Abstract

The effects of mixed feeding of boiled potato and waste activated sludge (WAS) on the performance of a microbial fuel cell (MFC) in treating solid potato waste were investigated. The coulombic efficiency (CE) of four MFCs fed with potato cubes containing 0, 48.7, 67.3 and 85.6% of boiled potato was 53.5, 70.5, 92.7 and 71.1%, respectively, indicating enhanced electricity generation and the existence of an optimum mixing ratio. The hydrolysis rate estimated using a first-order sequential hydrolysis model increased from 0.061 to 0.191 day−1, leading to shortening of the startup time for current density reaching its maximum from 25 to 5 days. The final chemical oxygen demand (COD) removal reached 85%. The CE of seven MFCs, fed with raw potato alone, sterilized/unsterilized WAS alone, and four mixed samples of raw potato with sterilized WAS at ratios of 2:1 and 4:1 and unsterilized WAS at 2:1 and 4:1, was found to be 6.1, 43.6, 0.3, 31.0, 16.5, 0.9 and 31.1%, respectively. The hydrolysis rate increased from 0.056 to 0.089 day−1, and the final COD removal changed from 39.5 to 89.6% following the order: potato alone > mixture of potato & WAS > sterilized WAS alone > unsterilized WAS alone.

HIGHLIGHTS

  1. Mixing boiled potato in with raw potato enhanced the electricity generation efficiency of MFC.

  2. Mixing boiled potato in with raw potato shortened the startup time for electricity generation.

  3. Mixing anaerobically cultured WAS could enhance hydrolysis of raw solid potato.

  4. Enhanced hydrolysis by mixing WAS significantly enhanced electricity generation.

  5. Mixing inactivated WAS enhanced electricity generation by serving as substrate.

INTRODUCTION

Potato is widely planted, processed and consumed around the world. According to the Food and Agriculture Organization (FAO), the world's total potato production was about 368 million tons in 2013. The wasted potato, which contains a higher amount of biodegradable organic matter than other staple crops, accounts for a considerable proportion of the total 1.3 billion tons of food waste generated before and during domestic and industrial consumption every year in the world (Gustavsson et al. 2011; Cook 2014).

On the other hand, the increase of waste activated sludge (WAS) generated from municipal wastewater treatment plants (MWTPs) due to consistent population growth and rapid urbanization is also a great cause of concern. The conventional methods for treatment and disposal of WAS include incineration, landfill, anaerobic digestion, composting, dumping without undergoing any destabilization treatment, and application as raw material for production of cement, bricks and asphalt (Chen et al. 2012). The expense of sludge treatment could reach about 60% of the total operational cost of most MWTPs (Canales et al. 1994).

Microbial fuel cell (MFC) is a technology that can convert organic matter into electricity (Logan et al. 2006). Many studies have been conducted using MFC to generate electricity from wastewater or waste biomass (including food waste, WAS and animal waste) (Jiang et al. 2009; Goud et al. 2011; Zhao et al. 2012). For food waste, most studies conducted so far used its leachate or crushed slurries (Goud et al. 2011; Durruty et al. 2012; Li et al. 2014), with direct use of its solid forms being very limited. For potato waste, since a significant part of it from cultivation, storage and processing is in solid forms, treatment in solid forms without being crushed into liquids or slurries is very meaningful. In a previous study by Du et al. (2015), potato cubes were used at three different sizes cut from fresh potato as the substrate. The authors compared the performance of MFC and the results indicated that decreasing the size of the potato cube could increase the rate of hydrolysis, hence elevating the performance of MFC. To improve the performance of MFC in treatment of potato waste with larger sizes of cube, methods that could accelerate hydrolysis are required.

Heat treatment can promote hydrolysis of food waste (Ariunbaatar et al. 2015). Since vegetable waste may contain cooked vegetables, it is reasonable to infer that mixed feeding of the raw potato with cooked potato may be a possible way to improve the performance of MFC. On the other hand, WAS after cultivation under anaerobic conditions may contain bacterial species that can perform hydrolysis. It is thus hypothesized that by feeding raw solid potato together with WAS, the treatment efficiency of MFC may be elevated.

The main objective of this study was to investigate the effect of mixed feeding of raw solid potato with (1) boiled potato and (2) WAS on the performance of MFC in treatment of potato waste. For the effects of boiled potato, four two-chamber MFCs were operated, in which four mixed samples of raw and boiled potato with different mass fractions for the boiled potato were fed respectively. For the effects of WAS, seven two-chamber MFCs were operated, to which mixed samples of raw potato cubes with sterilized and unsterilized WAS were introduced, respectively. Use was made of both sterilized and unsterilized WAS to answer: (1) if WAS could accelerate the hydrolysis of potato and then enhance electricity generation and potato removal, (2) if WAS could be involved only as additional substrate, and (3) if the bacteria in WAS could preferentially uptake the hydrolysis products of potato, hence reducing their availability for use by electrogenic bacterial species.

MATERIALS AND METHODS

Solid potato and WAS

White potato harvested in Hokkaido, which accounts for nearly 80% of the total potato production in Japan, was purchased from a supermarket in Gifu, Japan. It contains about 80% of starch, 1.5–2.3% of protein, 0.1–1.1% of fat and 0.6–0.8% of fiber (Vreugdenhil et al. 2011). A part of the purchased fresh potato was boiled to well-cooked level for eating. The differences in the content of water and organic matter, total solids (TS), volatile solids (VS) and chemical oxygen demand (COD) between the raw and boiled potato are presented in Table 1. Both the raw and boiled potato were cut into cubes with a length of 5 mm for direct use. The boiled potato cubes were mixed into the raw ones with a mass fraction of 0, 48.7, 67.3 and 85.6% (in wet weight), respectively.

Table 1

The content of water, organic matter, TS, VS and COD in raw and boiled potato, and the mass fraction of boiled potato in the potato feed to MFCs of this study

 Water content
 
Organic content
 
TS
 
VS
 
COD
 
 (%) (mg/g-wet) 
Raw potato 84.6 90.7 154.0 139.7 175.5 
Boiled potato 83.7 89.2 163.0 145.4 182.7 
Mass fraction of boiled potato in the mixed feed (%) 
 MFC 1 MFC 2 MFC 3 MFC 4 
Wet weight 48.7 67.3 85.6 
Dry weight 48.3 68.7 86.2 
 Water content
 
Organic content
 
TS
 
VS
 
COD
 
 (%) (mg/g-wet) 
Raw potato 84.6 90.7 154.0 139.7 175.5 
Boiled potato 83.7 89.2 163.0 145.4 182.7 
Mass fraction of boiled potato in the mixed feed (%) 
 MFC 1 MFC 2 MFC 3 MFC 4 
Wet weight 48.7 67.3 85.6 
Dry weight 48.3 68.7 86.2 

Organic content refers to the weight percentage of organic matter in the total dry solid potato (VS/TS).

For WAS, returned activated sludge collected from a MWTP in Gifu city was used. This plant was operated in anaerobic-aerobic mode for enhanced phosphorus removal. The collected sludge was settled for 4 hours and then cultured by addition of glucose in the dark without aeration for one week. After culturing, about half of the sludge was sterilized using NaClO at the concentration of 300 mg/L as Cl2. The sterilized WAS, together with the remaining half of the unsterilized WAS, were washed with Milli-Q water through mixing and centrifuging five times and were mixed separately with the raw potato cubes for the ratios of potato:WAS of 2:1 and 4:1 (in wet weight), respectively. The pH, TS, organic content, total COD (TCOD) and soluble COD (SCOD) of both the sterilized and unsterilized WAS are presented in Table 2.

Table 2

The characteristics of both the sterilized and unsterilized waste activated sludge (WAS)

Parameters Sterilized WAS Unsterilized WAS 
pH 6.90 6.91 
TS (%) 6.1 5.4 
Organic content (%, VS/TS) 75.2 75.2 
TCOD (mg/L) 1,769 1,569 
SCOD (mg/L) 350 315 
Parameters Sterilized WAS Unsterilized WAS 
pH 6.90 6.91 
TS (%) 6.1 5.4 
Organic content (%, VS/TS) 75.2 75.2 
TCOD (mg/L) 1,769 1,569 
SCOD (mg/L) 350 315 

TCOD: estimated value from the VS of the dry WAS by using 50% of VS as organic carbon; SCOD: measured value for the WAS solution as SCODCr after filtration through 0.2 μm membrane filter.

MFC configuration and operation conditions

Eleven two-chamber MFCs with a total volume of 0.25 L for each chamber (a working volume of 0.24 L) were assembled. 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. Cation exchange membrane was used to separate the two chambers (Feng et al. 2010). Inlet, outlet and a sampling port were designed for both chambers, and a port for aeration was designed for the cathode chamber. The anode and cathode were connected by a titanium wire. The external resistance used during the operation was 100 Ω.

Bacterial consortia were collected from MFCs operated in our previous study (Du et al. 2015). Inoculation was conducted using 20 mL of the bacterial consortia in 0.1 M phosphate buffer solution (PBS) that also contained trace minerals and vitamins (Du et al. 2015). After inoculation, the bacterial consortia were cultured using sodium acetate until current density reached stability, in order that the electricity-generating bacteria were fully enriched on the anode and the performance differences of the MFCs could be explained as the results of different feeding conditions to be compared in this study. The mixed samples of raw and boiled potato cubes, and those of raw potato cubes and WAS were added to the anode chamber of the MFCs, respectively, according to the designated quantities presented in Table 3. Then, PBS was filled into the anode chamber of each MFC until reaching its working volume of 0.24 L. In Table 3, the substrate loadings of all 11 MFCs (converted as CODCr) are also presented. All MFCs were placed on a horizontal shaker and shaken to make the anodic solution of the MFCs homogeneous. The temperature of the MFC operation was controlled at 30 °C. The pH of the anodic solution was adjusted to 7.0–7.1 by adding 1 N-NaOH or 1 N-HCl through the sampling port of each anode chamber every 4–6 days based on the results of monitoring with a micro pH electrode (ELP-036, DKK-TOA) that can be inserted into the sampling port. The adjustment of pH was made in order to maintain a stable and identical pH condition for all MFCs for comparison and for the activity of electricity-generating bacteria. The cathode chamber was fed with the PBS solution and aeration was conducted continuously with an air pump. The anodic solution was sampled through the sampling port with a volume of 1 mL for each sample. The schematic diagram of the MFC configuration is provided in Figure 1.

Table 3

The wet weight of raw and boiled potato cubes added into the reactors of MFC 1–4 and the wet weights of raw potato and WAS added into the reactors of MFC 5–11

 Raw and boiled potato cubes (g)
 
 Raw potato cubes and WAS (g)
 
MFC1 MFC2 MFC3 MFC4 MFC5 MFC6 MFC7 MFC8 MFC9 MFC10 MFC11 
Raw potato 2.5 1.3 0.8 0.4 Raw potato 5.0 – – 5.0 5.0 5.0 5.0 
Boiled potato – 1.2 1.7 2.1 Sterilized WAS – 5.0 – 2.5 – 1.25 – 
Unsterilized WAS – – 5.0 2.5 – 1.25 
Loading 1,830 1,864 1,879 1,891 Loading 3,360 1,769 1,569 4,245 4,145 3,802 3,752 
 Raw and boiled potato cubes (g)
 
 Raw potato cubes and WAS (g)
 
MFC1 MFC2 MFC3 MFC4 MFC5 MFC6 MFC7 MFC8 MFC9 MFC10 MFC11 
Raw potato 2.5 1.3 0.8 0.4 Raw potato 5.0 – – 5.0 5.0 5.0 5.0 
Boiled potato – 1.2 1.7 2.1 Sterilized WAS – 5.0 – 2.5 – 1.25 – 
Unsterilized WAS – – 5.0 2.5 – 1.25 
Loading 1,830 1,864 1,879 1,891 Loading 3,360 1,769 1,569 4,245 4,145 3,802 3,752 

MFC 1–4 were reactors for investigation of the effect of mixing boiled potato at the mass fraction of 0%, 48.7%, 67.3% and 85.6%; MFC 5–11 were reactors for investigation of the effect of mixing WAS: potato alone; sterilized/unsterilized WAS alone; potato & sterilized WAS (2:1); potato & unsterilized WAS (2:1); potato & sterilized WAS (4:1); potato & unsterilized WAS (4:1); Loading was organic loading as CODCr (mg/L). The two set of experiments were conducted separately at different time periods.

Figure 1

The schematic diagram of the MFC configuration.

Figure 1

The schematic diagram of the MFC configuration.

Analysis

Voltage was measured every minute using a digital multimeter and a data acquisition system (midi LOGGER GL200A, Graphtec Corporation, Japan). 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. In order to compare the performance, the coulombic efficiency (CE) was calculated according to Equation (1) (Logan 2007): 
formula
(1)
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. Total and soluble CODCr in the liquid phase of each anodic chamber were measured by the colorimetric method (DR/890 Colorimeter). For soluble CODCr, sampled solutions that had been filtered through a 0.2 μm membrane filter were used. Seven volatile fatty acids (VFAs), namely citrate, acetate, propionate, butyrate, isobutyrate, valerate and isovalerate in the filtered samples were also quantified using a high performance liquid chromatograph (Shimadzu Co., Japan) in order to evaluate the changes in the composition of the dissolved organic matter. The column used was Shim-pack SCR-102H (8 mm × 300 mm, Shimadzu), the eluent was 5 mmol/L of p-toluenesulfonic acid, and the flow rate was 0.8 mL/min. The concentrations of these seven VFAs were determined based on the standard curves generated in terms of the peak areas of their standard solutions with known concentrations.

Estimation of the first-order hydrolysis and degradation rate parameters

Consumption of the solid substrate in each MFC involving the combined biological reactions of hydrolysis and degradation could be described by the following first-order sequential reaction model (Li et al. 2005): 
formula
(2)
 
formula
(3)
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 cubes and WAS, 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 in the anodic solution: 
formula
(4)
kh and kd could be found by minimizing the differences between the calculated COD with Equation (4) and the observed COD according to the following definition: 
formula
(5)
where, Ci(obs) and Ci(cal) are observed and calculated COD concentration in the samples, respectively, and n is the total number of samples collected for COD measurements of each reactor.

RESULTS AND DISCUSSION

Mixed feeding effects of boiled potato cubes

The observed time profiles of current density are provided in Figure 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 to be effective in shortening the time needed for the current density to reach its maximum level (referred to hereafter as ‘peak time’) due to the increased rate of electricity generation in the initial time period. As can be seen from this figure, compared to the peak time of about 25 days needed for the MFC with the raw potato cubes alone (0% of boiled potato), the peak time for the MFC with 85.6% of boiled potato was markedly shortened to about 5 days. The reason was probably because increasing the amount of boiled potato provided more easily-biodegradable organic matter from the initial time of the MFC operation, thus shortened the startup time for electricity generation and enhanced the overall electricity generation efficiency (Ariunbaatar et al. 2015; Pang et al. 2015).

Figure 2

Effect of mixed feeding 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).

Figure 2

Effect of mixed feeding 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).

Based on the observed time profiles of current density as shown in Figure 2 and the time profiles of COD shown later in Figure 3(a), the CE of the four MFCs was estimated as 53.5, 70.5, 92.7 and 71.1%. The lowest CE (53.5%) was found for the feed with the raw potato alone (i.e., 0% of boiled potato) and the largest (92.7%) for the feed 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 cubes with sizes of 3, 5 and 7 mm (Du et al. 2015), as summarized in Table 4.

Table 4

COD removal, current density, maximum power density and coulombic efficiency of this study, together with those of authors' previous studies on treatment of vegetable waste by the same type of MFC

Substrate type Substrate form/ratio Initial COD (mg/L) COD removal (%) Current density (mA/m2Maximum power density (mW/m2Coulombic efficiency (%)  
Raw potato with different mass fractions of boiled potato (%) MFC 1 1,830 86.6 160 – 53.5 This study 
MFC 2 48.7 1,864 83.9 180 – 70.5 
MFC 3 67.3 1,879 84.1 254 – 92.7 
MFC 4 85.6 1,891 86.3 243 – 71.1 
Raw potato & sterilized/ unsterilized WAS MFC 5 Potato cubes 3,360 89.6 36 2.3 6.1 
MFC 6 Sterilized WAS 1,769 40.6 85 1.4 43.6 
MFC 7 Unsterilized WAS 1,569 39.5 0.3 
MFC 8 Potato cubes: sterilized WAS of 2:1 4,245 56.4 120 4.0 31.0 
MFC 9 4:1 3,802 73.7 90 4.0 16.5 
MFC 10 Potato cubes: unsterilized WAS of 2:1 4,145 71.1 10 0.9 
MFC 11 4:1 3,752 66.7 150 6.8 31.1 
Potato Digested liquid after removing debris by settling 833 91.1 100.2 3.5 17.3 Du et al. (2015)  
Tomato 833 89.6 86.7 3.4 21.2 
Lettuce 833 93.2 72.2 2.8 15.6 
Raw potato Solid cubes with side length of 3 mm 1,828 88.0 189.1 16.5 63.9 
5 mm 1,828 88.5 178.9 15.3 58.6 
7 mm 1,828 91.8 163.3 14.6 51.5 
Substrate type Substrate form/ratio Initial COD (mg/L) COD removal (%) Current density (mA/m2Maximum power density (mW/m2Coulombic efficiency (%)  
Raw potato with different mass fractions of boiled potato (%) MFC 1 1,830 86.6 160 – 53.5 This study 
MFC 2 48.7 1,864 83.9 180 – 70.5 
MFC 3 67.3 1,879 84.1 254 – 92.7 
MFC 4 85.6 1,891 86.3 243 – 71.1 
Raw potato & sterilized/ unsterilized WAS MFC 5 Potato cubes 3,360 89.6 36 2.3 6.1 
MFC 6 Sterilized WAS 1,769 40.6 85 1.4 43.6 
MFC 7 Unsterilized WAS 1,569 39.5 0.3 
MFC 8 Potato cubes: sterilized WAS of 2:1 4,245 56.4 120 4.0 31.0 
MFC 9 4:1 3,802 73.7 90 4.0 16.5 
MFC 10 Potato cubes: unsterilized WAS of 2:1 4,145 71.1 10 0.9 
MFC 11 4:1 3,752 66.7 150 6.8 31.1 
Potato Digested liquid after removing debris by settling 833 91.1 100.2 3.5 17.3 Du et al. (2015)  
Tomato 833 89.6 86.7 3.4 21.2 
Lettuce 833 93.2 72.2 2.8 15.6 
Raw potato Solid cubes with side length of 3 mm 1,828 88.0 189.1 16.5 63.9 
5 mm 1,828 88.5 178.9 15.3 58.6 
7 mm 1,828 91.8 163.3 14.6 51.5 

COD removal of each MFC was computed based on the volume loading, i.e., COD at the end of the treatment was the total COD including both suspended and dissolved COD remaining inside the reactor.

Figure 3

Effect of mixed feeding 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).

Figure 3

Effect of mixed feeding 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).

The time profiles of COD in the liquid phase of the anodic chamber are provided in Figure 3(a). 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 can 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. 2015; Zuo et al. 2015). COD increase in the initial period indicated that the releasing rate of organic matter from the added potato cubes due to hydrolysis was faster than the rate of degradation. In contrast, decrease of COD indicated that the rate of degradation exceeded the rate of hydrolysis or that the reaction of hydrolysis had already stopped.

The first-order hydrolysis (kh) and degradation (kd) rate parameters were estimated by fitting the observed COD concentrations with the calculated ones displayed as solid lines in Figure 3(a). As summarized in Table 5, 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%. This implied that mixing boiled potato effectively promoted the overall availability of the potato for bacterial use. The estimated kd varied over 0.053–0.079 day−1, a range much narrower than that of kh. This suggests that the impact of mixing boiled potato on degradation was comparatively smaller than the impact on hydrolysis. At the end of the operation, the total reduction of the potato, assessed by COD, reached 83.9–86.6%, indicating clearly that increasing the fraction of boiled potato did not affect the final overall removal of potato.

Table 5

The estimated rate parameters of hydrolysis and degradation for mixed feeding of the raw potato with boiled potato and WAS, respectively

Hydrolysis and degradation rates kh kd Error 
Raw potato with different mass fraction of boiled potato (%) 0.061 0.068 0.124 
48.7 0.086 0.056 0.107 
67.3 0.169 0.053 0.107 
85.6 0.191 0.079 0.151 
Raw potato – 0.056 0.083 0.180 
Sludge (sterilized) – 0.089 0.092 0.183 
Sludge (unsterilized) – 0.059 0.130 0.253 
Raw potato and sterilized WAS at different ratios 2:1 0.039 0.091 0.203 
4:1 0.071 0.177 0.192 
Raw potato and unsterilized WAS at different ratios 2:1 0.061 0.200 0.216 
4:1 0.089 0.088 0.149 
Hydrolysis and degradation rates kh kd Error 
Raw potato with different mass fraction of boiled potato (%) 0.061 0.068 0.124 
48.7 0.086 0.056 0.107 
67.3 0.169 0.053 0.107 
85.6 0.191 0.079 0.151 
Raw potato – 0.056 0.083 0.180 
Sludge (sterilized) – 0.089 0.092 0.183 
Sludge (unsterilized) – 0.059 0.130 0.253 
Raw potato and sterilized WAS at different ratios 2:1 0.039 0.091 0.203 
4:1 0.071 0.177 0.192 
Raw potato and unsterilized WAS at different ratios 2:1 0.061 0.200 0.216 
4:1 0.089 0.088 0.149 

kh: first-order hydrolysis rate parameter (day−1); kd: first-order degradation rate parameter (day−1); Error: the difference between calculated and observed COD concentrations defined by Equation (5).

Following the occurrence of hydrolysis, VFAs were generated. Among the seven VFAs analyzed (citrate, isobutyrate, acetate, propionate, butyrate, valerate and isovalerate), four (citrate, isobutyrate, acetate and propionate) were detected, as plotted in Figure 3(b) in the logarithmic scale. Citrate, being a major constituting organic acid in the potato tissue (Laties 1967), was detected at higher concentrations (143–242 mg/L) throughout the whole operation. Acetate, propionate and isobutyrate were detected only in some periods, with their concentrations being below 7 mg/L. Acetate is generally considered easier for biological degradation and was only detected in the periods where higher current density was recorded. Also, the time point where acetate was detected seemed to be earlier for the MFCs treating mixed raw and boiled potato than the MFC treating raw potato alone, reflecting the effect of intensified hydrolysis on acid generation. 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.

Mixed feeding effects of waste activated sludge

The observed time profiles of current density are provided in Figure 4. The maximum current density was about 36, 85 and 5 mA/m2 for the three MFCs used to treat potato alone, sterilized WAS alone and unsterilized WAS alone, respectively. For the four MFCs with the mixed samples of potato and sterilized WAS at the mass ratios of 2:1 and 4:1 and the mixed samples of potato and unsterilized WAS at the mass ratios of 2:1 and 4:1, the maximum current density observed was about 120, 90, 10 and 150 mA/m2, respectively.

Figure 4

Effect of mixing sterilized and unsterilized activated sludge (WAS) on the current density of MFCs treating raw potato cubes.

Figure 4

Effect of mixing sterilized and unsterilized activated sludge (WAS) on the current density of MFCs treating raw potato cubes.

Mixing the sterilized WAS into the raw potato cubes increased the current density; however, the extent of increase was found to be higher with the mass ratio of 2:1 than the ratio of 4:1, indicating that the sterilized WAS was used as an additional carbon source for electricity generation in the MFCs. For mixed feeding of raw potato cubes with unsterilized WAS, it is very interesting to notice that adding a larger amount of WAS (with the ratio of 2:1) significantly inhibited the electricity generation, while the addition of a smaller amount (with the ratio of 4:1) markedly increased electricity generation. This may thus indicate that some bacterial species in the cultured activated sludge could enhance the hydrolysis of potato, hence leading to more hydrolysis products being available for consumption by electrogenic bacteria. However, if the mixing ratio of the cultured activated sludge exceeds a certain level, the hydrolysis products can probably be more preferentially used by the increased number of ordinary heterotrophic bacterial species brought in by the sludge, rather than by the electrogenic bacteria. An optimum mixing ratio may exist, which needs investigation in future studies.

Based on the observed time profiles of current density shown earlier in Figure 4 and the time profiles of COD shown later in Figure 5(a), the CEs of the seven MFCs were estimated as 6.1, 43.6, 0.3, 31.0, 16.5, 0.9 and 31.1%, with the largest CE (43.6%) being associated with the MFC (MFC 2) fed with the sterilized WAS alone, and the smallest CE (0.3%) with the MFC (MFC 3) fed with the unsterilized WAS alone, as reported in Table 4. This indicated that activated sludge after sterilization could be well removed and used as an additional substrate for electricity generation. Moreover, compared to the CE (6.1%) of the MFC (MFC 5) fed with potato alone, the CE (31.1%) of the MFC (MFC 11) fed with the same amount of potato and a smaller amount of unsterilized WAS (the mass ratio of 4:1) was five times larger, indicating that mixing a certain amount of cultivated WAS could obviously improve the electricity generation efficiency of raw solid potato.

Figure 5

Effect of mixing sterilized and unsterilized activated sludge (WAS) on the behavior of COD (a) and VFAs (b) in the anodic solution of MFCs treating raw potato cubes. *(1)–(7): potato alone; sterilized/unsterilized WAS alone; potato & sterilized WAS (2:1); potato & unsterilized WAS (2:1); potato & sterilized WAS (4:1); potato & unsterilized WAS (4:1).

Figure 5

Effect of mixing sterilized and unsterilized activated sludge (WAS) on the behavior of COD (a) and VFAs (b) in the anodic solution of MFCs treating raw potato cubes. *(1)–(7): potato alone; sterilized/unsterilized WAS alone; potato & sterilized WAS (2:1); potato & unsterilized WAS (2:1); potato & sterilized WAS (4:1); potato & unsterilized WAS (4:1).

The time profiles of soluble COD in the liquid phase of the anodic chamber are provided in Figure 5(a). For all seven MFCs, COD revealed a similar trend with the MFCs treating raw and boiled potato, i.e., the concentration increased in the initial period and then gradually decreased. The kh and kd estimated by fitting the observed COD with the calculated COD, as displayed in Figure 5(a), are summarized in Table 5. kh varied over the range of 0.039–0.089 day−1. The treatment for potato alone displayed a relatively lower kh of 0.056 day−1. By mixing the sterilized WAS with the raw potato at a ratio of 4:1 (potato:WAS), kh increased to 0.071 day−1. However, for the treatment with the mixing ratio of 2:1, that is, if more sludge was mixed in, kh decreased to 0.039 day−1, indicating a decrease in the hydrolysis rate of potato. For the two treatments mixed with unsterilized WAS at 4:1 and 2:1, kh increased; however, the extent of increase was found to be larger for the treatment with the smaller amount of WAS (the ratio of 4:1). As can be seen from Table 5, by increasing the amount of WAS, kh decreased from 0.089 to 0.061 day−1, which indicates that mixing a smaller amount of the cultured WAS is more effective for enhancing the hydrolysis rate and thus the overall availability of the potato feed for bacterial use.

In treatment of kitchen waste, Li et al. (2016) reported a kh range of 0.075–0.125 day−1 in ordinary anaerobic fermentation reactors. In addition, using similar reactors fed with settled sludge from the primary sedimentation tank of a wastewater treatment plant, Feng et al. (2009a) studied the kinetic analysis of WAS hydrolysis and estimated a kh value of 0.011 day−1 and 0.072 day−1 when pH was increased to 10. The estimated kh (0.039–0.089 day−1) for the mixture of potato and WAS in the present study are closer to the values of the previous study for the treatment of kitchen waste, suggesting that hydrolysis in the MFC was processed at a pace similar to that in ordinary fermentation reactors.

For the degradation rate parameter kd, the estimated values fell in the range of 0.083–0.200 day−1, with the extent of differences (2.4 times) being similar to that of hydrolysis (2.3 times). At the end of the operation, the total COD removal was 89.6, 40.6, 39.5, 56.4, 73.7, 71.1 and 66.7% (shown in Table 4), displaying an order of potato alone > mixed potato with either sterilized or unsterilized WAS > sterilized WAS alone > unsterilized WAS alone. In the two MFCs fed respectively with the sterilized and unsterilized WAS alone, more than 50% of the initial COD loading still remained in suspended forms. This indicated that potato contains more biodegradable organic matter than WAS and mixing potato into WAS promoted the final WAS removal efficiency. The reason for the promoted removal of WAS was probably that mixing of potato and WAS balanced the overall carbon/nitrogen ratio of the mixture and promoted the activity of bacteria. Feng et al. (2009b) reported that increasing the substrate C/N ratio would benefit the anaerobic fermentation of complex organic matter, which supports our result since the C/N ratio of potato is generally higher than WAS.

The detected VFAs are plotted in Figure 5(b). Citrate and isobutyrate were detected at higher concentrations (62–105 and 6.3–169.5 mg/L) throughout the operation. Acetate, propionate, butyrate, valerate and isovalerate were only detected at some periods, with their concentrations being below 12 mg/L. For the MFC fed with the potato alone, acetate was detected only after 54 days' operation, with the concentration being lower than 1 mg/L. For the MFCs fed with mixed samples of potato and sterilized/unsterilized WAS, the concentration of acetate increased and the time period needed for the hydrolysis product (acetate) to emerge was shortened to 24 days. The concentration increases of acetate for the mixed feeding with sterilized WAS were probably a result of the effect of the sterilized WAS serving as an additional source of substrate. The increased concentration of acetate for the mixed feeding with the unsterilized WAS was probably due to the combined effect of WAS that worked both as the source of bacteria for hydrolysis of potato and the source of additional substrate.

CONCLUSIONS

The study revealed that mixing boiled potato in with the raw solid potato feed accelerated hydrolysis products at the initial stage of MFC operation, elevated the maximum current density and shortened the time needed for the current density to reach its maximum levels. The estimated CE confirmed the positive effect of the mixed feeding, and also indicated the existence of an optimum mixing ratio.

Mixing cultured WAS into raw solid potato could accelerate the hydrolysis rate of potato and increase the electricity generation efficiency. However, an optimal mixing ratio may exist, which needs clarification in future studies. Therefore, it can be concluded that the cultured WAS added in MFC treatment for raw solid potato functioned both as a bacterial source for hydrolysis of potato and a source of additional substrate for electricity generation.

ACKNOWLEDGEMENT

This research was supported in part by the Natural Science Foundation of Jiangsu Province for the project ‘Performance and mechanism of two chamber microbial fuel cells for simultaneous treatment of activated sludge and nitrogen containing wastewater’ (Project Number: BK20171017).

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