Liang, S., 2015. Waste Management. Anaerobic digestion of pre-fermented potato peel wastes for methane production, [Online]. 46, 197-200. Available at: https://www.researchgate.net/publication/282351165_Anaerobic_digestion_of_pre-fermented_potato_peel_wastes_for_methane_production [Accessed 1 March 2019].
This paper confirmed the use of waste potato, the second highest global wasted food as a waste material that produces methane when anaerobic digested.
This is important because, potato starch is the ingredient in IAM fuel that binds the materials together, also potato starch is the ingrediant that acts as a moisture barrier.
The individual ingredients are from waste streams, reducing environmental impact, financial costs of manufacture. The waste material is chosen to add purpose as a food tray and as a biofuel, when anaerobic digested of incinerated.
Abstract
This study investigated the feasibility of anaerobic digestion (AD) of potato peel waste (PPW) and its lactic acid fermentation residue (PPW-FR) for methane (CH4) production. The experimental results showed that about 60-70% CH4 content was obtained. The digester using PPW-FR as feedstock exhibited better performance and produced a highest cumulative CH4 production of 273L/kg VSfed, followed by 239L/kg VSfed using PPW under the same conditions. However, with increasing solid loadings of PPW-FR feedstock from 6.4% to 9.1%, the CH4 production was inhibited. The generation, accumulation, and degradation of volatile fatty acids (VFAs) in digesters were also investigated in this research.
Anaerobic digestion of pre-fermented potato peel wastes for methane
production
Shaobo Liang, Armando G. McDonald
⇑
Renewable Materials Program, Department of Forest, Rangeland and Fire Sciences, University of Idaho, 875 Perimeter Drive MS1132, Moscow, ID 83844, United States
article info
Article history:
Received 20 April 2015
Revised 19 September 2015
Accepted 21 September 2015
Available online 28 September 2015
Keywords:
Agricultural waste
Methane
Fermentation residue
Potato peel waste
Anaerobic digestion
abstract
This study investigated the feasibility of anaerobic digestion (AD) of potato peel waste (PPW) and its
lactic acid fermentation residue (PPW-FR) for methane (CH
4
) production. The experimental results
showed that about 60–70% CH
4
content was obtained. The digester using PPW-FR as feedstock exhibited
better performance and produced a highest cumulative CH
4
production of 273 L/kg VS
fed
, followed by
239 L/kg VS
fed
using PPW under the same conditions. However, with increasing solid loadings of
PPW-FR feedstock from 6.4% to 9.1%, the CH
4
production was inhibited. The generation, accumulation,
and degradation of volatile fatty acids (VFAs) in digesters were also investigated in this research.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Potatoes are a major crop for the United States Pacific Northwest
(Idaho, Washington and Oregon), with 12.5 million tons harvested in
2011 (PotatoPRO, 2015), and approximately 50% of the potatoes
were processed into fries and other products. Potato peel waste
(PPW) is the main co-product of steam peeling generating about
8% waste by weight (Mader et al., 2009), and waste disposal is
becoming a major issue. An alternative method has been developed
to convert PPW into mainly lactic acid with mixed microbial consor-
tia in batch and sequencing batch fermenters (Liang et al., 2015a,
2014, 2015c). This fermentation process can efficiently utilize easily
digestible starch in the PPW and leave the un-reacted cellulosic
material in the solids, called ‘‘PPW fermentation residue”
(PPW-FR), which is still a waste burden for disposal. A comprehen-
sive chemical characterization of PPW-FR reported previously that
contained approximate 20% cellulose/hemicellulose, 30% lignin
+ suberin, 25% protein, and 8% lipids (Liang and McDonald, 2014).
Due to its composition, the PPW-FR has been applied to produce
other value added bio-based products such as bioplastic composites,
pyrolysis crude bio-oil, and bio-char (Liang et al., 2015b; Wei et al.,
2015a). In addition, the abundant carbon and nutrients contents also
make its good feedstock for biogas production via anaerobic diges-
tion (AD) process (Dreschke et al., 2015; Parawira et al., 2007, 2004).
AD is a biochemical conversion process to decompose organic
materials into biogas (50–70% CH
4
) by mixed anaerobic microor-
ganisms in the absence of oxygen. The AD system has been widely
applied in the environmental management of wastewater sludge,
municipal solid waste, and livestock manure, with the primary
objective of waste reduction. The produced biogas as renewable
energy can also improve the operational profitability, reduce the
greenhouse gas (GHG) emission, and mitigate climate change
(Coats et al., 2013). It is reported that more than 2000 AD plants
are currently operated in the United States to produce biogas,
and an additional 11,000 sites could be developed in the future,
which in total could provide power supply for over 3 million
households and reduce between 4 and 54 million metric tons
CH
4
equivalent of GHG emissions by 2030 (USDA et al., 2014).
Parawira et al. (2007) compared the biogas production from PPW
using two-stage digesters with mesophilic and thermophilic con-
figurations, and results showed that higher CH
4
yield was obtained
under mesophilic condition.
In this study, we investigated the feasibility of biogas produc-
tion using PPW-FR as feedstock with different solid loadings to
explore the suitable condition. The raw PPW without lactic acid
fermentation was also employed in batch AD process as
comparison.
2. Materials and methods
2.1. Feedstock and inoculum
The PPW (Russet Burbank) sample was collected over a 2 h
period from a potato processing plant (JR Simplot Company,
Nampa, ID) in May 2012, mixed thoroughly and stored frozen
0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
⇑
Corresponding author.
E-mail address: armandm@uidaho.edu (A.G. McDonald).
Waste Management 46 (2015) 197–200
Contents lists available at ScienceDirect
Waste Management
journal homepage: http://www.elsevier.com/locate/wasman
(20 °C) in plastic containers before use. This PPW feedstock was
fermented to produce mainly lactic acid in a 1 L anaerobic
sequencing batch reactor under mesophilic (35 °C) conditions as
described previously (Liang et al., 2015c). The solid residue was
separated from fermentation broth by centrifugation at 5000 rpm
for 20 min, and the PPW-FR was obtained and stored in 20 °C
freezer prior to use. The AD inoculum was collected freshly from
the effluent of a 19 L mesophilic AD located in Department of Civil
Engineering, University of Idaho (Moscow, ID) that treated dairy
manure to produce biogas (Coats et al., 2012). The characteristics
of PPW, PPW-FR, and inoculum used in this experiment are given
in Table 1.
2.2. AD set-up
The experimental design of AD in this study is shown in Table 2.
Briefly, varying amounts of homogenized PPW and PPW-FR sam-
ples were mixed with sodium bicarbonate and water to adjust
pH to 7 and total solid (TS) content. After releasing of all the CO
2
bubbles, varying amounts of inoculum was added into each reac-
tor. All tests were conducted in batch mode using 800 mL serum
bottles fitted with a rubber septum and incubated in a water bath
at 35 °C for 40 days with manually shaking twice a day. The diges-
ters, PPW-1 and PPW-FR-1, have the same TS content of 6.4%, vola-
tile solid (VS) content of 5.2%, and feedstock/inoculum (F/I, the
amount of feedstock VS per amount of inoculum VS) ratio of 2.6.
Increasing TS, VS, and F/I ratios were conducted in digesters
PPW-FR-2 and PPW-FR-3. A blank AD reactor containing inoculum
(150 g) and water (150 g) was used as control. Biogas was collected
daily in a 1-L Tedlar
TM
gas sampling bag attached to the outlet of the
reactor with syringe needle and plastic tubing, and the produced
gas volume was quantified by the water displacement method.
An aliquot of liquid sample was withdrawn from the digester every
four days using a long needle syringe for analysis, and a solid sam-
ple was collected at the beginning and the end of digestion for
characterization.
2.3. Analytical methods
The TS content was determined by drying at 105 °C for 24 h, and
the VS content was based on the weight loss of TS at 550 °C till con-
stant weight was obtained. The C and N contents of solid samples
were determined using a CE-440 elemental analyzer (EAI Exeter
Analytical). Liquid and solid samples from the digesters were
mixed with water (10 volume) at room temperature for 30 min
prior to pH, ammonium nitrogen (NH
4
-N), total phosphorous (TP)
and volatile fatty acids (VFAs) measurement. The pH value was
measured with Orion-3-Star pH meter. The NH
4
-N and TP were
determined by Nessler method 4500-NH
3
and ascorbic acid
method 4500-P, respectively using a Beckman D640 spectropho-
tometer (Fullerton, CA) (APHA et al., 1998). VFAs were quantified
by gas chromatography (GC) using an Agilent 6890 instrument
(Palo Alto, CA) with an Alltech-Heliflex-AT
TM
Wax capillary column
(U 0.32 mm 30 m, Deerfield, IL) at 150 °C and flame ionization
detection (210 °C). The samples were acidified to pH 2 with
HNO
3
prior to injection. The composition of biogas (CH
4
and CO
2
)
in the gas sampling bags was analyzed using a Gow-Mac 350 ther-
mal conductivity GC (GOW-MAC Instrument, Bound Brook, NJ)
equipped with HaySep DB stainless steel packed column
(9.1 m 3 mm) operating at 30 °C with a detector temperature of
200 °C and helium as carrier gas (30 mL/min). Standard curves
were prepared from calibration standards of CO
2
and CH
4
in N
2
for quantification.
2.4. Statistical analysis
The AD experiments were conducted in duplicate and the
average values reported. The results were analyzed with SAS 9.3
software (SAS Inc., Cary, NC, USA) using a t-test with a threshold
p-value of 0.05.
3. Results and discussion
3.1. Methane production
The CH
4
content, CH
4
yield, and cumulative CH
4
production in
the four digesters with different substrates and conditions are
illustrated in Fig. 1. Among these digesters, the CH
4
content of
the biogas increased gradually to 60–70% after 8–10 days, except
for digester PPW-FR-3 (having the highest TS content of 9.1% and
VS content of 5.0%), which realized a biogas content of 65% on
day 14 (Fig. 1A). The slower increase in CH
4
composition was prob-
ably due to high organic loadings caused methanogenesis inhibi-
tion. The CH
4
contents in all digesters ultimately stabilized
around 65%, which was in accordance with previous studies using
potato waste as feedstock to produce biogas (Parawira et al., 2004).
It is known that the CH
4
production from lignocellulosic biomass
by AD usually generates low CH
4
content around 50–60% (Zhu
et al., 2010). The relative higher CH
4
contents obtained in this
study were probably due to the higher lipids contents of 2–8% in
the feedstock PPW and PPW-FR as compared to <1% in woody bio-
mass (Bayr et al., 2014). Lipids have lower oxidation state and
higher energy storage than carbohydrates and thereafter tend to
generate higher CH
4
content in AD system.
The digester PPW-1 and PPW-FR-1 with identical TS and VS
contents shared similar trends of daily CH
4
production (Fig. 1B).
A small peak was observed at the beginning of digestion in both
PPW-1 and PPW-FR-1, which was due to the quick conversion of
available VFAs into biogas, as shown in Fig. 2. After 4–6 days incu-
bation, the CH
4
production increased dramatically and reached the
highest daily yield of 16.8 and 20.8 L/kg VS
fed
on day 10 and 8,
respectively, in digester PPW-1 and PPW-FR-1. However, as
Table 1
Characteristics of the PPW, PPW-FR and inoculum.
Parameters PPW PPW-FR Inoculum
TS (% wb)
a
9.2 12.2 3.6
VS (% db) 82.0 79.5 80.1
Carbohydrates 39.3 22.4 –
Lignin 21.3 37.0 –
Lipids 2.0 7.7 –
Carbon (% db) 43.8 47.8 –
Nitrogen (% db) 4.1 4.1 –
C/N 10.7 11.9 –
pH 6.5 4.2 7.5
a
The TS content is wet basis (wb), and the VS, carbon, and nitrogen contents are
dry basis (db). Data are means of three samples (n = 3).
Table 2
Experimental design of anaerobic digestion of PPW and PFR.
Digester Feedstock
(g)
a
Inoculum
(g)
Water
(g)
TS (%) VS (%) F/I
PPW-1 150 150 0 6.4 5.2 2.6
PPW-FR-
1
115 150 35 6.4 5.2 2.6
PPW-FR-
2
150 150 0 7.9 6.3 3.4
PPW-FR-
3
180 120 0 9.1 7.3 5.0
Control 0 150 150 1.8 1.5 –
a
The feedstock and inoculum are wet basis, and the F/I ratio = the amount of
feedstock VS per amount of inoculum VS.
198 S. Liang, A.G. McDonald / Waste Management 46 (2015) 197–200
compared with digester PPW-FR-1, the daily CH
4
production for
digester PPW-FR-2 and PPW-FR-3 with higher TS and VS contents
were somehow inhibited, which generated a peak daily CH
4
yield
of only 7.7 and 5.9 L/kg VS
fed
, respectively. Similar results were
reported by Li et al. (2011) and Yao et al. (2013) in that CH
4
pro-
duction was decreased with increasing TS and VS contents with
corn stover and poplar residue as feedstock. This could be
explained by the hydrolysis or acidogenesis step being inhibited
by a high organic loading, or toxicity of free ammonia from the
degradation of protein (Chen et al., 2008).
The cumulative CH
4
production in four digesters, as illustrated
in Fig. 1C, exhibited significantly different results. The highest
cumulative CH
4
yield was obtained in digester PPW-FR-1 (273 L/
kg VS
fed
), which was 14% higher than that using PPW as feedstock
in digester PPW-1. A similar study found that a higher CH
4
poten-
tial was obtained from the lactic acid fermentation residue of
municipal organic solid (Dreschke et al., 2015). Coats et al.
(2012) studied the biogas production from dairy manure with a
two-stage AD process, and found that the additional pre-
fermentation in the first stage with short solid retention time of
4 days increased CH
4
content and enhanced the biogas production
as compared to single stage AD process. It is postulated that the
pre-fermentation process can loosen (or disrupt) the recalcitrant
components in the biomass, which make them susceptible to
microbial attack and assimilation by AD. Insufficient digestion
effects were obtained in digester PPW-FR-2 and PPW-FR-3, which
had cumulative CH
4
yields of 150 and 118 L/kg VS
fed
, respectively.
It should be noted that the CH
4
contents in these two digesters
reached a similar level with the other two digesters (PPW-1 and
PPW-FR-1).
3.2. Digester performance
VFAs are important intermediates of carbohydrate, lipids, and
protein degradation in AD process to CH
4
and CO
2
. The production
of VFAs can also lower the pH of the digester and inhibit the
methanogenesis process, depending on the alkalinity of the solu-
tion. Therefore, the balance between VFA generation and conver-
sion to biogas is critical to the stable operation of an AD. The
performance of VFA production, accumulation, and degradation
in digester PPW-1 and PPW-FR-1 are shown in Fig. 2. Acetic acid
was the most dominant VFA, followed by propionic and butyric
acids. No significant VFA accumulation existed after 40 days,
where the concentration of total VFA decreased from 4.98 to
1.45 g/L in digester PPW-FR-1, while digester PPW-1 had a slight
increase from 1.86 to 2.54 g/L. The highest VFA concentrations
were observed at 14.95 and 10.84 g/L on day 20 and 16, respec-
tively for digesters PPW-1 and PPW-FR-1. The produced acetic acid
was consumed quickly, while other VFAs were converted slowly
and exhibited a slight substrate inhibitory effect. These two diges-
ters shared a similar VFA profile (Fig. 2). The main difference
between the two digesters was that digester PPW-FR-1 responded
quicker on VFA production and degradation. Although relative
higher VFAs accumulation was observed in digesters at the end
of reaction, these VFAs can be treated as useful chemical precursors
for renewable fuels and biodegradable plastic polyhydroxyalka-
noate production (Vasquez et al., 2014; Wei et al., 2014, 2015b).
Table 3 gives the properties of three digesters (PPW-1, PPW-FR-
1, and blank) at the beginning and the end of 40 days digestion.
About 74.5% and 35.5% TS and VS reduction were obtained in
digester PPW-1, which was higher than that of 41.2% and 26.1%
reduction in digester PPW-FR-1, while only 23.3% and 0.3% reduc-
tions were observed in the control digester. This suggests that’s
that the PPW-1 was inhibited by the generated higher concentra-
tion VFAs as comparing to PPW-FR-1. At the same time, the pH val-
ues of digester PPW-1 and PPW-FR-1 were increased from 7.0 to
around 8.0, and the control AD remained at 7.5. The TP levels were
found to have no significant difference before and after digestion
and varied between 0.13 and 0.16 g/L, which was probably due
to the low P content (0.4%) in PPW (Mahmood et al., 1998).
Previous studies reported that PPW contained 17–25% protein
(Liang and McDonald, 2014; Liang et al., 2014). Protein degradation
in the AD process releases NH
4
-N, and in this study, the NH
4
-N con-
centration increased from 0.56 to 1.48 g/L in digester PPW-1 and
PPW-FR-1. It is known that the free ammonia (NH
3
-N)/NH
4
-N is
one of the main factor causing reactor failure, whereas the thresh-
old level of methanogenesis inhibition/toxicity varies with differ-
ent substrates and conditions (Yenigun and Demirel, 2013). It is
highly possible that at least the NH
3
-N/NH
4
-N concentration in
digester PPW-FR-1 was not high enough to cause an inhibitory
effect, while the exact threshold level for this AD systems will need
further investigation.
0
20
40
60
80
Methane content (%)
Time (day)
PPW-1
PPW-FR-1
PPW-FR-2
PPW-FR-3
0
5
10
15
20
25
Daily methane yield (L/kg VSfed)
Time (day)
0
100
200
300
400
Cumulative methane yield
(L/kg VSfed)
C
B
A
0 10203040
010203040
PPW-1 PPW-FR-1 PPW-FR-2 PPW-FR-3
Fig. 1. Methane (CH
4
) production in the various anaerobic digesters: (A) CH
4
content of biogas with reaction time; (B) daily CH
4
yield with time; and (C)
cumulative CH
4
yield.
S. Liang, A.G. McDonald / Waste Management 46 (2015) 197–200
199
feet1st 