Ci(obs) and Ci(cal) are observed and calculated COD concentration, respectively, and n is the total number of samples collected for COD measurement of each MFC. 3. Results and discussion 3.1. Electricity generation efficiency 3.1.1. Current density The observed time profiles of current density are shown in Fig. 2. The maximum current density reached about 160, 180, 254 and 243 mA/m2 for all four MFCs with 0, 48.7, 67.3 and 85.6% of boiled potato, respectively. Increasing the mass fraction of the boiled potato was also found effective in shortening the time needed for the current density to
reach its maximum level (referred hereafter as “peak time”). As could be seen from Fig. 2 that, compared to the peak time (about 25 days) needed for the MFC with the raw potato only (0% of boiled potato), the peak time (about 5 days) for the MFC with 85.6% of boiled potato was markedly shortened. The marked shortening of the peak time was due to the improved electricity generation efficiency in the initial stage of the MFC operation. Through linear regression analysis of the number of data that led to the largest correlation coefficient, the increasing rate of current density in the initial stage was estimated. As shown in Table 2, for all four MFCs with 0, 48.7, 67.3 and 85.6% of boiled potato, respectively, the estimated increasing rate values were 24.6, 97.0, 120.7 and 278.9 mA/m2/d, indicating clearly that, by increasing the mass fraction of the boiled potato, the startup time needed for MFC to reach its stable stage for electricity generation could be effectively shortened. The obvious differences in the maximum current density, the peak time and the increasing rate of the initial current density may indicate that, for practical applications, mixed feeding of raw and cooked potato is a likely approach for enhancing the overall performance of MFCs. Even within the period where current density reached higher levels, frequent drops in the current density were also observed. The drops were induced by the falling of pH occurring inside the anodic solution of each MFC and were effectively recovered by pH adjustment. Hydrolysis of organic solid during anaerobic fermentation generally leads to formation and accumulation of volatile fatty acids (VFAs) (Jiang et al., 2009). This was also observed in the study (data to be shown later) and was probably the major reason responsible for the observed pH drops. For all four MFCs, after 30 days, current density revealed a trend of gradual decreases following the consistent consumption of organic species that can be readily used for electricity generation (Venkata et al., 2010). This could be explained by the time profiles of COD and VFAs shown later in Fig. 5. 3.1.2. Anode potential The profiles of the anode potential determined against a saturated Ag/AgCl electrode are depicted in Fig. 3. For all four MFCs, within the examined external resistance range (50–30,000 Ω) the anode potential increased as the external resistance increased. The anode potential of the four MFCs with 0, 48.7, 67.3 and 85.6% of boiled potato recorded at 30,000 Ω reached −154, −337, −365 and −387 mV, respectively, revealing a trend of increases with the increases of the boiled potato. Such a trend was probably due to the increased availability of potato for electrogenic bacterial species. A trend of generally lower anode potential was associated with the MFC fed with the raw potato alone (0% of the boiled potato). This thus suggests that the overall availability of the raw potato for electrogenic bacteria was lower. Moreover, it was also noticed that the anode potential increased rapidly as the external resistance increased within the range below 4000 Ω. This implies that more effective electron discharge may occur below this external resistance (Goud et al., 2011). 3.1.3. CV and LSV The electrochemical behavior of MFCs was further evaluated using CV that measures the potential differences across the interface as well
Table 2 The initial increasing rate of current density and the estimated rate parameters for hydrolysis and degradation of different potato feeds. Parameters
Mass fraction of boiled potato (%) 0
rc kh kd
24.6 0.061 0.068
48.7 a
0.996 0.124b
97.0 0.086 0.056
67.3 a
0.991 0.107b
120.7 0.169 0.053
85.6 a
0.993 0.107b
278.9 0.191 0.079
rc: initial increasing rate of current density (mA/m2/day), kh: first-order hydrolysis rate parameter (day−1), kd: first-order degradation rate parameter (day−1). a Linear correlation coefficient. b The difference between calculated and observed COD concentrations defined by Eq. (5).
0.999a 0.151b
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
845
the mass fraction of the boiled potato on the electrochemical activity and power output of the MFCs. The LSV is used to delineate the efficiency of biocatalysts during operation (Srikanth and Venkata, 2012). For the compared four MFCs, the profiles of LSV were also generated. As shown in Fig. 4b, observable differences existed as the mass fraction of the boiled potato differed. The highest oxidation current (2.4 mA) was found in the MFC with 85.6% of boiled potato, followed by that in the MFC with 67.3% (1.9 mA), 48.7% (1.7 mA) and 0% (0.7 mA) of boiled potato. The gradual decreases of the oxidation current may indicate corresponding decreases in the electron discharge efficiency. Regarding the reduction current, the MFC with 48.7% of boiled potato showed the highest value (− 8.0 mA), followed by the MFCs with 0% (− 5.3 mA), 85.6% (−1.1 mA) and 67.3% (−0.1 mA) of boiled potato.
Fig. 3. Effect of increasing the mass fraction of boiled potato on the anode potential of MFCs treating potato cubes measured by varying the external resistance in 5–30,000 Ω when current density was stable (the mass fraction of boiled potato: 0, 48.7, 67.3 and 85.6% in wet weight).
as the redox of the components involved in the biochemical system (Jayarama, 1986; Nandy et al., 2015). As depicted in Fig. 4a, observable differences appeared as the mass fraction of the boiled potato differed. Among the four MFCs, the one with 85.6% of boiled potato showed the maximum oxidation and reduction current (38.9 mA and −20.1 mA); followed by the MFCs with boiled potato of 67.3% (35.0 mA and − 20.6 mA), 48.7% (33.1 mA and − 12.7 mA) and 0% (4.1 mA and −4.8 mA). The oxidation and reduction of components during the potential sweep causes the appearance of current peaks on the CV (Srikanth and Venkata, 2012). Distinct irreversible peaks were found on the sweeps with 48.7% and 67.3% of boiled potato (the peak potential was 0 V and −0.4 V, respectively). For the MFCs with 0% and 85.6% of boiled potato, irreversible oxidation peaks were also found (0.3 V and 0 V, respectively). All these evidenced the positive effect of increasing
3.1.4. Columbic efficiency Based on the observed current density shown earlier in Fig. 2 and the profiles of COD to be shown later in Fig. 5, the columbic efficiency of the four MFCs was estimated as 53.5, 70.5, 92.7 and 71.1%, with the smallest one being associated with the feed with only raw potato (i.e., 0% of boiled potato) and the largest one with the sample with 67.3% of boiled potato. The values of the latter three (70.5, 92.7 and 71.1%) were higher than those of MFCs treating raw potato with different sizes (Du and Li, 2015), mixed food waste of breakfast, lunch and dinner (Jia et al., 2013) and sweet potato wastewater (Cai et al., 2010), as summarized in Table 3. 3.2. Potato removal efficiency 3.2.1. Concentration changes of COD The time profiles of COD in the liquid phase of the anodic chamber are shown in Fig. 5a. For all four MFCs, COD revealed a similar trend: the concentration increased in the initial period and then gradually decreased. Such a trend is not typical and could be observed in anaerobic treatment of a variety of organic solids, such as activated sludge, food waste and vegetable waste (Ramdani et al., 2012; Grimberg et al.,
Fig. 4. Cyclic voltammograms (a) and extended linear sweep voltammetric profiles (b) of the anode of MFCs treating potato cubes with different mass fraction of boiled potato (0, 48.7, 67.3 and 85.6% in wet weight) when current density was stable.
846
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
Fig. 5. Effect of increasing the mass fraction of boiled potato on the behavior of COD (a) and VFAs (b) in the anodic solution of MFCs treating potato cubes with different mass fraction of boiled potato (0, 48.7, 67.3 and 85.6% in wet weight).
2015; Zuo et al., 2015). The observed COD increases indicated the releasing rate of organic matter from the added potato cubes due to hydrolysis was faster than the rate of degradation; while the followed COD decreases indicated the rate of degradation exceeded the rate of hydrolysis or that the reaction of hydrolysis had already stopped.
3.2.2. Hydrolysis and degradation rate parameters The first-order hydrolysis (kh) and degradation (kd) rate parameters estimated by fitting calculated COD concentrations with observed ones, as displayed in Fig. 5a, are summarized in Table 2. The estimated kh increased in the range of 0.061–0.191 day−1 as the mass fraction of the boiled potato increased from 0% to 85.6%, confirming that mixing boiled potato effectively elevated the overall availability of the potato feed for bacterial use. For the degradation rate parameter kd, the estimated values (0.053–0.079 day−1) differed within a range narrower than that of kh, suggesting the effect of increasing boiled potato on degradation was comparatively smaller than the effect on hydrolysis. It is also interesting to see that, compared to the increasing extent for the stimulated hydrolysis rate parameter of COD (2.9 times as the mass fraction of boiled potato increased from 0% to 85.6%), the increasing extent for the estimated rate of initial current density increases was more significant (11.3 times). This may thus suggest that the intensified hydrolysis could greatly improve the performance of MFCs and may also suggest that the increased hydrolysis products were more effectively used by electrogenic bacterial species than by ordinary anaerobic heterotrophic bacteria. The effective use of the hydrolysis products by electrogenic bacteria contributes to enhanced conversion of potato waste to cleanest energy (electricity), and the extent of adversary effects to water, soil, atmospheric and biospheric environments as well as humans, often accompanied with other disposal methods (such as landfill, composting, anaerobic digestion and carbonization), can be greatly alleviated. At the end of the operation of all four MFCs, the total use/consumption of the potato feed assessed by COD reached 86.6, 83.9, 84.1 and 86.3%, respectively; indicating clearly that increasing the fraction of boiled potato did not affect the final overall use/consumption of the potato fed to the MFCs. The obtained COD reductions were also higher than the documented ones with composite vegetable waste (Venkata et al., 2010) and composite food waste (Goud et al., 2011), as compared in Table 3. For the MFC with 0% of boiled potato, the observed COD reached its peak value after 22 days. For the three MFCs with 48.7, 67.3 and 85.6% of boiled potato, however, the time needed was shortened to 16, 12 and 8 days, respectively. The observed effect was a combined result of the effect of mixing boiled potato on hydrolysis and that on degradation. If the effect on hydrolysis alone was isolated for evaluation, the effect of the mixed feeding of raw potato with boiled one was found more prominent, particularly when the mass fraction of boiled potato increased from 0% to 67.3%, as illustrated by the estimations based on Eq. (3) and the first-order hydrolysis rate parameter kh given in Table 2 that:
Table 3 Comparison of the performance of MFCs of this study with previous ones treating different vegetable and food wastes. Substrate type
Substrate form
Mixed raw and boiled potato
Mass fraction of boiled potato (%) Two-chamber 0 48.7 67.7 85.6 Solid cubes with edge length (mm) Two-chamber 3.0 5.0 7.0 Liquid of masticated vegetable waste Single-chamber after removing pulp
Fresh potato
Composite vegetable waste
Composite food waste Mixed food waste of breakfast, lunch and dinner (in volume ratio of 1:2:2) Sweet potato wastewater
MFC type
Initial COD (mg/dm3)
COD removal (%)
Highest current density (mA/m2)
Columbic efficiency (%)
1830 1830 1830 1830
86.6 83.9 84.1 86.3
160.1 181.0 253.9 243.3
53.5 70.5 92.7 71.1
1830 1830 1830 700–2100
88.0 88.5 91.8 62.9
189.1 178.9 163.3 87.1–160
63.9 58.6 51.5 –
Present study
mg/dm3/day
Filtrate of masticated food waste
Single-chamber 1000–2600
44.3–64.8 211–390
–
Filtrate of masticated food waste
mg/dm3/day Single-chamber 2000–4900
77.2–86.4 –
23.5–27.0
Single-chamber 625–10,000
92.2
–
10.1–28.9
Filtrate of fermented sweet potato wastewater after removing ethanol
Du and Li (2015)
Venkata Mohan et al. (2010) Goud et al. (2011) Jia et al. (2013) Cai et al. (2010)
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
847
Fig. 6. Fluorescence EEMs of dissolved organic matter in the anodic solution of MFCs treating potato cubes with different mass fraction of boiled potato (0, 48.7, 67.3 and 85.6% in wet weight) at (1) the initial starting period (day 0); (2) the period with the highest current density; (3) the end of the MFC operation.
the estimated time lengths for the potato feed with the boiled fraction of 0, 48.7, 67.3 and 85.6% to be hydrolyzed for 50% (i.e., Ca/Ca0 = 0.5) are 11.4, 8.1, 4.1 and 3.6 days; and those for 95% (i.e., Ca/Ca0 = 0.95) are 49.1, 34.8, 17.7 and 15.7 days, respectively. Further increasing the boiled potato fraction from 67.3% to 85.6% also shortened the time needed for hydrolysis to occur; however, the extent of shortening was less significant as compared to the range of increases in the boiled potato fraction from either 0% to 48.7% or 48.7% to 67.3%. The estimated results, together with the observed profiles shown in Fig. 5, clearly indicated that mixing raw potato with boiled one could promote the occurrence of hydrolysis and that increasing the mass fraction of the boiled one could further accelerate the hydrolysis of potato. This result supports previous findings on the effect of heat pretreatment on anaerobic fermentation of composite food waste (Ariunbaatar et al., 2015) and activated sludge
(Pang et al., 2015) that heat treatment increased solubilization and/or hydrolysis of organic solids, leading to increases of soluble COD concentrations and their availability for anaerobic microorganisms. The effect of heat treatment is probably due to its capability to soften the potato cells through solubilization and release of pectin in the middle lamella (Vreugdenhil et al., 2007). 3.2.3. Concentration changes of VFAs Following the occurrence of hydrolysis, VFAs were generated. Among the seven VFAs (citrate, isobutyrate, acetate, propionate, butyrate, valerate and isovalerate) analyzed in this study, four (citrate, isobutyrate, acetate and propionate) were detected, as plotted in Fig. 5b in the logarithmic scale. Citrate, being also a major constituting organic acid in the potato tissue (Laties, 1967), was detected at higher
Table 4 Fluorescence intensity of all detected peaks in the fluorescence EEM of samples collected at the initial starting period, the period with the highest current density, and the end of the MFC operation. Mass fraction of boiled potato (%)
The initial starting period (day 0)
The period with the highest current density
The end of the MFC operation
0 48.7 67.3 85.6 0 48.7 67.3 85.6 0 48.7 67.3 85.6
Fluorescence intensity (QSU)a Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
28.2 16.8 18.3 23.2 64.9 74.4 62.2 55.1 15.4 15.4 16.7 14.4
46.2 29.6 33.9 42.5 103.0 117.4 101.6 93.7 24.3 23.7 23.4 23.4
146.1 117.3 122.5 131.0 492.2 436.2 321.7 245.4 197.9 146.9 168.0 131.0
135.9 98.2 108.5 113.2 422.6 518.8 401.0 292.7 109.0 95.0 107.2 78.0
– – – – 290.2 292.3 265.5 231.8 109.2 101.0 107.0 92.8
a Peak 1 (Ex/Em = 290/380): humic acid type substances; Peak 2 (Ex/Em = 250/369) and Peak 5 (Ex/Em = 280/310): large molecular weight peptides and proteins (microorganism related by-products); Peak 3 (Ex/Em = 220/298): lower molecular weight tyrosine-like aromatic amino acids; Peak 4 (Ex/Em = 220/352): lower molecular weight aromatic proteins and BOD-type substances.
848
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849
concentrations (220–340 mg/L) throughout the whole operation. Acetate, propionate and isobutyrate were detected only in some periods of the whole operation, with their concentrations being below 7 mg/L. Acetate is generally considered easier for biological degradation and was only detected in the period where higher current density was recorded as shown earlier in Fig. 2. The time when acetate was detected seemed to be earlier for the MFCs treating mixed raw and boiled potato than the sample of raw potato alone, reflecting the effect of intensified hydrolysis on the generation of acids for bacterial use. Although the net generation and consumption of acetate and propionate were not known, the observed profiles confirmed that their degradation occurred in all four MFCs. 3.2.4. Composition changes evaluated by fluorescence EEM The changes of soluble organic constituents in the anodic solution of the MFCs were also analyzed by three-dimensional fluorescence EEM (Jiang et al., 2010; Xiao et al., 2013). To interpret the changes, the fluorescence spectra of the raw and completely boiled potato, and the market-available standard reagents of the four VFCs detected in this study were also measured. As shown in Fig. 6, at the beginning of the operation (day 0), four peaks (Peak 1–Peak 4) corresponding respectively to the Excitation/Emission wavelengths (Ex/Em) of 290/380 nm, 250/ 369 nm, 220/298 nm and 220/352 nm appeared in all four MFCs. Peak 4 (Ex/Em = 220/352 nm), which is characterized as aromatic proteins (Chen et al., 2003), was also found in the raw and completely boiled potato, as well as in the individual standard solutions of the four marketavailable VFAs (data not shown). When each MFC was operated till the time point when current density reached its peak value, a new peak (Peak 5) (Ex/Em = 280/310 nm) appeared in the anodic solution and the fluorescence intensity of the four peaks existed from the beginning (day 0) obviously increased, as could be seen from Table 4, in which the fluorescence intensity of all detected peaks are summarized. The emerge of this new peak reflected the release of some dissolved organic constituents during hydrolysis of the potato solid and was probably corresponding to the larger molecular compounds of peptides and proteins (byproducts of microorganisms) (Reynolds and Ahmad, 1997; Jiang et al., 2010). The fluorescence intensity of the peaks slightly decreased as the mass fraction of boiled potato increased (Table 4). This probably indicated that some organic constituents were released and lost during boiling of the potato, and that the overall availability of the organic constituents detected by a spectrofluorometer was getting higher in the MFCs fed with boiled potato. The former indication could be supported by the slight decrease of the organic content (by 1.5%) for potato after boiling shown earlier in Table 1. The latter could be explained after referencing to literature reports by the followings: each fluorescence peak was a mixture of different organic constituents, with some being easier for degradation and some being difficult (Chen et al., 2003); and boiling accelerated the release and formation of easily degradable organic constituents and thus led to faster overall concentration decreases due to their degradation by bacteria (Ariunbaatar et al., 2015; Pang et al., 2015). At the operation end of all four MFCs, the fluorescence intensity of all peaks dropped to low values close to those at the beginning of the operation, indicating clearly that the emerged organic species were effectively degraded, particularly for the organic constituents reflected by Peak 1 - Peak 4. The peak at Ex/Em of 220/298 nm (Peak 3) showed the same tendency with the fluorescence spectra of the standard reagent of citrate (data not shown), verifying the observed result for VFAs (Fig. 5b) that citrate remained at the end of the operation. 4. Conclusions Increasing the ratio of boiled potato in the potato feed effectively promoted the occurrence of hydrolysis, elevated the maximum current density and shortened the time needed for the current density to reach the maximum. The estimated columbic efficiency suggested the
existence of an optimum mass fraction of the boiled potato since columbic efficiency decreased from 92.2% to 71.1% as the mass fraction further increased from 67.7% to 85.6%. Comparisons with previous studies indicated that the electricity generation efficiency with potato was generally higher than ordinary mixed food wastes and sweet potato wastewater. The results of this study demonstrated the capability of MFC in realizing simultaneous potato waste reduction and electricity generation, and the significant effects of mixed treatment with cooked potato; and thus suggested that MFC may serve as a more beneficial treatment method than other sustainable treatment options to water, soil, atmospheric and biospheric environments as well as humans. The findings obtained can also serve as important reference for further studies needed to identify the changes of the microbial structure (in both density and species) in the anode chamber, and to clarify the impacts of such important operation factors as the feeding concentrations, temperatures and mixed treatment with other types of organic waste. Verification experiments using larger reactors are also needed before MFC can be adopted for practical applications.
References Ariunbaatar, J., Panico, A., Yeh, D.H., Pirozzi, F., Lens, P.N., Esposito, G., 2015. Enhanced mesophilic anaerobic digestion of food waste by thermal pretreatment: substrate versus digestate heating. Waste Manag. 46, 176–181. Cai, X.B., Yang, Y., Sun, Y.P., Zhang, L., Xiao, Y., Zhao, H., 2010. Electricity generation from sweet potato fuel ethanol wastewater using microbial fuel cell technology. Huan jing ke xue = Huanjing kexue/[bian ji, Zhongguo ke xue yuan huan jing ke xue wei yuan hui" Huan jing ke xue" bian ji wei yuan hui.] 31, 2512–2517. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K., 2003. Fluorescence excitation-emission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 37, 5701–5710. Durruty, I., Bonanni, P.S., Gonzalez, J.F., Busalmen, J.P., 2012. Evaluation of potato-processing wastewater treatment in a microbial fuel cell. Bioresour. Technol. 105, 81–87. Du, H.X., Li, F.S., 2015. Size effects of potato waste on its treatment by microbial fuel cell. Environ. Technol. 37, 1305–1313. Du, H.X., Li, F.S., Yu, Z.J., Feng, C.H., Li, W.H., 2015. Nitrification and denitrification in twochamber microbial fuel cells for treatment of wastewater containing high concentrations of ammonia nitrogen. Environ. Technol. 37, 1232–1239. FAO, 2013. Food Wastage Footprint: Impacts on Natural Resources. FAO, Rome (ISBN 97892-5-107752-8). Feng, C.H., Ma, L., Li, F.B., Mai, H.J., Lang, X.M., Fan, S.S., 2010. A polypyrrole/anthraquinone-2,6-disulphonic disodium salt (PPy/AQDS)-modified anode to improve performance of microbial fuel cells. Biosens. Bioelectron. 25, 1516–1520. Goud, R.K., Babu, P.S., Venkata, M.S., 2011. Canteen based composite food waste as potential anodic fuel for bioelectricity generation in single chambered microbial fuel cell (MFC): bio-electrochemical evaluation under increasing substrate loading condition. Int. J. Hydrog. Energy 36, 6210–6218. Grimberg, S.J., Hilderbrandt, D., Kinnunen, M., Rogers, S., 2015. Anaerobic digestion of food waste through the operation of a mesophilic two-phase pilot scale digester-assessment of variable loadings on system performance. Bioresour. Technol. 178, 226–229. Gustavsson, J., Cederberg, C., Sonesson, U., Otterdijk, R.V., Meybeck, A., 2011. Global Food Losses and Food Waste: Extent Causes and Prevention. United Nations, Food and Agriculture Organization, Rome, Italy. Jayarama, R.S., 1986. Studies on Electrode Processes by Cyclic Voltammetry, Chronoamperometry, Chronopotentionmetry and Controlled Potential Coulometry. Sri Venkateswara University, Tirupati. Jia, J., Tang, Y., Liu, B.F., Wu, D., Ren, N.Q., Xing, D.F., 2013. Electricity generation from food wastes and microbial community structure in microbial fuel cells. Bioresour. Technol. 144, 94–99. Jiang, J., Zhao, Q., Wei, L., Wang, K., 2010. Extracellular biological organic matters in microbial fuel cell using sewage sludge as fuel. Water Res. 44, 2163–2170. Jiang, J., Zhao, Q., Zhang, J., Zhang, G., Lee, D.J., 2009. Electricity generation from bio-treatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 100, 5808–5812. Justin, C.C., 2014. Potatoes, milk, and the old world population boom. J. Dev. Econ. 110, 123–138. Laties, G.G., 1967. The inhibition of citrate, isocitrate and α-ketoglutarate oxidation in aged potato slices by γ hydroxyl α-ketoglutarate. Phytochemistry 6, 181–185. Li, Z., Haynes, R., Sato, E., Shields, M.S., Fujita, Y., Sato, C., 2014. Microbial community analysis of a single chamber microbial fuel cell using potato wastewater. Water Environ. Res. 86, 324–330. Logan, B.E., 2007. Microbial Fuel Cells. Jonh Wiley and Sons, Inc., New Jersey. Nandy, A., Kumar, V., Mondal, S., Dutta, K., Salah, M., Kundu, P.P., 2015. Performance evaluation of microbial fuel cells: effect of varying electrode configuration and presence of a membrane electrode assembly. New Biotechnol. 32, 272–281. Pang, L.N., Ni, J.R., Tang, X.Y., Chen, Q., 2015. Short-cut waste activated sludge fermentation and application of fermentation liquid to improve heterotrophic aerobic nitrogen removal by Agrobacterium sp. LAD9. Chem. Eng. J. 259, 911–917.
H. Du, F. Li / Science of the Total Environment 569–570 (2016) 841–849 Ramdani, A., Dold, P., Gadbois, A., Deleris, S., Houweling, D., Comeau, Y., 2012. Biodegradation of the endogenous residue of activated sludge in a membrane bioreactor with continuous or on-off aeration. Water Res. 46, 2837–2850. Reynolds, D.M., Ahmad, S.R., 1997. Rapid and direct determination of wastewater BOD values using a fluorescence technique. Water Res. 31, 2012–2018. Srikanth, S., Venkata, M.S., 2012. Change in electrogenic activity of the microbial fuel cell (MFC) with the function of biocathode microenvironment as terminal electron accepting condition: influence on overpotentials and bio-electro kinetics. Bioresour. Technol. 119, 241–251. Venkata Mohan, S., Mohanakrishna, G., Sarma, P.N., 2010. Composite vegetable waste as renewable resource for bioelectricity generation through non-catalyzed open-air cathode microbial fuel cell. Bioresour. Technol. 101, 970–976. Virdis, B., Rabaey, K., Rozendal, R.A., Yuan, Z.G., Keller, J., 2010. Simultaneous nitrification, denitrification and carbon removal in microbial fuel cells. Water Res. 44 (9), 2970–2980.
849
Vreugdenhil, D., Bradshaw, J., Gebhardt, C., Govers, F., MacKerron, D.K.L., Taylor, M.A., Ross, H.A., 2007. Chapter 24 Potato flavour and texture. Potato Biology and Biotechnology: Advances and Perspectives, p. 535 (Netherlands). Xiao, B.Y., Yang, F., Liu, J.X., 2013. Evaluation of electricity production from alkaline pretreated sludge using two-chamber microbial fuel cell. J. Hazard. Mater. 254-255, 57–63. Zhao, G., Ma, F., Li, W., Chua, H., Chang, C.C., Zhang, X.J., 2012. Electricity generation from cattle dung using microbial fuel cell technology during anaerobic acidogenesis and the development of microbial populations. Waste Manag. 32, 1651–1658. Zuo, Z., Wu, S., Qi, X., Dong, R., 2015. Performance enhancement of leaf vegetable waste in two-stage anaerobic systems under high organic loading rate: role of recirculation and hydraulic retention time. Appl. Energy 147, 279–286.