Biogas-1 Observed Biogas From Plants

8/2/2019 Biogas-1 Observed Biogas From Plants

1/9

Theoretical and observed biogas production from plant biomass of differentfibre contents

Ewa Klimiuk a, Tomasz Pokj a,*, Wojciech Budzynski b, Bogdan Dubis b

a Department of Environmental Biotechnology, University of Warmia and Mazury in Olsztyn, Soneczna Str. 45G, 10-709 Olsztyn, Polandb Department of Agrotechnology and Crop Management, University of Warmia and Mazury in Olsztyn, M. Oczapowskiego Str. 8, 10-719 Olsztyn, Poland

a r t i c l e i n f o

Article history:

Received 28 February 2010

Received in revised form 14 June 2010

Accepted 25 June 2010

Available online 8 August 2010

Keywords:

Anaerobic digestion

Energy crops

Agricultural biogas plants

Theoretical biogas potential

a b s t r a c t

The methane productivity of silage of four crop species Zea mays L., Sorghum saccharatum, Miscan-

thus giganteus and Miscanthus sacchariflorus was investigated. The experiments revealed that at a

hydraulic retention time of 60 days the volumetric methane yields from the Z. mays L. or S. saccharatum

silages were higher than those from the Miscanthusgiganteus or M. sacchariflorus silages because of the

higher crude fibre content in Miscanthus spp. However, at comparable lignin concentrations in the feed-

stock, methane productivity for M. sacchariflorus (0.19 0.08 L/g volatile solids) was twice that ofMiscan-

thus giganteus (0.10 0.03 L/g volatile solids). The efficiency of cellulose conversion varied from 83.6%

(S. saccharatum) to 52.1% (Miscanthusgiganteus), and hemicellulose from 88.9% (Z. mays L.) to 59.7%

(Miscanthusgiganteus). Conversion of cellulose and hemicellulose depended on the ratio of these poly-

saccharides to the lignin concentration of the feedstock.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Directive 2009/28/EC of the European Parliament and the

Council of 23 April 2009 endorsed a mandatory target of a 20%

share of energy from renewable sources in overall community

energy consumption by 2020. It also mandated a 10% minimum

target to be achieved by all Member States for the share of biofuels

in transport petrol and diesel consumption by 2020. Biogas pro-

duction is a key technology for the sustainable use of agricultural

biomass as a renewable energy source. Biogas can be produced

from a wide range of crops, animal manures and organic wastes,

and thus it offers high flexibility and can be adapted to the specific

needs of different locations and farm management. After anaerobic

digestion, the digestate is a valuable fertiliser for agricultural crops.

Germany is a European leader in using biogas technologies,

installing more than 3500 biogas plants with an overall electrical

capacity of more than 1000 MW during 20022007 (Demirel and

Scherer, 2009). According to Parawira et al. (2008), in Sweden dur-

ing 2004 the biogas-derived energy was 1.4 TWh, with further in-

creases to 25 TWh per year. It is expected that more than half of

this amount (14 TWh) will be from the agricultural sector. In

Poland, according to the Institute of Renewable Energy data, 156

biogas plants operated in 2006, and only one used agricultural

products (Granoszewski and Grabias, 2009). By 2020, however,

Polish government projects plan to have installed about 2000

new biogas plants with an overall electrical capacity of 2000

3000 MW.In an agricultural biogas plant, each group of substrates has a

specific potential for biogas production. The best properties are

raw-harvested plant material with low lignin content (Zubr and

Wise, 1989). The literature indicates that maize (Zea mays L.),

grasses (Poaceae), clovers (Trifolium), Sudan grass (Sorghum sudan-

ense) and fodder beet (Beta vulgaris L.) may be suitable for biogas

production (Gunaseelan, 1997; Tong et al., 1990; Weiland, 2006).

The evaluation of modern biogas plants in Germany, monitored

during 20022004, has shown that maize is the most commonly

used co-substrate in 80% of all agricultural biogas plants operated

with the fermentation of manure (Weiland, 2006). Key parameters

affecting biogas yield investigated so far, are the maize variety,

time of harvesting, mode of conservation and pre-treatment of

the biomass prior to the digestion process (Amon et al., 2007a,b).

Although biogas production from maize is the most efficient and

technically advanced option, it could result in severe competition

between energy and food supplies, which is probably not favour-

able in the long term. For that reason, a great deal of interest in

energy crops has been aroused in recent years. This interest has

focused on the use of agricultural wasteland and perennial crops.

On large farms, an important criterion in crop selection for biogas

production is the specific agriculture land available for production.

The variety of agricultural land and the need for a continuous

biomass supply for the biogas plants requires diversification in

the substrates supplied. The variety of plant biomass used not only

improves the operational management of biogas plants, but also

favours agroecosystem biodiversity.

0960-8524/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2010.06.130

* Corresponding author. Tel.: +48 89 5234161; fax: +48 89 5234131.

E-mail address: [email protected] (T. Pokj).

Bioresource Technology 101 (2010) 95279535

Contents lists available at ScienceDirect

Bioresource Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / b i o r t e c h
http://dx.doi.org/10.1016/j.biortech.2010.06.130mailto:[email protected]://dx.doi.org/10.1016/j.biortech.2010.06.130http://www.sciencedirect.com/science/journal/09608524http://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortechhttp://www.sciencedirect.com/science/journal/09608524http://dx.doi.org/10.1016/j.biortech.2010.06.130mailto:[email protected]://dx.doi.org/10.1016/j.biortech.2010.06.130


2/9

The complex structure of lignocellulosic materials is known to

be a crucial obstacle for transition from traditional cereal crops

to lignocellulosic feedstocks. However, in recent years there has

been a dynamic development of different methods of structural

digestion and recovery of lignin-encrusted sugars (Sun et al.,

1995; Zheng et al., 2009). This should increase the range of crop

types used for conversion into biogas in the future.

This present study investigated biogas productivity from plant

biomass of annual crops, i.e. maize (Z. mays L.), sugar sorghum

(Sorghum saccharatum) and perennial crops (Miscanthusgigan-

teus and Miscanthus sacchariflorus), without using manure or any

other co-substrate. The research sought to (i) determine whether

the hemicellulose and cellulose conversion into biogas, at deter-

mined hydraulic retention times (HRT), depends on the ratio of

their concentration to the lignin concentration in the feedstock;

(ii) estimate the methane digestion operational parameters, and

the specific biogas production rate and biogas yield; and (iii) com-

pare the estimated biogas production using elementary composi-

tion feedstock with the experimental values.

2. Methods

2.1. Field and laboratory investigations

Maize (Z. mays L.), sugar sorghum(S. saccharatum)and Miscanthus

spp. (Miscanthus giganteus and M. sacchariflorus) silageswere used

for biogas production. The plant biomass was obtained from field

experiments performedin 2008(for Miscanthus spp. this wasthesec-

ond year of use and the third year of vegetation) in the Production

and Experimental Station at Bacyny (533504900N, 1951020.300E),

University of Warmia and Mazury in Olsztyn. The crops were culti-

vated in typical lessive soil, of medium silt content, composed of

medium loam, havingthe IIIa bonitation class, and agricultural valu-

ation complex 2. The area of each single plot was 100 m2.

Harvesting of the maize (mid-early LG 3232) was carried out at

the BBCH 89 stage, the sugar sorgum (Sucrosorgo 506) at the milk-

waxen stage, the Miscanthusgiganteus (clone) when the firstlower leaves were beginning to dry, and the M. sacchariflorus at

the flowering phase. The crops were harvested in the first decade

of October by self-propelled harvesters equipped with cutting

drums that chopped the crops into pieces 23 cm in length. Next,

the raw-harvested crops were ensiled. Samples of the plant mate-

rials were concentrated in 200 L silos lined with foil for 90 d. For-

mic acid (85%) was added at a ratio of 5 g acid to 1 kg biomass.

2.2. Feedstock preparation

For standardisation of feedstock in the digestion process, the si-

lage was chopped in a cutting mill (Retsch SM100, Germany) and

passed through 1-mesh screen. Then the silage was stored in

plastic bags at 4 C. After chopping, the concentrations of totalsolids (TS) and volatile solids (VS) in the silage were 0.4248

0.028 g/g and 0.4016 0.022 g/g (Z. mays L.); 0.1412 0.053 g/g

and 0.1373 0.047 g/g (S. saccharatum); 0.3093 0.060 g/g and

0.2878 0.055 g/g (Miscanthusgiganteus); and 0.7007 0.048 g/

g and 0.6755 0.045 g/g (M. sacchariflorus), respectively.

2.3. Experimental set-up and experimental assumptions

The experiments were conducted in four parallel anaerobic,

continuously stirred tank-reactors (CSTRs) with a working volume

of 6 L. The stainless-steel reactors were equipped with a stirrer

with adjustable speed of rotation and a water jacket. Properly

mounted valves enabled reactor feeding, biogas and digestate col-

lection. The following silages were used as feedstock: Z. mays L.(series 1), S. saccharatum (series 2), Miscanthusgiganteus (series

3) and M. sacchariflorus (series 4). The experiments were performed

in duplicate.

Reactors were inoculated with anaerobic sludge fromthe sludge

digestion chambers of a municipal wastewater treatment plant in

Olsztyn (North Poland). Before feeding, the feedstock was mixed

with tap water to obtain a concentration of total solids of 8%. The

silages needed to be mixed with water in different ratios (w/w):

1:5.31 (Z. mays

L.), 1:1.77 (S. saccharatum

), 1:3.86 (Miscanthus

giganteus) and 1:8.76 (M. sacchariflorus).

The reactors were operated at 39 C. Once a day each reactor

was supplied with 100 mL of the feedstock after 100 mL of mixed

liquid had been withdrawn. The biogas was collected in Tedlar

sample bags. In all the series, a constant HRT of 60 days was as-

sumed. Relatively long HRT resulted from the high fibre content

of the Miscanthus spp. (Miscanthusgiganteus: neutral detergent

fibre (NDF) = 75.34 5.73% TS, acidic detergent fibre (ADF) =

47.48 4.02% TS; M. sacchariflorus: NDF = 84.77% 4.25% TS,

ADF = 54.25 3.18% TS).

2.4. Analytical methods

Analytical process control involved analysis of the silage, feed-stock, digestate, the liquid phase of the digestate and biogas pro-

duction and composition.

In the silage and digestate the following parameters were deter-

mined: TS, VS, water soluble carbohydrate (by the anthrone meth-

od; Daniels et al., 1994), NDF, ADF, lignin (acid detergent lignin

(ADL)) (by the Van Soest method; PN-EN ISO 13906:2009) cellu-

lose, hemicellulose, and elementary composition with reference

to carbon (C), nitrogen (N) and hydrogen (H). Hemicellulose con-

tent was calculated as the difference between NDF and ADF, while

cellulose was the difference between ADF and ADL. The C, N and H

content of the biomass was measured at the Institute of Organic

Chemistry, Polish Academy of Sciences in Warsaw (Poland) using

the vario EL III Element Analyzer (Elementar Analysensysteme

GmbH, Germany).

The TS and VS were determined for the feedstock. In the liquid

phase of the feedstock and digestate the following parameters were

determined: pH,chemical oxygendemand (COD) by thedichromate

method, ammonium nitrogen (N-NH4) and volatile fatty acids

(VFAs) by the distillation method, and alkalinity by the titration

method. The measurements were performed for filtered superna-

tant samples, previously centrifuged (8693gfor 10 min).

The parameters, including pH, alkalinity, TS, VS, COD, N-NH4and VFAs, were determined according to the standards methods

for the examination of water and wastewater (APHA, 1992).

2.5. Biogas characteristics

Biogas production and composition were measured daily in

averaged samples, collected in Tedlar sample bags. The biogas vol-

ume was measured following standard methods (APHA, 1992)

using apparatus consisting of a cylinder filled with a saturated

solution of sodium chloride combined with an equalising tank

equipped with a side tube. The composition of the biogas with ref-

erence to maximum methane (CH4) and carbon dioxide (CO2) con-

tents and to minimum oxygen concentration was measured using a

GA 2000+ automatic analyser (Geotechnic Instruments, UK).

3. Results and discussion

3.1. Characteristic of the feedstock and degree of organic digestion

Results of the chemical analysis of the silages (after dilutionwith tap water) are given in Table 1.

9528 E. Klimiuk et al. / Bioresource Technology 101 (2010) 95279535
http://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortech


3/9

From the data presented, it can be seen that total solids (TS)

in the feedstock did not exceed 8.8%. Silages of Z. mays L. and

S. saccharatum were characterised by acidic pH, whereas both

Miscanthus species exhibited a slightly alkaline pH, indicating little

ensilage. The VFAs and N-NH4 concentrations in the filtered super-

natant depended on the silage type. The highest VFAs concentra-

tion was for S. saccharatum, and the lowest for M. sacchariflorus.

Review of literature revealed that miscanthus can be preserved

and stored by ensiling (Lewandowski et al., 2000). In present study

low degree of ensiling ofMiscanthus spp. seemed to result from low

concentration of water soluble carbohydrate (WSC) and for M. sac-

chariflorus also low moisture content in fresh plant biomass. The

ensiling process is successful at minimum content of WSC equal

50g/kg (McDonald et al., 1991). Similarly, when dry matter of crop

exceeds 500 g/kg, the process of ensiling is also restricted, because

of impaired enzyme activity at lowmoisture content (Adogla-Bessa

and Owen, 1995). The WSC and dry matter (TS) content of Miscan-

thus giganteus and M. sacchariflorus used in our research were

7.9 2.27 g/kg, 35 2.83 g/kg and 45.1 1.44 g/kg, 65 4.24 g/kg,

respectively, and were kept behind the values needed for adequate

ensiling.

After digestion, the highest degree of organic removal

(expressed as volatile solids concentration) was 77.5 2.6% for S.

saccharatum silage (Fig. 1b). S. saccharatum contained a significant

concentration of water soluble carbohydrate (14.9 g/L), considered

to be easily available compounds for microorganisms. The sum ofthe hemicellulose, cellulose and lignin concentrations was 43.0 g/

L, and the conversion degree was 80.2%. Silage of S. saccharatum

contained more cellulose than that ofZ. mays L., with a lignocellu-

lose concentration of 35.1 g/L, and degree of conversion 74.3%. The

degree of conversion of hemicellulose was higher than that for cel-

lulose for all the silages tested (Fig. 1). The lowest efficiency of

hemicellulose and cellulose removal was for Miscanthusgigan-

teus. Lignin was not degradable during digestion (Fig. 1) and its

concentration in the digestate was comparable with its concentra-

tion in the feedstock for Z. mays L. and S. saccharatum silages. The

average lignin concentration for Miscanthus spp. was higher in the

digestate than in the feedstock. These differences were not statis-

tically significant (t-test for dependent samples: Miscanthus

giganteus, P= 0.1233; and M. sacchariflorus, P= 0.7811).There is a common view in scientific literature that lignin is not

biodegradable under anaerobic conditions. This is because the

extracellular enzymes required for depolymerisation of lignin need

molecular oxygen, and their oxidative reactions would not be

anticipated under anaerobic conditions (Hatakka, 1994; Jeffries,

1994). However, some authors have shown that lignin can be de-

graded under anoxic conditions, i.e. sulphate-reducing conditions

(Pareek et al., 2001). Earlier, Benner et al. (1984) demonstrated that

[14C]-lignocellulose preparations and synthetic [14C]-lignin incu-

bated with anoxic sediments were slowly degraded anaerobically

to 14CO2 and14CH4. A review of the literature shows that both cel-

lulose and hemicellulose can be anaerobically converted to CH4and CO2. However, the rate of degradation, especially of cellulose,

depends strongly on its state in the feedstock; if it is lignin-incrust-ed, then lignin prevents the access of cellulases to the cellulose

fibres. If the cellulose is mainly in a crystalline form, then cellulases

can attach to it, and hydrolysis can be relatively fast, leading to

propionate and butyrate formation (Jrdening and Winter, 2005).

The chemistry of grass lignocellulose varies considerably from that

of wood (Akin et al., 1995), and explains why it is currently as-

sumed that lignin concentration does not always reflect the degree

to which lignin inhibits cellulose bioavailability. The lignin in grass

is not as restrictive to microorganisms as the lignin in components

such as branches (Barlaz, 2006).

The present study showed that under defined operational con-

ditions (i.e. constant HRT), there was a relationship between the

efficiency of cellulose/hemicellulose removal (E) and the chemical

composition of the fibre in the feedstock, expressed as the ratio of

cellulose/hemicellulose to lignin concentration. For crops tested in

this study, the relationship is described using the empirical

equations:

E flnCCel=CADL or E flnCHem=CADL:

where CCel is the concentration of cellulose; CADL is the concentra-

tion of acid detergent lignin; and CHem is the concentration of

hemicellulose.

Experimental results were fitted to regression lines showing the

rectilinear regression coefficients for both polysaccharides (Fig. 2).

The efficiency of hemicellulose removal was higher, as indicated by

coefficient b. Moreover, the experiments demonstrated that the

effectiveness of cellulose/hemicellulose removal increased to-gether with an increase in the CCel/CADL and CHem/CADL ratios. Low

CCel/CADL and CHem/CADL ratios were typical for silages ofMiscanthus

spp., related to their high lignin contents. For the Z. mays L. and S.

saccharatum silages the ratios were high because of significantly

lower lignin concentrations. The CHem/CADL ratio of Miscanthus

spp. was approximately 1.31.7 times lower that of Z. mays L.

and S. saccharatum and the CCel/CADL ratio of Miscanthus spp. was

1.92.4 times lower that ofZ. mays L. and S. saccharatum. However,

further investigations are necessary to point out whether empiri-

cally determined equations can by applied for other crop species

and operational conditions.

Lignin concentration was not the only factor affecting the con-

version of the organic matter in the tested plants. The theoretical

biogas yield (biodegradable fraction, B) resulting from the biodeg-radation of the silages tested was compared with the empirical

results. Chandler et al. (1980) correlated the biodegradability of

various agricultural residues, determined by long-term digestion

studies, with the lignin content of the substrate, as determined

by sequential fibre analysis. They developed the following empiri-

cal relationship to estimate the B value of an organic substrate

from a lignin test:

B 0:83 0:028 ADL 1

where ADL is expressed as % VS.

For Z. mays L. and S. saccharatum silages, the B values calculated

from Eq. (1) (74.6% and 73.9% VS, respectively) were comparable

with the values obtained experimentally, 75.5% and 77.5% VS,

respectively. For Miscanthusgiganteus and M. sacchariflorus theexperimental values were 29.4% and 36.3% VS about half of the

Table 1

Characteristics of the feedstocks fed to reactors in series 14 (standard deviation from the mean value is given in parentheses).

Parameter Unit Series 1 Series 2 Series 3 Series 4

Z. mays L. S. s accharatum Miscanthus giganteus M. sacchariflorus

pH 4.3 (0.16) 3.75 (0.04) 7.46 (0.042) 7.37 (0.113)

Volatile fatty acids mg/L 1457 (150.2) 2648 (133.3) 600 (12.7) 274 (2.1)

Ammonium nitrogen mg N-NH4/L 110.6 (10.9) 112 (0.177) 66.5 (0.071) 22.4 (0.092)

Total solids % 8.4 (0.79) 8.5 (0.38) 8.8 (1.41) 8.8 (0.68)

Volatile solids % 7.8 (0.74) 7.8 (0.61) 7.9 (1.32) 8.5 (0.64)

E. Klimiuk et al. / Bioresource Technology 101 (2010) 95279535 9529


4/9

calculated B values of 59.0% and 61.0% VS, respectively. These

differences could be the result of an insufficiently long HRT for

the Miscanthus spp. silage in the CSTR reactor, which limited the

degree of conversion.

Benner et al. (1984) tested specifically radiolabelled [14C-lig-

nin]-lignocellulose and [14C-polysaccharide]-lignocellulose pre-

pared from a variety of marine and freshwater wetland plants

including a grass, a sedge, a rush and a hardwood. They demon-strated that lignocellulose derived from herbaceous plants was

degraded more rapidly than lignocellulose from hardwood. After

246 days, 16.9% of the lignin and 30.0% of the polysaccharide com-

ponents of the lignocellulose derived from the grass were de-

graded. However, only 1.55% of the lignin and 4.15% of the

polysaccharide components of the lignocelullose derived from

hardwood were degraded to gaseous end products.

Cellulose degradability is not always directly correlated

with lignin concentration. Tong et al. (1990) reported the metha-nogenic conversion of white fir and wood grass to be 9% and

0

20

40

60

80

100

0

20

40

60

80

100

Volatile solids Water soluble

carbohydrates

Hemicellulose Cellulose Lignin

E(%)

g/L

0

20

40

60

80

100

0

20

40

60

80

100

Volatile solids Water soluble

carbohydrates


E(%)

g/L

0

20

40

60

80

100

0

20

40

60

80

100

Volatile solids Water solublecarbohydrates

Hemicel lulose Cel lu lose Lignin

E

(%)

g

/L

0

20

40

60

80

100

0

20

40

60

80

100

Volatile solids Water solublecarbohydrates


E

(%)

g

/L

silage

digestate

a b

c d

Fig. 1. Concentration of volatile solids, water soluble carbohydrate and fibre in tested silages and digestate (g/L) and the efficiency of their removal (E): (a) series 1

(Z. mays L.); (b) series 2 (Sorghum saccharatum); (c) series 3 (Miscanthusgiganteus); and (d) series 4 (M. sacchariflorus). Error bars represent standard deviation.

y = 39,531x + 14,133

R2 = 0,992

0

20

40

60

80

100

1 1.25 1.5 1.75 2

lnCHem

/CADL

EHem(

%)

y = 39,86x - 8,0881

R2 = 0,9941

0

20

40

60

80

100

1.5 1.75 2 2.25 2.5

lnCCel

/CADL

ECel

Fig. 2. The relationships between the efficiency of hemicellulose (EHem) and cellulose (ECel) removal and the ratio of hemicellulose and cellulose concentration to lignin

concentration (CHem/Ci and CCel/CADL, respectively). Error bars represent standard deviation.



5/9

66%, respectively, despite similar lignin concentration in both sub-

strates. Based on the decomposition of cellulose in grass and paper,

Eleazer et al. (1997) showed that lignin concentration alone was

not a good predictor of cellulose bioavailability.

3.2. Characteristic of the digestate liquid phase

Maintenance of suitable environmental conditions is extremely

important for the performance of CH4 digestion. Some of the envi-

ronmental factors that affect methanogenesis require stringent

monitoring and control to avoid process failure. Among them,

pH, alkalinity and VFAs concentration have been investigated.

These factors play leading roles in building up the selective pres-

sure, which elicits and stabilises a biological community, and they

are important mainly because of the critical nature of the syn-

trophic relationship between the microorganisms in the digester.

Anaerobic bacteria, especially methanogens, exhibit a charac-

teristic sensitivity to extremes of pH. In an effective working diges-

ter, pH drop can be countered by natural processes, such as

alkalinity and the consumption of VFAs by methanogens. Alkalinity

is the equilibrium between CO2 and bicarbonate ions that provides

buffer capacity in anaerobic digestion, preventing significant and

rapid changes in pH. Therefore, buffering capacity is proportional

to bicarbonate concentration. It should be noted that to maintain

an adequate buffering capacity, and to keep the pH at a safe and

stable level (Fig. 3a), the digesters fed withZ. mays L. and S. saccha-

ratum silages required the addition of 5 M NaOH solution through-

out the operation. This explains the higher alkalinity (Fig. 3b),

despite the high concentration of VFAs in the digestate liquid

phase, of these feedstocks (Fig. 3d). During digestion of Z. mays L.

and S. saccharatum silage, VFAs concentration in the digestate liquid

phases were 2420.8 612.1 and 1555.7 377.9 mg C2H4O2/L, respec-

tively;higherthan for Miscanthus spp.silage. Similarly, theconcentra-

tion of soluble organic compounds (measured as COD) was highest in

thedigestate liquid phase fromZ. mays L. silage(Fig. 3c). For the other

silages, it varied between 1761 285 (Miscanthusgiganteus) and

1290 119 mg COD/L (M. sacchariflorus).During fermentation, high molecular weight compounds (i.e.

polysaccharides) are hydrolysed to monomers, and then digested

to organic acids. Acetic acid, H2 and CO2 produced at the acetogen-

esis phase are utilised by methanogens as substrates and converted

into CH4. Consumption of volatile acids is dependent on the equi-

librium between acidogens and acetogens and this can be easily

upset by changes in the operational and environmental conditions.

When the activity of hydrolytic bacteria converting cellulose and

hemicellulose to soluble intermediates, and the acetogenic bacteria

and methanogens remain at equilibrium, then short-chain acids

are converted to CH4 and CO2. Under unsustainable conditions,

short-chain carboxylic acids accumulate in the liquid phase of

the digestate. If the acids are present at high concentrations they

alter the pH and cause microbial stress and methanogenesis inhi-bition. Siegert and Banks (2005) reported that glucose fermenta-

tion was inhibited at total VFAs concentration greater than 4 g/L.

To overcome accumulation of VFAs and in order to transform all

soluble organic compounds into biogas it seems to employ two-

stage digestion. Efficacy of such process configuration is confirmed

in practice (Weiland, 2006).

Acetic acid is usually present at higher concentrations than

other fatty acids during anaerobic digestion (Wang et al., 1999).

Essentially, the methanogens cannot metabolise the acetate pro-

duced by acetogenic organisms until the number of methanogenic

organisms has increased sufficiently. This is especially true of feed-

stocks that are rapidly hydrolysed. With poorly-degradable feed-

stocks, the hydrolysis stage is more likely to be the limiting step,

as confirmed in the present study. For the Miscanthus spp. silage,the hydrolysis of polysaccharides proceeded slowly because of lig-

nin incrustation, limiting the rate of further digestion phases. The

concentration of organic acids in the digestate liquid phase was

low, whereas the pH remained near neutral, without the need to

add further chemicals (Fig. 3a).

Nitrogen plays an important role in supplying the basic cellular

building blocks for cell growth and the synthesis of enzymes and

cofactors. Additionally, fermenting nitrogenous compounds con-

tribute to the stable neutral pH of the CH4

-digesting liquor by

the release of NH4 ions. The investigations demonstrated that indi-

vidual silages were characterised by different C/N ratios (Table 3).

The highest C/N ratio was for the M. sacchariflorus silage (C/

N = 103.3), and the lowest (C/N = 19.1) was for S. saccharatum,

which had the highest N-NH4 concentration (447.8 51.6 mg/L)

in the digestate liquid phase. During anaerobic digestion of organic

matter, the rate of C consumption was 2530-times higher than

that of N. For that reason, to maintain optimal growth conditions,

the C/N ratio should be between 20:1 and 30:1 (Malik et al.,

1987). If the organics in the silage were completely converted into

biogas, then the N concentration of the Miscanthus spp. would be

insufficient for proper microorganism growth.

The research showed, however, that the degree of organic con-

version into biogas was 29.3% for Miscanthusgiganteus and 36.3%

for M. sacchariflorus, resulting in only partial use of the N, which

was reflected in the N-NH4 concentration. The N-NH4 concentra-

tion in the digestate liquor was high at 297 and 220 mg N-NH4/L

for Miscanthusgiganteus and M. sacchariflorus, respectively

(Fig. 3e).

3.3. Biogas production rate and biogas yield

At a constant HRT of 60 days, the organic loading rate (OLR) in

the following series varied from approximately 1.30 g VS/L d (Z.

mays L., S. saccharatum and Miscanthusgiganteus) to 1.42 g VS/

L d (M. sacchariflorus) (Table 2).

The biogas production rate (rb) and methane production rate

(rm) for Z. mays L. silage were comparable with the rates for S. sac-

charatum silage, and were correlated with low concentrations offibrous materials, specifically lignin. Specific biogas production

from silage of M. sacchariflorus was higher than for Miscanthus

giganteus. The Miscanthus spp. had a similar lignin content; how-

ever, the cellulose concentration in M. sacchariflorus silage (41.9%

TS) was higher than in Miscanthusgiganteus (31.9% TS). There

was a similar trend for the biogas yield coefficient, Yb, and the

methane yield coefficient, Ym. The coefficient values for theZ. mays

L. and S. saccharatum silages were nearly twice those for the M. sac-

chariflorus silage. Likewise, for the specific biogas production rate,

Miscanthusgiganteus was characterised by a lower Yb value than

M. sacchariflorus. The literature suggests that for Z. mays L. silage

the Ym value is 0.280.42 L/g VS, for winter wheat straw 0.28 L/g

VS (HRT 82 days), for rice straw 0.23 L/g VS (HRT 73 days) and

for rape straw 0.19 L/g VS (HRT 42 days) (Zubr and Wise, 1989).In the present study, biogas production rate and methane yield

coefficient ofM. sacchariflorus were almost twice higher than those

ofMiscanthusgiganteus, while the Ym value (0.19 L/g VS) was the

same as for rape straw (Zubr and Wise, 1989). Uellendahl et al.

(2008) reported twice higher methane yield for Miscanthus

giganteus amounted to 0.2 L/g VS. The difference seemed to result

from distinct assumptions of investigation employed by the

authors. They provided batch experiments compared with contin-

uously operated reactors in present study.

3.4. Comparing calculated biogas production and experimentally

obtained values

The amount of biogas produced is closely related to theelementary composition of the biomass. Some models have been



6/9

developed to estimate biogas composition from the chemical com-

position of organic substrates in a feedstock, including C, H, N and

sulphur, and there are suitable equations available (Nyns, 1986).

However, these models do not estimate the CH4 yield when the de-

gree of conversion of organic substrates is not complete, which is

common in practice. In the present study, biogas production and

composition was estimated as the difference between the theoret-

ical value calculated on the basis of elementary silage composition

(Qtheor), the potential amounts of biogas in the digestate (Qwaste)

and in the digestion liquor (Qalkal), according to the following

equation:

Qestimated Qtheor Qwaste Qalkal 2

where Qestimated is the theoretical biogas production obtained under

established experiment conditions, Qtheor is the theoretical biogasproduction calculated on the basis of feedstock composition, Qwaste

6

6.4

6.8

7.2

7.6

8

Z. mays L. S. saccharatum M. gigantheus M. saccharif lorus




pH

0

1000

2000

3000

4000

5000

6000

7000


Alkalinity(mgCaCO3

/L)

0

2000

4000

6000

8000

C

OD(mgO2

/L)

0

2000

4000

6000

8000

VFAs(mgC2

H4

O2

/L)

0

100

200

300

400

500

600

Ammoniumn

itrogen(mgN-NH

4/L)

a

c

b

d

e

Fig. 3. Chemical parameters of digestate liquid phase after digestion of silage of Z. mays L., Sorghum saccharatum, Miscanthusgiganteus and M. sacchariflorus: (a) pH;

(b) alkalinity; (c) chemical oxygen demand (COD); (d) volatile fatty acids (VFAs); and (e) ammonium nitrogen. Error bars represent standard deviation.

Table 2

Parameters characterizing biogas productivity in series 14 (standard deviation from the mean value is given in parentheses).

Parameter Unit Series 1 Series 2 Series 3 Series 4

Z. mays L. Sorghum saccharatum Miscanthusgiganteus M. Sacchariflorus

Feedstock loading (FX) g VS/d 7.84 7.85 7.93 8.52

Organic loading rate (OLR) g VS/L d 1.31 (0.12) 1.31 (0.10) 1.32 (0.22) 1.42 (0.12)

Methane content in biogasa % 58.1 (5.7) 57.1 (4.8) 48.2 (6.3) 55.3 (5.5)

Biogas production rate (rb)a L/L d 0.82 (0.26) 0.79 (0.18) 0.31 (0.07) 0.45 (0.12)

Methane production rate (rm)a L/L d 0.47 (0.19) 0.44 (0.13) 0.13 (0.03) 0.27 (0.10)

Biogas yield coefficient (Yb)a L/g VS 0.57 (0.21) 0.59 (0.14) 0.23 (0.06) 0.32 (0.08)

Methane yield coefficient (Ym)a L/g VS 0.33 (0.13) 0.33 (0.10) 0.10 (0.03) 0.19 (0.07)

a Volume of biogas and methane were expressed at normal temperature (C) and pressure (101 kPa).



7/9

is the potential biogas production from the digestate, and Qalkal is

the potential amount of CO2 bound in the form of bicarbonates in

the digestate liquid phase.

Calculated values ofQestimated were then compared with the val-

ues obtained experimentally.

3.4.1. Theoretical biogas production (Qtheor)

The chemical composition (as % TS) of the silages investigated,determined on the basis of elementary composition, are given in

Table 3.

DJesus et al. (2006) reported for Z. mays L. silage the following

elementary composition: 43.40% C, 6.17% H, 46.70% O and 1.02%, N,

and were comparable with the values reported in the present

study. The C concentration in S. saccharatum silage was similar to

a mean value of 41.9% given by Monti et al. (2008). Sweet sorghum

was cultivated in the experimental farm of Bologna University

(44330N, 33m a.s.l) in soil containing: 1.8% organic matter, 27.0%

sand, 39.0% silt and 34.0% clay. The crop was sown in late April

and harvested in September. A higher C concentration (45.08%),

but lower H (5.76%) and N (0.85%) concentrations in S. saccharatum

biomass grown in Italy (Umbria region) and also harvested in the

autumn but without ensilage was found by Piskorz et al. (1998).The elementary composition of Miscanthus biomass corresponded

to values reported by Lewandowski and Kicherer, 1997. According

to the authors carbon and hydrogen content (% dry weigh) of

Miscanthusgiganteus ranged 47.849.7 and 5.645.92, respec-

tively. Cited values referred to plant cultivated in haplic luvisol soil,

composed of loamy sand located in Durmersheim (Upper Rhine

Valley, Germany), and harvested in February when the plantation

was in the third ratoon.

To calculate the amounts of CH4 and CO2 that it is possible to

obtain from one mole of biomass with a known elementary com-

position, ORourkes equation was applied (Nyns, 1986):

CaHbOcNd 4a b 2c 3d

4

H2O !

4a b 2c 3d

8

CH4

4a b 2c 3d

8

CO2 dNH3 3

The molar composition of the biogas obtained from one mole of si-

lage tested, is expressed by the following equations:

Z: mays L: : C56:51H112O48:07N 5:3H2O ! 29:9CH4

26:4CO2 NH3 4

S: saccharatum : C22:33H40:18O19:82N 3:1H2O ! 10:9CH4

11:5CO2 NH3 5

Miscanthus giganteus : C36:53H51:92O29:25N 9:7H2O ! 17:1CH4

19:5CO2 NH3 6

M: sacchariflorus : C121:87H192:78O96:82N 26H2O ! 60:5CH4

61:4CO2 NH3 7

The mass of CH4 (FCH4;theor) and CO2 (FCO2;theor) theoretically ob-

tained during silage digestion, assuming complete conversion of

organics into biogas, was calculated using the formulas below:

FCH4 ;theor MCH4

Mfeedstock FX g CH4=d 8

FCO2 ;theor MCO2

Mfeedstock FX g CO2=d 9

where FX is the organics load fed to the reactor, MCH4 is the molar

mass of CH4, MCO2 is the molar mass of CO2, and Mfeedstock is the mo-

lar mass of the individual silages.

Assuming the density of CH4 as 0.717 g/L and that of CO2 as1.978 g/L (at 0 C, 101 kPa), the total volume was calculated as Ta

ble

3

Elem

entarycompositionofsilagesusedinexperimentsassubstratesanddigestate(standarddeviation

from

themeanvalueisgiveninparentheses).

E

lement

Unit

Z.mays

L.

Sorghumsaccharatu

m

Miscanthus

gigante

us

M.sacchariflorus

Silage

Digestate

Silage

Digestate

Silage

Digestate

Silage

Digestate

C

arbon

%

43

.13(0

.191)

40

.88(0

.117)

41

.91(0

.042)

32

.70(0

.191)

45

.09(0

.042)

38

.95(0

.141)

45

.44(0

.113)

33

.79(0

.134)

H

ydrogen

%

7.1

2(0

.021)

6.2

8(0

.309)

6.2

9(0

.106)

5.5

4(0

.028)

5.3

4(0

.127)

6.0

2(0

.014)

5.9

9(0

.014)

5.5

3(0

.106)

O

xygen

%

48

.88(0

.318)

49

.10(0

.244)

49

.62(0

.177)

58

.61(0

.163)

48

.13(0

.071)

52

.58(0

.318)

48

.14(0

.092)

58

.19(0

.057)

N

itrogen

%

0.8

9(0

.106)

3.7

3(0

.086)

2.1

9(0

.113)

3.1

6(0

.057)

1.4

4(0

.014)

2.4

6(0

.191)

0.4

4(0

.035)

2.5

0(0

.028)

C

hemicalformula

C56

.51

H112

O48

.07N

C12

.78

H23

.56

O11

.51

N

C22

.33

H40

.18

O19

.82N

C12

.07

H24

.54

O16

.23

N

C36

.53

H51

.92

O29

.25

N

C18

.51

H34

.33

O18

.74

N

C121

.87

H192

.78

O96

.82

N

C15

.77

H30

.94

O20

.37

N



8/9


9/9

Miscanthusgiganteus or M. sacchariflorus silages because of high-

er lignin content in Miscanthus spp. The results indicate that anaer-

obic digestion of silage of Miscanthus spp. under mesophilic

conditions needs longer HRT than 60 d or applying methods of lig-

nin-encrusted sugars recovery (e.g. alkaline pre-treatment).

References

Adogla-Bessa, T., Owen, E., 1995. Ensiling of whole-crop wheat with cellulase-

hemicellulase based enzymes. 1. Effect of crop growth stage and enzyme on

silage composition and stability. Anim. Feed Sci. Technol. 55, 335347.

Akin, D.E., Rigsby, L.L., Sethuraman, A., Morrison III, W.H., Gamble, G.R., Eriksson, K.-

E.L., 1995. Alterations in structure, chemistry, and biodegradability of grass

lignocellulose treated with the white rot fungi Ceriporiopsis subvermispora andCyathus stercoreus. Appl. Environ. Microbiol. 61, 15911598.

American Public Health Association (APHA), 1992. Standard Methods for the

Examination of Water and Wastewater, 18th ed. American Public Health

Association, Washington, DC.

Amon, T., Amon, B., Kryvoruchko, V., Machmller, A., Hopfner-Sixt, K., Bodiroza, V.,

Hrbek, R., Friedel, J., Ptsch, E., Wagentristl, H., Schreiner, M., Zollitsch, W.,

2007a. Methane production through anaerobic digestion of various energy

crops grown in sustainable crop rotations. Bioresource Technol. 98, 32043212.

Amon, T., Amon, B., Kryvoruchko, V., Zollitsch, W., Mayer, K., Gruber, L., 2007b.

Biogas production from maize and dairy cattle manure influence of biomass

composition on the methane yield. Agr. Ecosyst. Environ. 118, 173182.

Barlaz, M.A., 2006. Forestproducts decomposition in municipal solid waste landfills.Waste Manage. 26, 321333.

Benner, R., Maccubbin, A.E., Hodson, R.E., 1984. Anaerobic biodegradation of the

lignin and polysaccharide components of lignocellulose and synthetic lignin by

sediment microflora. Appl. Environ. Microbiol. 47, 9981004.

Chandler, J.A., Jewell, W.J., Gossett, J.M., Vansoet, P.J., Robertson, B.J., 1980.

Predicting methane formation biodegradability. Biotechnol. Bioeng. Symp. 10,

93107.

DJesus, P., Boukis, N., Kraushaar-Czarnetzki, B., Dinjus, E., 2006. Gasification of corn

and clover grass in supercritical water. Fuel 85, 10321038.

Daniels, L., Hanson, R.S., Phillips, J.A., 1994. Metabolism. Chemical analysis. In:

Gerhardt, P., Wood, W.A. (Eds.), Methods for General and Molecular

Bacteriology. American Society for Microbiology, Washington, pp. 512554.

Demirel, B., Scherer, P., 2009. Bio-methanization of energy crops through mono-

digestion for continuous production of renewable biogas. Renew. Energ. 34,

29402945.

Directive 2009/28/EC of the European Parliament and of the Council of 23 April

2009 on the promotion of the use of energy from renewable sources and

amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC.

Eleazer, W.E., Odle, III, W.S., Wang, Y.-S., Barlaz, M.A., 1997. Biodegradability ofmunicipal solid waste components in laboratory-scale landfills. Environ. Sci.

Technol. 31, 911917.

Granoszewski, K., Grabias, M., 2009. Potencja rozwoju instalacji biogazowych w

Polsce. Available from: .

Gunaseelan, V.N., 1997. Anaerobic digestion of biomass for methane production: a

review. Biomass Bioenerg. 13, 83114.

Hatakka, A., 1994. Lignin-modifying enzymes from selected white-rot fungi:

production and role in lignin degradation. FEMS Microbiol. Rev. 13, 125135.

Jeffries, T.W., 1994. Biodegradation of lignin and hemicelluloses. In: Ratledge, C.

(Ed.), Biochemistry of Microbial Degradation. Kluwer Academic Publishers,

Netherlands, pp. 233277.

Jrdening, H.-J., Winter, J., 2005. Environmental Biotechnology Concepts and

Applications. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.

Lewandowski, I., Clifton-Brown, J.C., Scurlock, J.M.O., Huisman, W., 2000.

Miscanthus: European experience with a novel energy crop. BiomassBioenerg. 19, 209227.

Lewandowski, I., Kicherer, A., 1997. Combustion quality of biomass: practical

relevance and experiments to modify the biomass quality of

Miscanthusgiganteus. Eur. J. Agron. 6, 163177.Malik, R.K., Singh, R., Tauro, P., 1987. Effect of inorganic nitrogen supplementation

on biogas production. Biol. Waste. 21 (2), 139142.

McDonald, P., Henderson, A.R., Heron, S.J.E., 1991. The Biochemistry of Silage,

second ed. Chalcombe Publications, Aberystwyth, UK.

Monti, A., Di Virgilio, N., Venturi, G., 2008. Mineral composition and ash content of

six major energy crops. Biomass Bioenerg. 32, 216223.

Nyns, E.J., 1986. Biomethanation processes. Microb. Degrad. 8, 207267.

Parawira, W., Read, J.S., Mattiasson, B., Bjrnsson, L., 2008. Energy production from

agricultural residues: high methane yields in pilot-scale two-stage anaerobic

digestion. Biomass Bioenerg. 32, 4450.

Pareek, S., Azuma, J.-I., Matsui, S., Shimizu, Y., 2001. Degradation of lignin and lignin

model compound under sulfate reducing condition. Water Sci. Technol. 44 (2

3), 351358.

Piskorz, J., Majerski, P., Radlein, D., Scott, D.S., Bridgwater, A.V., 1998. Fast pyrolysis

of sweet sorghum and sweet sorghum bagasse. J. Anal. Appl. Pyrol. 46, 1529.

PN-EN ISO 13906:2009. Animal feeding stuffs. Determinationof acid detergent fibre

(ADF) and acid detergent lignin (ADL) contents.

Siegert, I., Banks, C., 2005. The effect of volatile fatty acid additions on the anaerobic

digestion of cellulose and glucose in batch reactors. Process Biochem. 40, 3412

3418.

Sun, R., Lawther, J.M., Banks, W.B., 1995. Influence of alkaline pre-treatments on the

cell wall components of wheat straw. Ind. Crop. Prod. 4, 127145.

Tong, X., Smith, L.H., McCarty, P.L., 1990. Methane fermentation of selected

lignocellulosic materials. Biomass 21, 239255.

Uellendahl, H., Wang, G., Mller, H.B., Jrgensen, U., Skiadas, I.V., Gavala, H.N.,

Ahring, B.K., 2008. Energy balance and costbenefit analysis of biogas

production from perennial energy crops pretreated by wet oxidation. Water

Sci. Technol. 58.9, 18411847.

Wang, Q., Kuninobu, M., Ogawa, H.I., Kato, Y., 1999. Degradation of volatile fatty

acids in highly efficient anaerobic digestion. Biomass Bioenerg. 16, 407416.

Weiland, P., 2006. Biomass digestion in agriculture: a successful pathway for the

energy production and waste treatment in Germany. Eng. Life Sci. 6, 302309.

Zheng, M., Li, X., Li, L., Yang, X., He, Y., 2009. Enhancing anaerobic biogasification ofcorn stover through wet state NaOH pretreatment. Bioresource Technol. 100,

51405145.

Zubr, J., Wise, D., 1989. Field crops as renewable sources of convertible energy for

methanogenic fermentation. Int. Biosyst. 3, 165171.

http://www.ieo.pl/http://www.ieo.pl/

Documents

Biogas-1 Observed Biogas From Plants