ORIGINAL PAPER

Biological hydrogen production by anaerobic digestionof food waste and sewage sludge treated using variouspretreatment technologies

Seungjin Kim • Kwangkeun Choi •

Jong-Oh Kim • Jinwook Chung

Received: 30 May 2012 / Accepted: 17 January 2013 / Published online: 7 February 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The purpose of this study was to enhance

the efficiency of anaerobic co-digestion with sewage

sludge using pretreatment technologies and food

waste. We studied the effects of various pretreatment

methods (thermal, chemical, ultrasonic, and their

combination) on hydrogen production and the char-

acteristics of volatile fatty acids (VFAs) using sewage

sludge alone and a mixture of sewage sludge and food

waste. The pretreatment combination of alkalization

and ultrasonication performed best, effecting a high

solubilization rate and high hydrogen production

(13.8 mL H2/g VSSconsumed). At a food waste:pre-

treated sewage sludge ratio of 2:1 in the mixture, the

peak hydrogen production value was 5.0 L H2/L/d. As

the production of hydrogen increased, propionate

levels fell but butyrate concentrations rose gradually.

Keywords Food waste �Hydrogen � Sewage sludge �Solubilization � Volatile fatty acids (VFAs)

Introduction

Reports on sustainable treatment systems that minimize

energy consumption have encouraged the use of anaer-

obic biological systems for intensive organic waste

treatment due to their low operational cost (Letting et al.

1979; Jetten et al. 1997). Systems that are based on

anaerobic biological processes have traditionally been

adopted to stabilize primary and secondary waste sludge

(Parkin and Owen 1986). Based on their advantages,

such as high biogas production and waste stabilization,

anaerobic systems are an appropriate organic waste

management method. Further, biogas recovery from

organic waste is the most important source of alternative

energy (Piccinini 2008).

However, anaerobic digestion of sewage sludge is

inefficient, because the typical ratio of volatile solids

(VS) to total solids (TS) is low (0.4–0.6). The

biological hydrolysis of sludge is the rate-limiting

step (Wang et al. 1999; Tiehm et al. 2001). To reduce

the impact of this step, pretreatment of sludge is

required, such as thermal (Li and Noike 1992; Tanaka

et al. 1997; Sawayama et al. 1996), alkaline (Penaud

et al. 1999; Ray et al. 1990; Lin et al. 1997), ultrasonic

(Wang et al. 1999; Tiehm et al. 2001), microwave

(Coelho et al. 2011; Eskicioglu et al. 2007), and

mechanical disintegration (Nah et al. 2000).

S. Kim � J. Chung (&)

R&D Center, Samsung Engineering Co. Ltd.,

415-10 Woncheon-Dong, Youngting-Gu,

Suwon, Gyeonggi-Do 443-823, Korea

e-mail: jin-wook.chung@samsung.com

K. Choi

910 U-Tower Heungduk-Dong, Kiheung-Gu, Yongin,

Gyeonggi-Do 446-982, Korea

J.-O. Kim

Department of Civil Engineering, Gangneung-Wonju

National University, Gangneung-Daehangno 120,

Gangneung, Gangwon-Do 210-702, Korea

123

Biodegradation (2013) 24:753–764

DOI 10.1007/s10532-013-9623-8

Another alternative is co-digestion, which is one of

the most effective systems for the treatment of organic

waste, generating various types of organic substrates as

a homogeneous mixture that is input into the reactor

(Braun 2002). Co-digestion of organic wastes improves

the CH4 yield and process performance due to the

synergy between substrates, which offsets the lack of

nutrients (Del Borghi et al. 1999; Mata-Alvarez et al.

2000; Braun 2002; Viotti et al. 2004).

However, anaerobic co-digestion processes require

the proper conditions with regard to substrates. Among

organic wastes, the anaerobic digestion of sewage

sludge is ineffective due to its low VS content, and the

anaerobic treatment of food waste is inefficient due to

its extremely high biodegradability, resulting in the

accumulation of volatile fatty acids (VFAs). Co-

digestion with the optimal dose of organic wastes

using high-rate anaerobic digestion technologies is a

promising method of improving these low digestion

efficiencies.

This study was performed to overcome the low

efficiency of conventional anaerobic digestion of

sewage sludge by co-digestion using sewage sludge

with pretreatment and food waste as a co-substrate.

Materials and methods

Characteristics of substrate

Seed sludge was taken from a full-scale thermophilic

anaerobic digester at a local municipal wastewater

treatment plant (WWTP) that was treating sewage

sludge as substrate; the collected sludge was boiled at

90 �C for 15 min to eliminate non-spore formers and

promote the germination of spores, after which the

levels of active hydrogen producers (hydrogenotrophic

bacteria) in the bioreactor increased, leading to greater

potential hydrogen production (Cohen et al. 1985).

The substrate was prepared individually to generate

a mixture of sewage sludge and food waste. Sewage

sludge from the thickener of a local municipal WWTP

was filtered through a stainless steel sieve (pore size

2.0 mm). The food waste was sampled from a self-

service restaurant. The chief constituents of the food

waste were classified as grain, vegetable, and meat.

Samples were processed in an electric blender after

dehydration in a drying oven at 105 �C. The charac-

teristics of the substrate are summarized in Table 1.

Pretreatment of sewage sludge

We measured the solubilization efficiency after apply-

ing the various pretreatment technologies to sewage

sludge—thermal treatment, ultrasonication, alkaliza-

tion, acidification, and a combination of alkalization

and ultrasonication. For the thermal treatment, we

applied heat for 30 min at 120 �C using a high-

temperature and high-pressure wet sterilizer, after

which we cooled the sample at room temperature and

measured the solubilization efficiency.

For alkalization and acidification, we performed the

solubilization at pH 12 and pH 2 using 3 M NaOH and

3 M HCl, respectively. Solubilization with ultrasonic

waves was conducted for 30 min using a digital

ultrasonic homogenizer (20 kHz, Bandelin, Germany).

For solubilization using a combination of alkaliza-

tion and ultrasonication, we performed 2 experiments:

ultrasonication after alkalization and alkalization after

ultrasonication. Details on the solubilization treat-

ments are shown in Table 2.

As discussed, we input the data from the various

solubilization technologies into Eq. (1) and obtained

the solubilization efficiency:

Solubilization efficiency ¼ SCODf

TCODf

; ð1Þ

where, SCODf is the soluble chemical demand

(SCOD) value after pretreatment (mg/L) and TCODf

is the total chemical oxygen demand (TCOD) value

after pretreatment (mg/L).

Accordingly, the extent of solubilization of sewage

sludge progresses as the solubilization efficiency

increases; further, use of the resulting solubilized

sewage sludge accelerate the degree of hydrolysis in

the anaerobic digestion rate.

Table 1 Characteristics of sewage sludge and food waste as

substrate

Item Unit Sewage sludge Food waste

pH 7.7 ± 0.2 4.6 ± 0.2

Alkalinity mg/L 3,720 ± 650 0.3 ± 0.1

VS mg/L 10,550 ± 760 129,300 ± 8750

TS mg/L 12,650 ± 1,890 183,530 ± 6,880

SCOD mg/L 383 ± 73 84,280 ± 11,230

TCOD mg/L 10,050 ± 1870 164,670 ± 5,530

754 Biodegradation (2013) 24:753–764

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Hydrogen production with various pretreatment

technologies

After the sewage sludge was treated using the various

solubilization technologies, we mixed the prepared

seed sludge and solubilized sludge in the reactor (5 L),

as shown in Fig. 1, at at ratio of 1:1 (v/v) and examined

the production of biological hydrogen after injecting a

buffer solution of 1 M phosphate, pH 8.

Hydrogen production with a mixture of sewage

sludge and food waste

In measuring hydrogen production by a mixture of

food waste and sewage sludge that was pretreated with

alkalization and ultrasonication, we used the same

reactor as in the previous experiment with food

waste:solubilized sewage sludge ratios of 1:1, 2:1,

3:1, and 1:3. The mixed substrate and prepared seed

sludge were added to that reactor at a ratio of 1:1 (v/v).

The temperature and agitation speed were 37 ± 1 �C

and 150 rpm, respectively. All experiments were

batch-type and performed for 48 h at pH 5.5 ± 0.1.

Hydrogen production at various pHs

In measuring hydrogen production as a function of pH, we

mixed food waste and sewage sludge that was solubilized

by a combination of alkalization and ultrasonication at a

ratio of 2:1 (v/v) and also mixed them with prepared seed

sludge at a ratio of 1:1 (v/v) in an effective volume of 3 L.

To meet the anaerobic conditions, we purged the mixture

with N2 sufficiently. We conducted the batch-type

experiment for 48 h at pH 4, 5, 6, 7, and 8 using 3 M

KOH and 3 M HCl at 150 rpm and 37 ± 1 �C.

Analysis

The gas was measured immediately after its genera-

tion using a wet gas meter (Model W-NK-0.5,

Table 2 Experimental conditions for various pretreatment technologies

Ultrasonication Alkalization Acidification Heating Ultrasonication ?

alkalization

Alkalization ?

ultrasonication

pH control – 12 2 – 12 12

Ultrasonic (amplitude) 120 – – – 120 120

Temperature (�C) 25 25 25 120 25 25

Treatment time (min) 30 30 30 30 30 (15 ? 15) 30 (15 ? 15).

Fig. 1 Schematic of batch-type experimental apparatus for biohydrogen production

Biodegradation (2013) 24:753–764 755

123

SHINAGAWA, Japan) to prevent the partial pressure

of the hydrogen that was generated inside the reactor

from affecting the efficiency of hydrogen production

(Hawkes et al. 2002; Mizuno et al. 2000). We also

analyzed total suspended solids (TSS), volatile sus-

pended solids (VSS), TCOD, and SCOD at constant

intervals per standard methods (APHA 1998).

To analyze the characteristics of biological hydro-

gen production by solubilization conditions for sew-

age sludge and a mixture of food waste and sewage

sludge, we measured hydrogen and VFAs simulta-

neously. The preparation of samples for the determi-

nation of VFAs by gas chromatography (GC) was

based on Manni and Caron’s procedure (Manni and

Caron 1995). Briefly, the samples were acidified to pH

2 using 65 % nitric acid. One-milliliter aliquots were

shaken with 1 mL diethyl ether for *10 min, and the

ether phases were transferred to 4-mL flasks, to which

a small amount of anhydrous sodium sulfate was

added. Five-hundred-microliter portions of the ether

phase were transferred to new 4-mL flasks and 150 lL

diazomethane was added.

A series of VFA standards for the calibration curves

were prepared as described above. Calibration curves

were obtained using five aqueous acid solutions—

acetic, propionic, butyric, valeric, and caproic—at

5–1,000 mg/mL. GC analyses were performed on an

Agilent 7820A GC (Agilent Technologies, Korea),

equipped with a flame ionization detector and a DB-23

capillary column (30 m, 0.25 mm I.D., 0.25-lm film

thickness, Alltech, Poland). The injector and detector

temperatures were both 170 �C. The carrier gas was

argon. The analyses were performed using the follow-

ing program: 5 min at 30 �C and a linear gradient from

30 to 130 �C at 10 �C per min. In each case, 2 lL of

sample was injected (flow split 1:10).

Results and discussion

Pretreatment of sewage sludge

Solubilization efficiency by pretreatment technology

To use the sewage sludge as substrates for biological

hydrogen production, we applied various solubilization

technologies to sewage sludge alone or in combination.

As shown in Fig. 2, the value of the final SCOD

increased for all methods, and the combined application

of each solubilization technology effected greater

efficiency than the individual methods. For application

of ultrasonication (30 min at 20 kHz) and alkalization

(30 min at pH 12 with NaOH) alone for each case, the

efficiency of solubilization was approximately 0.2 and

0.3, respectively. For ultrasonication alone, a treatment

time of 30 min was the most effective, because the

solubilization did not improve beyond this point (data

not shown). In addition, the efficiency of solubilization

peaked when NaOH of 2 g/L-sludge was injected,

because increased quantities of alkali result in high

viscosity of sewage sludge and lack of agitation; thus,

the injected alkali did not affect the flocs or microor-

ganisms in sewage sludge (data not shown). Therefore,

the injection of alkali (based on NaOH) at 2 g/L-sludge

is the most cost-effective and efficient solubilization

method.

As shown in Fig. 2, the most extensive effects of

solubilization occurred with an initial alkalization step

for sewage sludge, followed by ultrasonication. Alka-

lization can decompose flocs of sewage sludge easily

compared with ultrasonication, and after the collapse

of flocs by alkalization, the effect of solubilization was

maximized by the cavitation effects of ultrasonic

waves. We hypothesize that at the initial stage of

solubilization, the alkalization step caused the col-

lapse in flocs, and subsequently, the cavitation effect

by ultrasonic waves destroyed the cellular walls of

each microorganism. Accordingly, ultrasonication

after alkalization is the more effective order of this

combination of methods. On the other hand, alkaliza-

tion, combined with heat or acidification, was seldom

considered because solubilization efficiencies by heat

and acidification alone were significantly lower than

those by ultrasonication alone.

The comparative results on the combined application

of alkalization and ultrasonication are shown in Fig. 3.

Solubilization of sewage sludge was performed with

1 g-NaOH/L-sludge and ultrasonic waves (20 kHz) at

the initial stages of the experiment and with 2 g-NaOH/

L-sludge when the solubilization efficiencies did not

increase. When alkalization occurred after ultrasonica-

tion, the solubilization efficiency was *0.18 after

20 min versus 0.50 when ultrasonication was performed

after alkalization.

The solubilization efficiency of the combination

of ultrasonication and alkalization with 1 g-NaOH/L-

sludge did not differ compared with the individual

methods, implying that more energy is needed to

756 Biodegradation (2013) 24:753–764

123

deconstruct the cellular walls of microorganisms

after the collapse of flocs, although ultrasonication

and alkalization are effective in collapsing sludge

flocs. Accordingly, when the effects of solubilization

failed to change further, the addition of NaOH was

increased to 2 g, and the solubilization efficiency

improved to 0.6. This result implies that the decon-

struction of cellular walls is accelerated by additional

alkalization. In addition, with alkalization being

performed after ultrasonication, VSS at the initial

stages of solubilization decreased slightly, but when

the solubilization did not increase any further, this

decline ceased. With ultrasonication after alkaliza-

tion, however, the reduction rate in VSS increased

gradually. Thus, ultrasonication after alkalization is

the most optimal technology for solubilization of

sewage sludge.

Hydrogen production by pretreatment technology

Figure 4a shows biological hydrogen production with

sewage sludge that has been pretreated with various

solubilization technologies; at higher solubilization

efficiencies, the efficiency of hydrogen production

increased. Compared with the control (without pre-

treatment), however, the efficiency of hydrogen pro-

duction after thermal treatment was higher, although

the solubilization efficiencies were similar. This low

efficiency was attributed to the low amounts of VFAs

that were produced in anaerobic digestion. Conse-

quently, the hydrogen productivity peaked with ultra-

sonication after alkalization, followed by alkalization

after ultrasonication. This result suggests that a

combination of technologies for solubilization

increases the production of hydrogen versus each

individual method.

Figure 4b shows the concentrations of the acetic

acid, butyric acid, and propionic acid that were

produced during hydrogen production. In all cases

except for the control, large amounts of VFAs were

produced, indicating that anaerobic digestion pro-

gressed sufficiently. In biological hydrogen produc-

tion, the ratio of butyric acid to acetic acid (B/A ratio)

and propionic acid concentration are indicators of

hydrogen production; higher B/A ratios and lower

concentrations of propionic acid reflect higher effi-

ciency of biological hydrogen production (Han and

Shin 2004; Chen et al. 2002; Payot et al. 1998),

because thermal treatment of anaerobic sludge is

predominated by spore-forming microorganisms,

most of which are Clostridia species, which produce

hydrogen during butyric acid production. Conse-

quently, the concentration of butyric acid rises among

VFAs (Lay et al. 1999).

In addition, to maintain the high concentration of

hydrogen in a fermentor, no methanogens or bacteria

Fig. 2 Effect of various pretreatment technologies on solubilization of sewage sludge

Biodegradation (2013) 24:753–764 757

123

that produce propionic acid should exist, because the

hydrogen that is produced by Clostridia species is

consumed by methanogens and propionic acid-pro-

ducing bacteria for the generation of methane or

propionic acid (Sparling et al. 1997). Therefore, among

VFAs, at lower concentrations of propionic acid and

higher B/A ratios, the efficiency of hydrogen produc-

tion increase. Consequently, as shown in Fig. 4a, b, we

obtained the highest B/A ratio, lowest concentration of

propionic acid, and highest efficiency of hydrogen

production with ultrasonication after alkalization.

In the control, the B/A ratio was higher than with

the thermal treatment, but the efficiency of hydrogen

production was lower, because the concentration of

VFAs was greater and more butyric acid was produced

in the thermal treatment. In addition, compared with

acidification and thermal treatment, we predicted that

the amounts of hydrogen that was produced would be

low due to the lower efficiency of solubilization.

However, the hydrogen that was produced with

thermal treatment was higher than with acidification,

implying a higher B/A ratio in the thermal treatment

versus acidification and greater hydrogen production

by Clostridia species.

Mixture of food waste and pretreated sewage

sludge

Hydrogen production by pretreatment technology

To improve the efficiency of hydrogen production, we

mixed pretreated sewage sludge and food waste that

contained high organic matter. After applying various

solubilization technologies to sewage sludge, we

mixed solubilized sewage sludge and food waste at

the same ratio (1:1 (v/v)) and mixed them with co-

substrate and prepared sludge for seeding at a ratio of

1:1 (v/v) at pH 5.5 ± 0.1.

As shown in Fig. 5a, when food waste and solubi-

lized sewage sludge were mixed, the yield of hydrogen

with all solubilization technologies was significantly

higher than with solubilized sewage sludge alone. Also,

the highest yield of hydrogen with ultrasonication after

alkalization was 13.8 mL H2/g-VSSconsumed, 4 times

greater than with solubilized sewage sludge alone. It is

difficult to use sewage sludge alone as substrate for

hydrogen production, because sewage sludge contains a

low content of organic matter. However, food waste was

added to sewage sludge, and the dissolved organic

Fig. 3 Effect of combined treatment with alkalization and ultrasonication on solubilization and VSS reduction in sewage sludge

758 Biodegradation (2013) 24:753–764

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content and alkalinity increased simultaneously due to

the complementary reaction of the low alkalinity of food

waste and low organic content of sewage sludge.

Therefore, co-substrates can have antagonistic activities

due to the characteristics of the two wastes, resulting in

improved hydrogen yield.

The production of hydrogen from organic matters

during anaerobic digestion occurred primarily at the

stage of acidification, at which time organic acid was

produced. With the combination treatment, the B/A

ratio peaked, and most metabolism occurred in the

path of hydrogen production. The combination treat-

ment effected the lowest concentration of propionic

acid, whereas propionic acid levels peaked in the

thermal treatment, during which less hydrogen is

produced. Accordingly, although the efficiency of

Fig. 4 Biological hydrogen production (a) and VFA production (b) with sewage sludge treated by various pretreatment technologies

Biodegradation (2013) 24:753–764 759

123

solubilization increases with thermal treatment of

sewage sludge, the production of hydrogen is inhibited

by relatively high levels of propionic acid.

As shown in Table 3, in this study the VFA yields

of pretreated sewage sludge ranged from 0.25 to

1.43 g VFA-COD/g TOCD, which is 2–10 times

higher than in earlier reports, but lower than those of

mixture conditions 0.011–0.199 g VFA-COD/g TOCD.

Compared with VFA yield by codigestion of food waste

and sewage sludge, VFA yield by sewage sludge

decreased but there was little difference in hydrogen

production.

Hydrogen production at various pHs

Because the organic acid that is produced significantly

affects pH, pH is one of the environmental factors that

Fig. 5 Biological hydrogen production (a) and VFA production (b) from mixed food waste and sewage sludge

760 Biodegradation (2013) 24:753–764

123

affect the production of hydrogen. As shown in

Fig. 6a, the highest amount of hydrogen was 1.8 L

H2/L/d at pH 5.5 (1.6 L H2/L/d at pH 5.0). Below pH 4

and above pH 7, hydrogen was hardly produced,

similar to other studies (Fang and Liu 2002; Lay 2000;

Fan et al. 2004).

We examined the changes in organic acid that was

produced during hydrogen production. As shown in

Fig. 6b, the production of hydrogen began after 3 h,

peaking at 16 h of operation and declining after 18 h.

With regard to VFA composition, because the pro-

duction rate of butyric acid increased, rather than

acetate acid, it had more advantageous effects on the

production of hydrogen. In addition, after 18 h, as the

production rate of propionic acid increased, the

metabolism shifted from the production of hydrogen

toward its consumption.

Hydrogen production of mixture

Based on previous experiments, the efficiency of

hydrogen production increased in a mixture of food

waste and sewage sludge versus sewage sludge alone.

Figure 7a shows the change in hydrogen production and

B/A ratio at various ratios of food waste and solubilized

sewage sludge. When food waste and sewage sludge

were mixed at ratios of 1:1 and 2:1, 1.8 and 5.0 L H2/L/d

was produced, respectively; the production rate of

hydrogen increased as the concentration of dissolved

organic matter increased. When the ratio rose to 3:1, little

hydrogen was produced, possibly due to the adverse

effects of hydrogen-producing bacteria and the high level

of salinity or excessive production of organic acid

(Mizuno et al. 2000; Kim 2005; van Ginkel et al. 2001).

One of the reasons why a mixture of food waste and

sewage sludge was used as co-substrate was to dilute the

concentration of salinity in food waste; the concentration

of salinity is shown as percentages in parentheses, such

as 1:1 (0.6 %), 2:1 (1.1 %), 3:1 (2.1 %), and 1:3 (0.3 %).

Figure 7b shows the change in VFAs at the time of

hydrogen production at various ratios of food waste

and sewage sludge. VFA levels at ratios of 2:1 and 3:1

peaked at 48,675 and 47,901 mg-COD/L, respec-

tively, indicating that the content of organic matter and

production efficiency of VFAs is high when the rate of

food waste is relatively high. However, the produced

quantity of hydrogen was minimal when the ratio was

3:1, caused by high contents of propionic acid and a

low B/A ratio. Therefore, when a mixture of food

waste and sewage sludge is used as substrate at a ratio

of 3:1, VFAs can be produced sufficiently, but this

ratio is inappropriate as substrate for the production of

hydrogen due to the low productivity of hydrogen.

Thus, the optimal ratio of food waste and sewage

sludge for the production of hydrogen was 2:1.

Conclusions

In this study, various pretreatment technologies of

sewage sludge and co-digestion with food waste were

examined to improve anaerobic digestion efficiency

and hydrogen production. The most important results

are:

1) Pretreatment, combined with alkalization and

ultrasonication, is ideal for treating sewage

sludge, effecting a solubilization rate and elution

rate of organic material of 0.9 and 0.076/min,

respectively.

Table 3 Comparison with

VFA yields of previous

research and this study

Feed source g VFA COD/g TCOD References

Sewage sludge 0.058–0.14 Yuan et al. (2009)

Sewage sludge 0.095–0.19 Ubay-Cokgor et al. (2005)

Sewage sludge 0.05–0.11 Banister and Pretorius (1998)

Sewage sludge 0.044–0.14 Ucisik and Henze (2008)

Primary sludge 0.19 Ucisik and Henze (2008)

Sewage sludge ? primary sludge 0.081 Ucisik and Henze (2008)

Sewage sludge 0.125 This study

Pretreated sewage sludge 0.250–1.43 This study

Food waste ? pretreated sewage sludge 0.011–0.199 This study

Biodegradation (2013) 24:753–764 761

123

2) The highest hydrogen production values of sew-

age sludge alone and mixed food waste and

sewage sludge were 4.3 mL H2/g VSSconsumed

and 13.8 mL H2/g VSSconsumed with the combi-

nation pretreatment with alkalization and ultra-

sonication.

3) The optimal pH for hydrogen production was

5.0–5.5, and the hydrogen production rate was

1.6–1.8 L H2/L/d at a food waste:pretreated

sewage sludge ratio of 1:1 (v/v).

4) Hydrogen production peaked at 5.0 L H2/L/d

when the ratio of food waste and pretreated

sewage sludge was 2:1 (v/v).

5) At higher hydrogen production rates, propionate

concentration was relatively lower, but butyrate

and acetate concentrations were significantly

Fig. 6 Biological hydrogen production from mixture of food waste and sewage sludge at various pHs (a) and butyrate/acetate ratios in

the range of pH 5.0–5.5 (b)

762 Biodegradation (2013) 24:753–764

123

higher, indicating that the chief metabolic path-

way relies on hydrogen production.

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