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Production of methane from sugar beet silagewithout manure addition by a single-stageanaerobic digestion process

B. Demirel, P. Scherer�

Lifetec Process Engineering, Faculty of Life Sciences, Hamburg University of Applied Sciences, Lohbrugger Kirchstrasse 65,

21033 Hamburg, Germany

a r t i c l e i n f o

Article history:

Received 23 July 2007

Received in revised form

10 September 2007

Accepted 15 September 2007

Available online 23 October 2007

Keywords:

Anaerobic digestion

Biogas

Biomass

Methane

Renewable energy

Sugar beet

nt matter & 2007 Elsevieioe.2007.09.011

thor. Tel.: +49 40 42875635Paul.Scherer@rzbd.haw-h

a b s t r a c t

Single-stage continuous anaerobic conversion of sugar beet silage without manure to

methane was investigated in this experimental work, using a laboratory-scale mesophilic

anaerobic biogas digester. The sugar beet silage had an extreme low pH of 3.3. The reactor

was operated in a hydraulic retention time (HRT) range of between 95 and 15 days, and an

organic loading rate (OLR) range of between 0.937 and 6.33 g�1 VS l�1 d�1. The highest

specific gas production rate (spec. GPR) of 0.72 l g VS�1 d�1 could be obtained at 25 days of

HRT, with an average methane content of about 63%, at a pH of around 6.8. Since sugar beet

silage without the leaves is a poor substrate, in terms of the availability of the nutrients and

the buffering capacity, external supplementation of nitrogen and buffering agents has to be

regularly performed, in order to achieve a stable and an efficient process. Sodium or

potassium hydrogen carbonate addition seemed to function best in our case, among the

other agents used, to provide adequate buffering capacity and to keep the digester pH

stable during the operation. Use of a new harvest (a new charge of substrate) also affected

the spec. GPR values significantly.

& 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Single-phase and high-rate two-phase anaerobic digestion

processes have often been used to treat soluble and solid

types of domestic and industrial wastes [1]. Biomass can

biologically be converted to methane and hydrogen by the

anaerobic digestion process.

There already exists recent literature about the applications

and benefits of the anaerobic digestion process to produce

renewable energy from various sources of biomass [2–7].

Furthermore, there also exist several works about continuous

anaerobic digestion of sugar beets for production of methane

[8–12]. On the other hand, for each particular biomass type to

be used, without any manure or sludge addition, the effects of

r Ltd. All rights reserved.

5; fax: +49 40 428756359.amburg.de (P. Scherer).

both operational and environmental parameters on the

process performance of the anaerobic biogas digester

have to be individually determined, in order to achieve

a high conversion efficiency, since each substrate, even

different harvests of the same substrate, has its unique

characteristics.

Therefore, the primary objective of this study was to

investigate and determine the optimum operational (HRT

and OLR) and environmental (pH, the appropriate amount of

macro–micro-nutrients that should be available and the

adequate buffering capacity) conditions during long-term

fermentation of sugar beet silage to methane, without

addition of manure. Sugar beet was chosen as the sole

substrate, because it has gained interest as the regulatory

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price was decreased by the EU in 2006, and now many farmers

are looking for an alternative use.

2. Materials and methods

2.1. Description of the reactor system

A laboratory-scale, single-stage continuous digester was used

in this experimental work. The schematic configuration of the

anaerobic biogas reactor is given in Fig. 1. The description of

the reactor was previously reported [13]. The reactor was

inoculated with 1/3 of sewage sludge, 1/3 of swine manure

and 1/3 of a compost suspension (without solids). Tempera-

ture was kept at 41–42 1C during the entire operation. The

reactor was fed once a day, manually. Biogas production was

measured daily, using a Milligascounters type MGC10 (Ritter,

Bochum, Germany). Methane (CH4) and carbon dioxide (CO2)

compositions (v/v) were measured online, using infrared

sensors (BlueSens Gaz Analyzer, Herten, Germany). Tempera-

ture, pH and redox potential (ORP) were also continuously

measured online.

QIR

QIR

QIR

M

C

C

TIR

Biogas Reactor

Effluent

Moisture

pH

Redox

Feed

CH4 / CO2

Gas

Temperature

10cm

QIR

Fig. 1 – Configuration of the laboratory-scale reactor.

Q ¼ quality of a measured value; M ¼motor; W ¼ weight;

T ¼ temperature; R ¼ recorded values; I ¼ instrument;

C ¼ controller.

2.2. Substrate

Sugar beet silage (without the leaves) was used as the mono-

substrate. The general characteristics of the sugar beet silage

are given in Table 1. More data about the characteristics of the

sugar beet silage can also be found elsewhere [14]. Two

harvests of sugar beet silage were used during the entire

experimental work, and both harvests were obtained from

Soltau, Germany. Parallel analyses were carried out for

determination of each parameter. The substrate was stored

at 4 1C until further use. Since sugar beet silage is a poor

substrate, in terms of nitrogen (N), phosphorus (P) and

buffering capacity, nitrogen was regularly provided to the

feed by external addition of ammonium hydrogen carbonate

(NH4HCO3) or ammonium chloride (NH4Cl), while sodium

hydrogen carbonate (NaHCO3), potassium hydrogen carbo-

nate (KHCO3), potassium hydroxide (KOH) or sodium carbo-

nate (Na2CO3) were used as buffering agents to provide

alkalinity and to keep the reactor pH stable. Stock solutions

of minerals were also prepared and added to the substrate,

to provide phosphate (5.2 mM, Na and K salts), calcium

(Ca, 1 mM), magnesium (Mg, 2 mM), zinc (Zn, 10mM), manganese

(Mn, 2mM), copper (Cu, 2mM), wolfram (W, 1mM), cobalt

(Co, 5mM), nickel (Ni, 10mM), selenium (Se, 0.4mM), molybdenum

(Mo, 2mM) and sulphur (S, 0.5mM). All chemicals were reagent

grade, obtained from commercial sources (Merck, Darmstadt,

Germany).

2.3. Analytical methods

Mixed samples were regularly drawn from the reactor, and

measured to determine volatile solids (VS), volatile sus-

pended solids (VSS), ammonium (NH4+), phosphate (PO4

3�),

volatile fatty acids (VFAs), alcohols and alkalinity. The VSS

content was measured according to DIN methods (DIN 38414-8)

[15], while alkalinity, ammonium and phosphate were

measured according to standard methods [16]. Total VS was

defined as the sum of VSS and volatile dissolved solids

(volatile dissolved solids was the sum of VFAs, lactic acid and

Table 1 – Characterization of sugar beet silage as mono-substrate

Parameter Unit Range Average

pH 3.27–3.38 3.34

Volatile solids (VS) % 18.44–19.83 19.1

Ammonium (NH4+) mg l�1 55–85 70.3

Phosphate (PO43�) mg l�1 2–15a/575–590b 8.5a/583b

Acetic acid mg l�1 13,965–28,732 21,657

Propionic acid mg l�1 247–3083 1476

Isobutyric acid mg l�1 29–294 137

Butyric acid mg l�1 113–146 129.7

Isovaleric acid mg l�1 32–63 47.5

Valeric acid mg l�1 7–70 38.5

a Substrate charge 1.b Substrate charge 2.

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0.75

1.00

VS

-1 d

-1)

60

80

4)

(%) 3

4

l-1 d

-1)

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alcohols). VFAs and alcohols were determined using a HP 5890

Series II GC with a flame ionization detector and a BP 21

Bonded FFAP Fused Silica column. Hydrogen (H2) was used as

the carrier gas. Injection and detector temperatures were 240

and 260 1C, respectively.

0 20 40 60 80 100 1200.00

0.25

0.50

sp

ec

. G

PR

(l

g

HRT (days)

0

20

40

vol. GPR

spec. GPR

CH4

Me

tha

ne

(C

H

0

1

2

vo

l. G

PR

(l

Fig. 2 – Specific gas production rate (spec. GPR), volumetric

gas production rate (vol. GPR) and the methane (CH4) content

of digester biogas under steady-state conditions at different

hydraulic retention time (HRT) levels (for HRT of 25 days,

results obtained only with substrate charge 2 are

given here).

0 20 40 60 80 100 1200

1500

3000

4500

6000A

lkalin

ity (

mg

CaC

O3 l

-1)

HRT (days)

6.0

6.5

7.0

7.5

8.0

Alkalinity

pH

pH

Fig. 3 – Values of pH and alkalinity at different hydraulic

retention time (HRT) levels during mesophilic anaerobic

digestion of sugar beet silage (for the old and new harvests

there exist two different steady-state conditions at 25 days

of HRT; no alkalinity data are available for operation at 38

3. Results and discussion

A summary of the operational and environmental parameters

achieved at steady-state conditions for the entire experi-

mental study are given in Table 2. In Fig. 2, specific gas

production rate (spec. GPR), volumetric gas production rate

(vol. GPR) and methane content (%) of the digester biogas at

different HRT levels are displayed. The average values of pH

and alkalinity at steady-state conditions at different HRT

levels are shown in Fig. 3, while the concentrations of

ammonium (NH4+) and phosphate (PO4

3�) during anaerobic

digestion of sugar beet silage are given in Fig. 4.

The continuously driven anaerobic biogas digester was

firstly operated at a HRT of 38 days, and external supplemen-

tation of stock solutions was started during this phase of the

experiments (reactor day 1259). Before these experiments

were started, the reactor was operated with fodder beet silage

[2]. At 38 days of HRT, the anaerobic reactor exhibited spec.

GPR and vol. GPR levels of 0.5 l g�1 VS d�1 and 1.203 l l�1 d�1,

respectively, with an average pH of 6.75. The methane content

of the digester biogas ranged between 46% and 57%, with an

average of 53%. However, towards the end of period 1b the

reactor pH started to decline, to 6.3–6.4, due to low buffering

capacity of sugar beet silage (after reactor day 1500). No

external addition of buffering agents was carried out at 38

days of HRT. In order to maintain a stable reactor pH, without

addition of any external buffering agents, HRT was increased

from 38 to 95 days, in the second period (2b). However, in spite

of a high HRT of 95 days, the digester pH still remained in a

range of between 6.3 and 6.4. Therefore, ammonium hydro-

gen carbonate addition was started, in order to increase both

the buffering capacity and the ammonium (NH4+) content of

the reactor. The ammonium content and the buffering

capacity (total alkalinity) of the reactor ranged from 150 to

215 mg l�1, and 1176 to 1478 mg CaCO3 l�1, respectively, before

days of HRT).

Table 2 – A summary of the observed operational and environmental conditions for anaerobic digestion of sugar beetsilage

Period HRT(days)

Reactordays

Feeding(mL)

spec. GPR(l g VS�1 d�1)

OLR(g VS l�1 d�1)

VSS reactor(%)

Temp.(1C)

Redoxa

(mV)

1b 38 1441–1488 150 0.50 2.447 2.50 42 �265

2b 95 1568–1608 60 0.40 0.937 2.10 42 �267

3b 50 1613–1656 114 0.53 1.780 2.25 42 �281

4b 25 1659–1695 228 0.49 3.560 2.65 42 �288

4c 25 1794–1849 228 0.72 3.968 1.88 42 �272

5c 15 2024–2066 380 0.54 6.330 1.16 42 �296

a Corrected redox potential (ORP) values by �230 mV reference electrode are reported here.b Substrate charge 1.c Substrate charge 2.

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external supplementation began. After external supply was

started, the reactor pH gradually increased, from 6.3 and 6.4,

to 6.75, in about 2 weeks, and the steady-state conditions

could then be attained. At 95 days of HRT, the spec. GPR and

vol. GPR levels were 0.4 l g�1 VS d�1 and 0.37 l l�1 d�1, respec-

20 40 60 80 100

0

200

400

600

800

1000

1200

1400

SCh-1

Change of substrate charge at HRT of 25 days

SCh-1SCh-2

Co

nc

en

tra

tio

n (

mg

l-1

)

HRT (days)

Ammonium

Phosphate

Fig. 4 – Concentrations of ammonium (NH4+) and phosphate

(PO43�) in a mesophilic anaerobic biogas reactor during the

entire operation with sugar beet silage without manure

addition using substrate charges SCh-1 and SCh-2.

Table 3 – A comparison of reactor output with substratecharge 1 and 2, both at 25 days of HRT operation

Parameter Substratecharge 1

Substratecharge 2

Spec. GPR (l g�1 VS d�1) 0.49 0.72

Vol. GPR (l l�1 d�1) 1.743 2.863

Methane (CH4)

content (%)

51–61 58–67

pH 7.27 6.79

1800 1810 1820 1830 180.00

0.25

0.50

0.75

1.00

1.25

1.50

Methane

spec. GPR

sp

ec.G

PR

(l

gV

S-1

d-1

)

Reactor operation

Fig. 5 – Spec. GPR, pH and methane values before (during steady

and during the failure period (on reactor day 1860 at 15 days of

tively, with a pH of about 6.9. These low levels of spec. and vol.

GPR could be attributed to a relatively high HRT employed

during this period of reactor operation. The methane content

of digester biogas ranged between 49% and 58%, with an

average of 53%. Alkalinity ranged between 2548 and

5145 mg CaCO3 l�1, while the ammonium concentration var-

ied from 678 to 1380 mg l�1. Change in HRT affected both

specific and volumetric GPR values adversely. However,

variation in HRT did not provide a more stable reactor pH,

without the supplementation effect of the buffering agents.

Addition of NH4HCO3 obviously provided a stable reactor pH,

with a high amount of buffering capacity, but seemed to have

no profound effect on increased biogas production rate.

In the following period (3b), HRT was decreased from 95 to

50 days. At steady-state conditions at 50 days of HRT, the

spec. GPR and vol. GPR levels of 0.53 l g�1 VS d�1 and

0.944 l l�1 d�1 were obtained, respectively, with a pH of 7.1

(on average). The methane content of digester biogas varied

between 50% and 58%, with an average of 54%. The alkalinity

ranged from 3440 to 4743 mg CaCO3 l�1, while the ammonium

concentration varied between 1087 and 1320 mg l�1. HRT

variation only had a slight effect on spec. GPR. Besides, the

methane content also remained almost the same. In the

following run (4b), HRT was further adjusted from 50 to 25

days. During steady-state conditions, the spec. and vol. GPR

levels were 0.49 l g�1 VS d�1 and 1.743 l l�1 d�1, respectively,

with an average reactor pH of 7.27. The methane content of

digester biogas varied between 51% and 61% (57% on average),

and the alkalinity ranged from 5250 to 6825 mg CaCO3 l�1.

During this period, NH4HCO3 addition was reduced, due to

high ammonium concentrations (between 2060 and

2674 mg l�1). Change in HRT from 50 to 25 days seemed to

affect only spec. GPR slightly. These steady-state data at 25

days of HRT were obtained during use of the substrate charge

1 (Table 2). However, during the steady-state operation at 25

days of HRT, we had to use a new charge of substrate (a new

harvest-substrate charge 2), because the old charge of

substrate ran out (reactor day 1695). Eventually, the reactor

40 1850 1860

pH

(days)

5.0

5.5

6.0

6.5

7.0

7.5

8.0

pH

0

20

40

60

80

100

Meth

an

e (

CH

4)

perc

en

tag

e (

%)

-state conditions at 25 days of HRT before reactor day 1860)

HRT).

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output was obviously affected by this variation. During the

operation with the new charge of substrate at 25 days of HRT

(period 4c), external addition of NH4HCO3 was ceased (Fig. 4).

In order to provide adequate buffering capacity and a stable

reactor pH, sodium carbonate, calcium carbonate and potas-

sium hydroxide were used. However, calcium carbonate

addition caused excessive foam formation in the digester,

which led to clogging of gas outline and condensate trap.

Sodium carbonate was also avoided later, since sodium ions

could have a positive effect on methanogenesis [17,18].

The spec. and vol. GPR levels were 0.72 l g�1 VS d�1 and

2.863 l l�1 d�1, respectively, with an average reactor pH of 6.79,

at steady state for 25 days of HRT, using substrate charge 2,

with KOH addition. The methane content of digester biogas

varied between 58% and 67% (63% on average), and the

alkalinity ranged from 1580 to 2975 mg CaCO3 l�1. The

ammonium concentration also varied between 77 and

505 mg l�1. A slightly higher OLR was used with substrate

charge 2, because it had a higher VS content than that of

substrate charge 1. The new charge of substrate used

provided higher specific and volumetric gas production rates,

Table 4 – Results of the batch tests at HAW Hamburg

Substrate NaHCO3 addition(mM)

N

Reactor effluent (blank) –

Reactor effluent+ SBS** –

Reactor effluent (blank) 30

Reactor effluent+ SBS** 30

Reactor effluent (blank) 60

Reactor effluent+ SBS** 60

Reactor effluent (blank) 90

Reactor effluent+ SBS** 90

Reactor effluent (blank) –

Reactor effluent+ SBS** –

Reactor effluent (blank) –

Reactor effluent+ SBS** –

*Three-fold experiments were carried out at 37 1C; incubation time was 2

**SBS ¼ sugar beet silage.

0

2000

4000

6000

8000

10000

12000

1800 1820 1840 1860 1880 1900 1920

Reactor operation (days)

VF

A c

on

cen

trati

on

(mg

l-1

)

Total VFA

HAC

HPRO

HISOBUT

HBUT

HISOVAL

HVAL

Fig. 6 – Concentrations of VFA during failure at 15 days of

HRT and the following recovery periods during anaerobic

digestion of sugar beet silage.

and a higher methane content in digester biogas produced

(Table 3).

Higher ammonium concentrations between 2060 and

2674 mg NH4+ l�1 due to external addition during substrate

charge 1 feeding caused free ammonia (NH3) concentrations

ranging between 464 and 593 mg l�1. During substrate charge

2 feeding, lower ammonium concentrations could be main-

tained in the reactor, ranging from 77 to 505 mg l�1, causing

even lower NH3 concentrations between 109 and 17 mg l�1.

The formula reported by Gallert and Winter was used to

calculate NH3 concentrations from measured NH4+ concentra-

tions [19]. Previously, stable anaerobic digestion of swine

manure has been reported for ammonia concentrations at

6000 mg N l�1, and inhibition of the biogas process occurred at

a free ammonia concentration of approximately 1100 mg N l�1

[20]. Furthermore, it was recently reported that the optimum

growth conditions for Methanosaeta concilii, which is the most

ammonia-sensitive methanogen, were in the range of

250–1100 mg NH4+ l�1 [21]. Therefore, it was unlikely to con-

clude that high ammonium or free ammonia concentrations

had an adverse effect on anaerobic digestion of sugar beet

silage.

In the following period (5c), HRT was decreased from 25 to

15 days (reactor day 1850). No external ammonium supply

was used at 15 days of HRT, in order to find out the minimum

amount of ammonium concentration required for anaerobic

conversion of sugar beet silage to methane. It was earlier

reported that the ammonia concentration had to be kept in

excess of at least 40–70 mg N l�1, to prevent reduction of

bacterial activity [22]. KOH was still used to control digester

pH. Within 10 days, the ammonium concentration in the

reactor depleted, and in spite of pH control by KOH, the

reactor pH declined to 5.5 (on reactor day 1860). The spec.

GPR, pH and methane values before (during steady-state

conditions at 25 days of HRT, before reactor day 1860) and

during the failure period (on reactor day 1860 at 15 days

of HRT) are displayed in Fig. 5. The concentrations and

aCl addition(mM)

KHCO3 addition(mM)

Gas yield(ml STP)

– – 835

– – 1126

– – 894

– - 4962

– – 875

– – 5021

– – 743

– – 5500

60 – 887

60 – 2633

– 60 1001

– 60 2761

6 days [23].

ARTICLE IN PRESS

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B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 2 0 3 – 2 0 9208

distribution of VFA during this failure and the following

recovery periods are also shown in Fig. 6. The maximum

concentrations of acetic (HAC) and propionic (HPRO) acids

were determined to be 6238 and 3514 mg l�1, respectively (Fig.

6). Even an alkalinity level of 2900 mg CaCO3 l�1 could not

prevent decrease of pH, when the bacterial activity was

inhibited. In order to recover the activity of the reactor at

operation day 1861, HRT was firstly adjusted from 15 to 100

days. Then, it was further adjusted to 75, 50 and 25 days.

During this recovery phase, firstly NH4HCO3, and then KOH

were used to provide N and buffering capacity. After stable

conditions had been maintained, HRT was finally adjusted to

15 days again.

During operation at 15 days of HRT, KOH and KHCO3 were

used to provide buffering capacity, while ammonium chloride

(NH4Cl) was used to maintain the adequate amount of N

concentration in the reactor. Potassium hydrogen carbonate

(KHCO3) seemed to function better to provide the adequate

buffering capacity and to keep the reactor pH stable,

compared with KOH and Na2HCO3, during continuous experi-

ments. On the contrary, batch tests showed that Na2HCO3 was

more favoured than KHCO3. In our laboratory at HAW

Hamburg, NaHCO3 showed a stimulating effect over KHCO3,

in batch bottle tests carried out for a period of 3 weeks

(Table 4). The methods for these batch tests at HAW Hamburg

have been previously reported [23].

At steady-state conditions at 15 days of HRT, the specific

and volumetric GPR levels were determined to be only

0.53 l g�1 VS d�1 and 3.357 l l�1 d�1, respectively, with an aver-

age digester pH of 7.13. The methane composition of digester

biogas varied between 54% and 67% (60% on average). The

average concentrations of alkalinity and ammonium were

3268 mg CaCO3 l�1 and 163 mg l�1, respectively. A brief sum-

mary of similar works from the literature is given in Table 5.

However, most of these research works focused on anaerobic

digestion of sugar beet pulps.

Variation in concentration of ammonium (NH4+) was depen-

dent on external additions of ammonium hydrogen carbonate

or ammonium chloride. Therefore, sharp decreases could

sometimes be observed during the operation (Fig. 4). External

addition of phosphate to the substrate was always the same

(1 mM), so the variation in reactor phosphate concentration

was dependent only on the substrate. The new harvest

(substrate charge 2) had a higher PO43� content than that of

substrate charge 1. Therefore, PO43� concentration increased

after introduction of the new harvest (after reactor day 1695,

at 25 and 15 days of HRT). Decreases in reactor VSS content at

25 and 15 days of HRT, respectively, could be attributed to

higher substrate feeding flow rates (Table 2). There was no

effluent recycling system installed in the experimental set-

up; therefore, some bacterial washout at higher flow rates

could be expected. Furthermore, we could observe no

correlation between the redox potential (ORP) values and

the reactor behaviour during the entire operation.

4. Conclusions

During single-phase mesophilic anaerobic conversion of

sugar beet silage as a mono-substrate (without addition of

ARTICLE IN PRESS

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 2 0 3 – 2 0 9 209

manure) to methane, which had an extreme low pH of 3.4, the

highest spec. GPR and methane content in digester biogas

were obtained at a HRT of 25 days and a pH of around 6.8.

Since sugar beet silage without the top (leaves) is a poor

substrate, in terms of nutrient availability and buffering

capacity, external supplementation of nutrients, especially

nitrogen, and buffering agents (to provide adequate amount

of alkalinity) has to be carried out regularly, to achieve a

stable and an efficient digestion process. In order to provide

adequate buffering capacity and to keep pH stable, potassium

hydrogen carbonate seemed to function quite well. Besides,

change in substrate charge, with a relatively higher content

of phosphate (use of a different harvest), also affected

the reactor process performance significantly during the

operation.

Acknowledgements

The authors would like to express their gratitude to Nils

Sharfenberg, Christian Rosner, Karsten Lehmann, Olaf

Schmidt and Monika Unbehauen for their help and support.

This project was supported by the BMBF KFZ 03SF 03171.

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