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Anaerobic digestion of onion residuals using a mesophilicAnaerobic Phased Solids Digester

Rowena T. Romano, Ruihong Zhang*

Department of Biological and Agricultural Engineering, University of California, Davis, 2030 Bainer Hall, One Shields Avenue, Davis,

CA 95616-5294, USA

a r t i c l e i n f o

Article history:

Received 25 February 2010

Received in revised form

7 June 2011

Accepted 17 June 2011

Available online 5 August 2011

Keywords:

Onion

Anaerobic Phased Solids Digester

Biogas

Anaerobic digestion

Food processing residuals

Energy

* Corresponding author. Tel.: þ1 530 752 953E-mail address: rhzhang@ucdavis.edu (R.

0961-9534/$ e see front matter ª 2011 Elsevdoi:10.1016/j.biombioe.2011.06.036

a b s t r a c t

The anaerobic digestion of onion residual from an onion processing plant was studied

under batch-fed and continuously-fed mesophilic (35 � 2 �C) conditions in an Anaerobic

Phased Solids (APS) Digester. The batch digestion tests were performed at an initial loading

of 2.8 gVS L�1 and retention time of 14 days. The biogas and methane yields, and volatile

solids reduction from the onion residual were determined to be 0.69 � 0.06 L gVS�1,

0.38 � 0.05 L CH4 gVS�1, and 64 � 17%, respectively. Continuous digestion tests were carried

out at organic loading rates (OLRs) of 0.5e2.0 gVS L�1 d�1. Hydrated lime (Ca(OH)2) was

added to the APS-Digester along with the onion residual at 16 mg Ca(OH)2 gVS�1 to control

the pH of the biogasification reactor above 7.0. At steady state the average biogas yields

were 0.51, 0.56, and 0.62 L gVS�1 for the OLRs of 0.5, 1.0, and 2.0 gVS L�1 d�1 respectively.

The methane yields at steady state were 0.29, 0.32, and 0.31 L CH4 gVS�1 for the OLRs of

0.5, 1.0, and 2.0 gVS L�1 d�1 respectively. The study shows that the digestion of onion

residual required proper alkalinity and pH control, which was possible through the use of

caustic chemicals. However, such chemicals will begin to have an inhibitory effect on the

microbial population at high loading rates, and therefore alternative operational parame-

ters are needed.

ª 2011 Elsevier Ltd. All rights reserved.

1. Introduction digestion processes (i.e., single-phase and two-phase systems)

Food processing residuals contain biodegradable material and

can cause many environmental problems if disposed of into

a landfill, including the release of greenhouse gases, volatile

organic compounds, odors into the atmosphere, and seepage

and runoff of leachate towater resources. An environmentally

safer approach may be through the controlled treatment of

these residuals via anaerobic digestion. Anaerobic digestion

produces a biogas, containing methane (CH4) and carbon

dioxide that can be used as a source of renewable energy. Food

processing plants can utilize the biogas to meet the energy

demands of their facilities. Different types of anaerobic

0; fax: þ1 530 752 2640.Zhang).ier Ltd. All rights reserve

have been previously investigated for digestion of food

processing residuals [1e7]. Since the hydrolysis and bio-

gasification phases of anaerobic digestion are separated in

a two-phase system, the design offers more flexibility in

handling variable loading. Two-phase systems may also

provide more process stability over a single-phase system

when digesting solid vegetable residuals [7], and are suitable

for residuals containing high amounts of soluble organics [8].

A new solids digestion system, an Anaerobic Phased Solids

(APS) Digester [9] was developed at the University of

California, Davis (UCDavis), and has been successfully applied

for conversion of various organic solid residuals [10e12].

d.

Hydrolysis Reactor

Biogasification Reactor

Biogas recirculationfor mixing Onions

Pump

Return liquid

Biogas to gas meter

Biogas to gas meter

Screens

Biomediapellets

Fig. 1 e Schematic of a batch-fed APS-Digester system used

for the treatment of onion residual.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 1 7 4e4 1 7 9 4175

The system uses a solid bed reactor as the hydrolysis reactor

and an anaerobic sequencing batch reactor (ASBR) or an

anaerobic mixed biofilm reactor (AMBR) as the biogasification

reactor. Previous studies of the APS-Digester were concen-

trated on the batch digestion of solid organic residuals using

several hydrolysis reactors coupled to one biogasification

reactor. However, for rapidly degradingmaterials such as food

processing residuals, coupling one hydrolysis reactor to one

biogasification reactor may be an appropriate approach.

A fresh cut onion processing facility located in Southern

California currently produces 91e136 wet mton of onion

waste per day. The onion residual, consisting of cull onions,

skins, flesh, and top and bottom trims, is disposed at a landfill

and therefore justifies the need to find environmentally-

friendly alternatives. The objectives of this study were to

determine the digestibility of onion residuals in terms of

biogas yield, methane yield, and volatile solids reduction. The

digestibility of the onion residual was studied using a two-

phase APS-Digester, first under batch-fed operation then

under continuous-fed operation.

2. Materials and methods

2.1. Characterization of onion residual

Fresh onion residual was collected by a Southern California

onion processor and delivered to the Bioenvironmental

Engineering Lab at the University of California, Davis (Davis,

CA). The onion residual was received as diced pieces about

0.64e1.3 cm2. The onion residual was analyzed for moisture

content (MC), total solids (TS), volatile solids (VS), fixed solids

(FS), and bulk density in the Bioenvironmental Engineering

Lab using standard methods [14]. A dried sample of onion

residual was also prepared in the Bioenvironmental Engi-

neering Laboratory by vacuum drying the material for 24 h

at 55 �C and then grounding the material using a mortar and

pestle. Further analysis on the dried sample was outsourced

to the University of California, Davis, Agricultural and Natural

Resources Analytical Laboratory, for which their methods are

described and listed on the website http://anlab.ucdavis.edu/.

The sample was analyzed for carbon (C) content, nutrients

(N, P, K, S, and B), minerals (Ca, Mg, Zn, Mn, Fe, and Cu), and

fibers (ADF, NDF, and lignin).

2.2. Batch digestion tests

Two mesophilic (35 � 2 �C) laboratory scale batch digestion

experiments were carried out using an APS-Digester system

as shown in Fig. 1. Hydrolysis and fermentation of the organic

material occurred in the first stage (hydrolysis reactor), and

products from the solubilization of the onion residual in the

first stagewere further converted to biogas in the second stage

(biogasification reactor). The hydrolysis reactor had a 2.2-L

total volume and a 2-L working volume, and operated as

a leach bed system. A plastic screen having openings of 2 mm

in diameter was fitted at the top of the reactor to keep the

onions submerged in the liquid. Another screenwas also fitted

at the bottom of the reactor to allow collection and transfer of

the solubilized products from the hydrolysis reactor to the

biogasification reactor. The biogasification reactor had a 4-L

total volume with a 3-L working volume, and operated as an

anaerobic mixed biofilm reactor (AMBR). The biogasification

reactor contained biomedia pellets to encourage biofilm

formation and the retention of methanogens. The biomedia

pellets were polyethylene cylinders of approximately 10 mm

in diameter and height (Kaldnes Miljoteknologi AS, Norway)

and have a density of 0.95 g cm�3. The pellets moved with the

reactor liquid during the mixing phase, but would float to the

top of the reactor during the settling phase. Some pellets were

observed to become entrapped in the sludge bed, but most

of the pellets floated to the top of the reactor even when

sludge was apparent on the pellet. The pellets occupied about

25% of the reactor volume.

The reactors were seeded with inoculum from a working

mesophilic anaerobic digester at the Davis Wastewater

Treatment Plant (Davis, CA). In each batch test, the onion

residual (14.4 � 0.25 gVS) was loaded into the hydrolysis

reactor and digested for 14 days until daily biogas production

was negligible. During the batch test, the liquid in the hydro-

lysis and biogasification reactors recirculated at a rate of

84 mL every 2 h (i.e., 12 times per day) to achieve a total

transfer rate of 1 L d�1. At each recirculation, liquid was

transferred from the lower compartment of the hydrolysis

reactor to the bottom of the biogasification reactor, and the

same amount of liquid was returned to the top of the hydro-

lysis reactor from the upper part of the biogasification reactor.

The liquid from the hydrolysis reactor contained the solubi-

lized organic compounds produced from the degradation of

the onion residual. After each liquid recirculation, the bio-

gasification reactor was thoroughly mixed for 2 min using the

headspace gas, and the contents were allowed to react and

settle for 2 h until the next recirculation.

The biogas produced from each reactor was directed to

a wet gas tip meter and the biogas volume was recorded daily

[13]. Daily liquid samples (2 mL) were taken from the hydro-

lysis and biogasification reactors and measured for pH. At the

end of digestion, the methane and carbon dioxide content of

the biogas from each reactor was measured. Biogas compo-

sition was measured using gas chromatography (Hewlett

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 1 7 4e4 1 7 94176

Packard 5890) equipped with an Alltech Carbosphere 80/

100 column (Deerfield, Illinois) and a thermal conductivity

detector. The carrier gas was helium (He) at a flow rate of

80 mL/min. The GC operating conditions were: oven temper-

ature at 100 �C, and injection and detector temperatures at

120 �C.

2.3. Continuous digestion tests

For the continuous digestion study, the same biogasification

reactor was used as in the batch system, but the top of the

hydrolysis reactor was modified with a gas tight tube to allow

onions to be loaded into the reactor on a daily basis (Fig. 2). To

load the onions, a ball valve at the bottom of the tube was

initially closed while the tube was filled with the onion solids.

After loading, a push rod having an o-ringwas placed on top of

the loaded onions, sealing the tube from the atmosphere. The

ball valve was then opened and the onions were pushed into

the reactor with the push rod. The ball valve was then closed

and the push rod was removed. The hydrolysis reactor had

a total volume of 1.5 L and a working volume of 1 L. The

hydrolysis reactor was fed daily with the onion residual, and

the same amount of effluent was removed from the bio-

gasification reactor. Between feeding events, liquid was

circulated between the hydrolysis and biogasification reactors

at a rate of 84 mL every 2 h (i.e., 12 times per day) for a total

transfer rate of 1 L per day. The hydrolysis reactor had

a hydraulic retention time of 1 day and the biogasification

reactor had a hydraulic retention time of 3 days. Three organic

loading rates (OLRs) (0.5, 1.0, and 2.0 gVS L�1 d�1) were tested,

during which the pH and biogas production from both reac-

tors were measured daily. At each OLR, when biogas produc-

tion was steady, which occurred after about 10 days of

continuous digestion, biogas samples were taken from both

reactors and analyzed for methane and carbon dioxide

contents using GC/TCD. Statistical analysis (one-tailed t-test)

was conducted on four observations of biogas yield and

Liquid to biogasification reactor

Liquid from Biogasification reactor

Biogas to gas meter

Push rod

O-ring seal

Onion residual

Feed tube Ball valve

Fig. 2 e The hydrolysis reactor of the continuous system

was equipped with a feeding tube to allow loading of onion

residual under anaerobic conditions.

methane yield at each OLR using the Microsoft Excel Data

Analysis Toolkit.

3. Results and discussion

3.1. Characteristics of onion residual

The carbon to nitrogen ratio (C:N) and carbon to phosphorus

ratio (C:P) of the onion residual were 21.6 and 103.5, respec-

tively (Table 1). The ratios were considered appropriate for

anaerobic bacteria and therefore no additional nutrients were

added to the onion residual. The onion residual had a mois-

ture content of 92.6%, a bulk density of 1.04 g/mL, and a vola-

tile solids (VS) content of 96.1% of the total solids (TS). The

fiber content, which consists of cellulose and hemicellulose,

comprised 10% (dry basis) of the onion residual. Other

nutrient and mineral contents of the onion residual for

microbial growth are shown in Table 1.

3.2. Batch digestion tests

In the batch digestion tests, biogas production in the bio-

gasification reactor increased rapidly until day 6 of digestion,

having produced about 90% of the total biogas collected by

day 14 (Fig. 3). By day 14 the biogas production rate had slowed

down to less than 0.05 L gVS�1 d�1 and the reaction was

stopped. Althoughmore biogasmay have been produced if the

reaction was carried out longer, the small increase in biogas

production was considered negligible for this study. In the

hydrolysis reactor, biogas production ceased after 3 days at

which time the pH dropped to 6.0. The pH in the hydrolysis

reactor later gradually increased to 6.9 by the end of digestion

due to the return of the liquid from the biogasification reactor

and continual removal of organic acids from the hydrolysis

reactor. The average cumulative biogas yield from the

hydrolysis reactor was only 0.13 � 0.06 L gVS�1, and the

methane and carbon dioxide contents were 30% and 70%,

respectively. In the biogasification reactor the average

Table 1 e Chemical composition of onion residual (eachvalue is the average of two analyses and is reported ondry weight basis).

Component Value Unit

C 39.32 %

N 1.82 %

P 0.38 %

K 1.30 %

S 4135 ppm

B 18.5 ppm

Ca 0.48 %

Mg 0.13 %

Zn 26 ppm

Mn 10 ppm

Fe 37 ppm

Cu 13 ppm

Fiber 10.0 %

Lignin 0.4 %

0.00

0.04

0.08

0.12

0.16

0.20

0 2 4 6 8 10 12 14Digestion Time (d)

Cum

ulat

ive

Biog

as Y

ield

(L g

VS)

0.0

0.2

0.4

0.6

0.8

1.0Bi

ogas

Pro

duct

ion

Rat

e (L

gVS

d) Biogas Production Rate

Cumulative Biogas Yield

Fig. 3 e Average biogas yield and biogas production rate

from both hydrolysis and biogasification reactors for the

batch digestion of onion residual.

5.5

6.0

6.5

7.0

7.5

8.0

0 10 20 30 40 50 60 70 80 90Time (d)

pH

Fig. 4 e Biogasification reactor pH during continuous-

feeding operation for onion residual.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 1 7 4e4 1 7 9 4177

cumulative biogas yield was 0.56 � 0.002 L gVS�1, and the

methane and carbon dioxide contents were 61% and 39%,

respectively. The average cumulative biogas and methane

yields from the entire digester system were determined to be

0.69 � 0.06 L gVS�1. After digestion the hydrolysis reactor

was emptied and measured for remaining VS. The VS

reduction was determined to be 62 � 17%. The methane yield

from the onion residual was similar to the methane yield

from onion peels (0.4 L gVS�1) reported by Gunaseelan [15],

and fell within the range of 0.25e0.58 L CH4 gVS�1 reported

for food processing residuals for various fruits and vege-

tables [1,2,16,17].

0.0

0.2

0.4

0.6

0.8

1.0

0.5 1 2Organic Loading Rate (gVS L d )

Biog

as a

nd M

etha

ne Y

ield

(L g

VS)

Biogas YieldMethane Yield

Fig. 5 e Average biogas and methane yield from

continuous-fed digestion of onion residual. Error bars

represent the standard deviation from four sub-samples

during steady state.

3.3. Continuous digestion tests

The APS-Digester was initially loaded with onion residual at

0.5 gVS L�1 d�1, however after 7 days of continuous loading,

the pH of the biogasification reactor dropped to 6.5. For the

unobstructed growth of methanogens and stable biogas

production, the biogasification reactor should be maintained

in the pH range of 6.8e7.8. The APS-Digester was continually

loaded at 0.5 gVS L�1 d�1 but the liquid recirculation rate

between the hydrolysis and biogasification reactors was

decreased from 1 L d�1 to 0.5 L d�1 on day 9 to increase the

pH and retention time of the liquid in the biogasification

reactor to 6 days. The strategy helped to increase the pH in

the biogasification reactor to 6.75 however the pH again

decreased to 6.4 after a few days of continuous feeding (Fig. 4).

The liquid recirculation rate was further reduced to 0.33 L d�1

on day 18 of digestion, which increased the retention time of

the liquid in the biogasification reactor to 9 days but did not

help to increase the pH of the biogasification reactor. The

alkalinity of the biogasification reactor was suspected to be

causing the pH of the reactor to drop despite increasing the

retention time. The alkalinity was measured to be about

900 mg CaCO3 L�1, which was considered low for a proper

buffering capacity. The initial alkalinity of the biogasification

reactor was not measured. A decision was made to add

chemicals to control and stabilize the pH of the bio-

gasification reactor at 2500 mg CaCO3 L�1. Calcium carbonate

(6.4 g) was added to the biogasification reactor on day 22 of

digestion to increase the alkalinity to 2500 mg CaCO3 L�1. To

maintain the buffering capacity of the system, alkalinity was

added to the APS-Digester with the onions. The CaCO3 was

later replaced by hydrated lime (Ca(OH)2) on day 35 of diges-

tion to avoid the release of extraneous carbon dioxide from

carbonate. Hydrated lime was mixed with the onions fed to

the hydrolysis reactor at an amount of 16 mg Ca(OH)2 gVS�1

(1.2 mg Ca(OH)2 g�1 wet onion). The pH in the biogasification

reactor increased to 7.2 and the pH in the hydrolysis reactor

increased to 7.0 from 5.5. When the biogasification reactor

pH appeared stable, the liquid recirculation rate was returned

to 0.5 L d�1, and eventually to 1.0 L d�1 on day 50 of digestion.

When biogas production was stable, methane content of the

biogas was measured before increasing the OLR from

0.5 gVS L�1 d�1 to 1.0 gVS L�1 d�1 on day 63 of operation. The

OLR was further increased to 2.0 gVS L�1 d�1 on day 77 of

operation.

At OLRs of 0.5 and 1.0 gVS L�1 d�1 the pH in the hydrolysis

reactor was maintained above 6.8 and the biogas had high

methane content. In general, the biogas and methane yields

slightly increased as the OLR increased from 0.5 gVS L�1 d�1 to

2.0 gVS L�1 d�1 (Fig. 5). When the OLR was increased to

2.0 gVS L�1 d�1 the hydrolysis reactor pH decreased below 6.5,

Table 2 e Statistical analysis (one-sided t-test) forsignificant difference (a [ 0.05 and hypothesized meandifference [ 0) in biogas and methane yields betweendifferent organic loading rates of onion residual in theAPS-Digester.

Compare treatments (OLRs) p-Values

Biogas yield

0.5 1.0 0.003

0.5 2.0 0.020

1.0 2.0 0.115

Methane yield

0.5 1.0 0.005

0.5 2.0 0.262

1.0 2.0 0.699

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 1 7 4e4 1 7 94178

and methanogenic activity was negatively affected as the

methane content of the biogas dropped from 46% to 23%.

The decrease in methane content likely resulted in the

insignificant differences in methane yields between the OLRs

of 0.5 and 2.0 gVS L�1 d�1 and between the OLRs of 1.0 and

2.0 gVS L�1 d�1 (Table 2). Biogas yields at an OLR of

0.5 gVS L�1 d�1 were significantly lower ( p < 0.05) than the

biogas yields at OLRs of 1.0 gVS L�1 d�1 and 2.0 gVS L�1 d�1,

suggesting that at the low OLR the microorganisms may

not have been performing optimally. However biogas yields

between the OLRs of 1.0 and 2.0 gVS L�1 d�1 were not sig-

nificantly different ( p > 0.05), which suggests that although

the methanogenic microorganisms were compromised the

hydrolytic carbon dioxide producing microorganisms are

active, resulting in the insignificant difference. The consis-

tently significantly higher biogas andmethane yield at an OLR

of 1.0 gVS L�1 d�1 than at 0.5 gVS L�1 d�1 suggests that at

1.0 gVS L�1 d�1 the reactor conditions were most favorable.

The digester loading rate was further increased to

3.0 gVS L�1 d�1, however after 3 days of continuous loading the

pH in the hydrolysis reactor dropped to 5.5, and after 9 days

of continuous loading the pH in the biogasification reactor

dropped below 6.5. As the loading rate increases a propor-

tional increase in alkalinity using Ca(OH)2 may eventually

create a condition that will further inhibit the bacteria.

Moderate inhibitions of calcium on microorganisms are

shown to occur between 2500 and 4500 mg L�1 [18]. Therefore,

as the amount of Ca(OH)2 added to the system increases with

the increase in OLR, accumulation of calcium in the anaerobic

digester will likely occur. At a loading rate of 1.0 gVS L�1 d�1,

the total calcium concentration of the biogasification reactor

was measured to be 1240 mg L�1, and thus as the OLR

increases to 2.0 gVS L�1 d�1 and 3.0 gVS L�1 d�1, the calcium

concentration could possibly reach 2400 mg L�1 and

3800 mg L�1, which is near the upper limit of moderate inhi-

bition on microorganisms according to literature. Therefore

as the organic loading rate increases additional Ca(OH)2should not be added as this can cause inhibiting conditions

for the microorganisms. Accumulation of onion solids was

also noticeable in the hydrolysis digester. Operation of the

continuous digester was stopped to consider alternative

methods to treat the onion residual.

From the data on the continuously-fed digester operating

at 2.0 gVS L�1 d�1, the design and electricity savings of

a commercial anaerobic digester treating 91 mton d�1 (wet)

onion residual were calculated. A total working volume of

3219 m3 would be required for the hydrolysis and bio-

gasification reactors. Assuming a conservative biogas yield of

0.6 L gVS�1 at 50% methane content, and 30% electric

generator efficiency, a 240 kW generator will be required to

produce 5758 kW h d�1 of electricity. At a price of 0.10$ kW h�1

the biogas generated from treating the onion residual through

anaerobic digestion would save $576 d�1 on electricity. In

addition, assuming a 60% recovery of residual heat from the

engine-generator can be recovered, recoverable heat would

amount to 28,994 MJ d�1. The recovered heat is likely suffi-

cient to provide the required energy to heat the reactors,

however the amount of heat required will depend on many

factors such as the material selected for the reactor and

insulation, the configuration of the reactor (e.g., cylindrical,

rectangular), and the environmental conditions where the

reactor is located.

4. Conclusions

Onion residual was digested in an APS-Digester under batch-

fed and continuous-fed operations without additional nutrient

requirements. Under batch-fed conditions, the total biogas

yield from the digester system was 0.69 � 0.06 L gVS�1. In

the hydrolysis reactor, the biogas yield, and methane and

carbon dioxide contents of the biogas were 0.13 � 0.06 L

gVS�1, 30%, and 70%, respectively. In the biogasification

reactor, the biogas yield, and methane and carbon dioxide

contents of the biogas were 0.56 � 0.002 L gVS�1, 61%, and

39%, respectively. Under continuous-feeding operation at an

OLR of 2.0 gVS L�1 d�1, the daily biogas and methane yields

were 0.62 � 0.03 L gVS�1 and 0.31 � 0.02 L gVS�1. The APS-

Digester system offered the flexibility of controlling the

loading rate of organic acids from the hydrolysis reactor to

the biogasification reactor, thereby allowing pH control

without disruption of the continuous operation of the

digester system. However, alkalinity was found necessary to

control the pH of the biogasification reactor pH above 7.0 to

maintain an environmental conducive for the growth of

methanogens.

The study showed that an anaerobic digestion of onion

residuals is possible, provided that the system alkalinity

and pH can be appropriately maintained. The study found

limitations to using caustic chemicals to maintain proper

alkalinity and pH as the increased concentrations of

calcium could negatively affect the microbial consortia

necessary for anaerobic digestion. Changes to the feedstock

and operational parameters are areas that need to be

further explored for successful anaerobic digestion of onion

residuals.

Acknowledgments

The authors would like to thank Karl Hartman (UC Davis) for

his assistance in this project, and Gills Onions (Oxnard, CA)

for their financial support.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 1 7 4e4 1 7 9 4179

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