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What type of digester configurations should be employed to produce biomethane

from grass silage?

Abdul-Sattar Nizami a,b, Jerry D. Murphy a,b,*a Department of Civil and Environmental Engineering, University College Cork, Cork, Irelandb Environmental Research Institute, University College Cork, Ireland

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

1.1. Focus of the paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

1.2. Anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

1.3. Grass: a new way towards renewable energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

1.4. Biomethanation of grass silage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

1.5. Design of anaerobic digester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

1.6. Properties of grass silage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1559

2. Potential digester configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1561

2.1. One-stage versus two-stage digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1561

2.2. Dry versus wet digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1561

2.3. Batch versus continuous digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

2.4. High-rate digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

3. Digester configurations suitable for grass silage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

3.1. Wet continuous digester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562

3.2. Leach bed system connected with high-rate digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15633.3. Dry continuous digester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1564

3.4. Batch digester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1564

4. Arguments for and against different digester configurations for grass silage digestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565

4.1. Biogas production per unit of grass silage based on volatile solids destruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565

4.2. The wet continuous two-stage process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565

Renewable and Sustainable Energy Reviews 14 (2010) 1558–1568

A R T I C L E I N F O

Article history:

Received 8 December 2008Accepted 8 February 2010

Keywords:

Grass silage

Biogas

Biomethane

Anaerobic digester

A B S T R A C T

Grass is an excellent energycrop;it maybe classified as a high yielding,low energyinput, perennial crop.

Over 90% of Irish agricultural land is under grass; thus farmers are familiar with, and comfortable with,

this crop as opposed to a ‘‘new energycrop’’such as Miscanthus. Ofissue therefore isnot the crop, but the

methodology of generating energy from the crop. Numerous farmers across Europe (in particular

Germany and Austria) use grass silage as a feedstock for biogas production; in a number of cases the

produced biogas is scrubbed to biomethane and used as a transport fuel or injected into the natural gas

grid. Many Irish farmers are considering converting from conventional farming such as beef production

to grass biomethane production. Numerous technologies and combinations of such technologies are

available; from one-stage batch dry systems to two-stage wet continuous systems; from one-stage

continuous wet systems to two-stage systems incorporating a batch dry reactor coupled with a second

stage high-rate reactor. This paper reviews work carried out both in the scientific literature and in

practice at commercial scale.

2010 Elsevier Ltd. All rights reserved.

Abbreviations: COD, chemical oxygen demand; CSTR, continuously stirred tank reactor; D-value, digestibility-value; HRT, hydraulic retention time; ME, metabolizable

energy; MSW, municipal solid waste; N, nitrogen; OFMSW, organic fraction of municipal solid waste; OLR, organic loading rate; UASB, upflow anaerobic sludge blanket; VS,

volatile solids; VFA, volatile fatty acid.

* Corresponding author at: Department of Civil and Environmental Engineering, University College Cork, Cork, Ireland. Tel.: +353 21 4902286; fax: +353 21 4276648.

E-mail address: [email protected] (J.D. Murphy).

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4.3. The dry batch process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565

4.4. The dry continuous process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565

4.5. The leach bed system combined with UASB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1565

5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566

5.1. Potential improvements in anaerobic digesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566

5.2. Research required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566

1. Introduction

1.1. Focus of the paper

The number of on-farm digesters is increasing significantly

across Europe. There is tremendous potential for these on-farm

digesters using grass silage as a bioenergy feedstock, especially in

Ireland where 91%of agriculturalland is under grass [1]. Thebiogas

generated may be used on site as a source of combined heat and

power. Alternatively it may be upgraded to biomethane and

distributed via the natural gas grid where better efficiencies and

markets may be available. For example grass biomethane offersgreat potential as a transport fuel [1]; it may also be used as a

source of renewable heat to existing housing stock connected to

the gas grid. The industry in Ireland is in its infancy, however

numerous technology providers are selling their wares; many

facilities are at planning stage and numerous farmers are

interested in the industry due to the low farm family income

associated in particular with beef farming. The big issue is what

type of digester should be utilized. This paper reviews the present

state-of-the-art anaerobic digester configurations for high solid

content feedstocks, and their application to grass silage. This paper

has an ambition of explaining and evaluating various anaerobic

digester configurations.

1.2. Anaerobic digestion

The process of anaerobic digestion has now become a more

attractive source of renewable energy due to reduced technological

cost and increased process efficiency [2]. A plethora of substrates

such as wastewaters [3], animal wastes [4] and sewage sludge [5]

are extensively used for anaerobic digestion [6]. Additionally,

during the last few years, the use of lignocellulosic substrates [7]

and feedstocks with a high solids content such as the OFMSW

(organic fraction of municipal solid waste) [8], crops [9], crop

residues [10] and grass silage [11,12] has received considerable

attention particularly in Europe [13]. Various researchers have

reviewed and compared various digester types suitable for

digesting solid wastes [14,15]. Digesters which are optimized

for OFMSW may not be ideal for grass silage because the volatile

solid content of grass silage is of the order of 92% where as the

volatile solid content of OFMSW may be as low as 60% [1]

1.3. Grass: a new way towards renewable energy

In Ireland [16] and generally in temperate regions, grassland is

the most predominant form of land use providing most of the feed

requirements for ruminants [17] either through grazing or after

conservation as hay or, more recently, silage [18]. Despite its

ubiquity in European lands, there is still a considerable risk of its

conversion into surplus land [19] if the land is not used

productively. Nevertheless usage of grassland as a renewable

source of energy through biogas production will contribute

significantly to the protection of the environment, due to the

ability of grass to sequester carbon into the soil matrix [20].

Additionally, many socio-economic benefits [21] can be achieved

without harming the food industry [22]; this is particularlytrue for

Ireland, where land area available to grow grass is 10 times more

than for arable land. Furthermore, to affect a 10% reduction in

emissions from the agriculture sector, the National Climate Change

Strategy for Ireland [23] recommended reductions in the national

herd. This eventually, in combination with the preservation of

grassland, will necessitate grass growth as a new source of

renewable energy in Ireland [1].

1.4. Biomethanation of grass silage

Optimal digestion of grass silage is an area still under active

research. Most of the work on optimising grass silage digestion is

conducted at laboratory and pilot trials. The interest in using grass

silage as a feedstock for bioenergy and biorefinery systems is due to

its high yield potential in terms of methane production per hectare

[1]; howeverits lignin andcellulose content [24] makes it suitable as

a multiple source of energy and products [25]. Grass silage is the

most important substrate after maize silage for biogas plants in

Germany [26] and one of the most usedco-substrates in agricultural

biogas plants between 2002 and 2004 in Germany [27]. Still the use

of biomethane from substrates like grass silage in Europe is modest

compared to other rawmaterials[28]. Thehigh potentialof methane

production from grasssilage hasbeen shown in thestudiesof Amonet al. [29], Mahnert et al. [12] and Lehtomaki et al. [30].

1.5. Design of anaerobic digester

The optimization of digester design i.e. higher OLR (organic

loading rate), reduced HRT (hydraulic retention time) and higher

methane yields is of great importance [31]. Operational parameters

such as HRT, mixing, number of tanks and temperature [14] along

with the properties of the feedstock [32] form the basis of digester

design. Moreover, in digesting lignocellulosic substrates such as

grass silage, the dry matter content, the solubility and hydrolysis

rates, play a critical role [33]. A detailed description of operational

aspects of various digester types was undertaken by Hobson and

Wheatley in 1993 [34]. There is a need to examine the digester

configuration as applied to grass silage in the 21st Century. Various

digester configurations are employed which use different

approaches such as one-stage or two-stage digesters [35], wet or

dry/semi-dry digesters [15], batch or continuous digesters [36],

attached or non-attached biomassdigesters [37], high-ratedigesters

[38] and digesters withcombination of different approaches (Fig.1).

1.6. Properties of grass silage

Grass silage is wet (less than 20% dry solids content) ordry (20–

40% dry solids) depending on whether it is wilted, weather

conditions at time of harvesting and storage conditions (baled or

pit) [1]. In Ireland the dry solids content of grass silage is of the

order of 20% for pit silage and 30% for baled silage. The D-value

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–1568 1559



Fig. 1. Possible combination of various digester types.

Table 1Comparison of different digester configurations for high solid content feedstocks [14,34,41,42].

Criteria One-stage versus two-stage

digesters

Dry versus wet

digesters

Batch versus continuous

digesters

High-rate

bioreactors

One-stage Two-stage Dry Wet Batch Continuous

Biogas production Irregular and

discontinuous

Higher and stable Higher Less and

irregular

Irregular and

discontinuous

Continuous Continuous

and higher

Solid content 10–40% 2–40% 20–50% 2–12% 25–40% 2–15% <4–15%

Cost Less More Less More Less More More

Volatile solids destruction Low to high High 40–70% 40–75% 40–70% 40–75% 75–98%

HRT (days) 10–60 10–15 14–60 25–60 30–60 30–60 0.5–12

OLR (kgVSm3 d1) 0.7–15 10–15 for

second stage

12–15 <5 12–15 0.7–1.4 10–15

Table 2

Comparison of process weaknesses and benefits of various digester types [14,44–46].

System Strengths Weaknesses

One-stage versus

two-stage digesters

One-stage Simpler design Higher retention time

Less technical failure Foam and scum formation

Low cost

Two- stage Efficient s ubs trate degr adation

owing to recirculation of digestate

Complex and expensive to build and

maintain

Constant feeding rate to second stage Solid particles need to be removed

from second stageMore robust process

Less susceptible to failure

Dry versus wet digesters Dry Higher biomass retention Complex handling of feedstock

Controlled feeding Mostly structured substrates are used

Simpler pretreatment Material handling and mixing is difficultLower parasitic energy demands

Wet Good operating history Scum formation

Degree of process control is higher High consumption of water and energy

Short-circuiting

Sensitive to shock loads

Batch versus

continuous digesters

Batch No mixing, stirring or pumping Channeling and clogging

Low input process and mechanical needs Larger volume

Cost-effective Lower biogas yield

Continuous Simplicity in design and operation Rapid acidification

Low capital costs Larger VFA (Volatile Fatty Acid)

production

High-rate bioreactors Higher biomass retention Larger start-up times

Controlled feeding Channeling at low feeding rates

Lower investment cost

No support material is needed

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–15681560



reflects the digestibility and may be defined as the organic matter

digested by the cow divided by the dry matter digested. The ME

(metabolizable energy) value is defined as the energy available for

the cow. In Scotland values for grass silage cut in early June were

found tohavea D-value of67% and anME value of10.7 MJ kg1; for

late cut silage (cut in late June) values of 65% and 10.4 MJ kg1

were recorded [39]. Thus early cut grass silage is more digestible

than late cut silage. In Southern Ireland D-values and ME values

would be considerably higher [39].

2. Potential digester configurations

2.1. One-stage versus two-stage digesters

In one-stage digestion all themicrobiological phases of anaerobic

digestion occur in one tank [37]. In two-stage digestion different

microbial phases can be separated [14]. Conversely two-stage

digestion may allow both stages to be complete microbial processes

with the second stage incorporating storage of digestate and

remedial gas collection [35]. When the microbial phases are

separated the hydrolytic and acidification phases may occur in

the first reactor and acetogenesis and methanogenesis occur in the

second reactor [9]. The concept of two-stage digestion is driven by

optimization of the digestion process [40], resulting in potentially

higher yields of biogas in smaller digesters (Table 1). Parawira et al.

[43] scrutinized pilot and lab-scale two-stage systems for anaerobic

digestion of MSW, agricultural residues and market waste. The one-

stage system is still popular at industrial scale because of the

simplicity in operation, reduced costs and lesser technical problems

(Table 2). Process reviews wereundertaken by Weiland et al. [47] on

one-stage digesters and by Demired and Yenigun [48] on two-stage

digesters. However the scientific literature is relatively sparse on

one-stage digestion of grass silage [49], which is the normal

application for commercial scale. Most of the studies conducted at

laboratory and pilot scale use two-stage digesters (Table 3), which

are not available at commercial scale. In the one-stage process,

either dry batch systems or wet continuous systems are used [15],

whereas in the two-stage process, continuous and wet processes are

preferred (Figs. 2–4).

2.2. Dry versus wet digesters

Vandevivere et al. [14] classify dry and wet systems as follows.

Digesters, in which the feedstock used consists of 20–40% dry

matter, are known as dry anaerobic digesters; those with less than

20% dry matter are classified as wet digesters. Therefore,

pretreatment (i.e. pulping and slurrying) is required for grass

silage in wet digesters. Currently, one-stage dry continuous (Fig. 3)

and dry batch digesters (Fig. 4) are relatively new and innovativedigesters used for MSW, biowaste and grass silage; their use is

Table 3

Performance data of different anaerobic digesters applied for silage/grass digestion.

Studies Digester

characteristics

Operating

temperature

(8C)

Mono/

co-digestion

Retention

time

(days)

Characteristics of

substrate

Biogas yield

(m3 /kg VS

added)

Methane yield

(m3 /kg VS

added)

Prochnow

et al. [11]

Continuous digester,

Laboratory scale

35 Co-digestion 18–36 Extensive grassland cut,

silage and hay

0.5–0.55 Not reported

Continuous digester,

Farm scale

20 Extensive grassland

cut, silage


Baserga and

Egger [50]aBatch digester,

Laboratory scale

35 Co-digestion 25 Intensive grassland cut,

first cut in June, fresh

and ensiled


Extensive grassland cut,

first cut in August, fresh,

silage and hay


Extensive grassland cut,

silage and hay

0.5–0.6 Not repor te d

Baserga [51]a Continuous digester,

Farm scale

35 Co-digestion 20 Extensive grassland cut, silage 0.5–0.55 Not reported

Mah nert et al. [52];

Mahnert [53]aBatch digester,

Laboratory scale

35 Mono-digestion 28 Three grass species, first cut

in mid-May, fresh ad ensiled

0.65–0.86 0.31–0.36

Semi-continuous

digester,

Laboratory scale

35 Mono-digestion 28 Three grass species,

second cut, ensiled

0.56–0.61 0.3–0.32

Amon et al. [54]a Batch digester,

Laboratory scale

37–39 Mono-digestion 59 Intensive forage mixture of

grassland and clover, ensiled,

Mid-May (before anthesis i.e.

when the flower is readyfor pollination)

0.53 0.37

End of May (anthesis) 0.47 0.32

Mid-June (after anthesis) 0.42 0.29

Lemmer and

Oechsner [55]aSemi-continuous

digester,

Laboratory scale

and farm scale

37 Co-digestion 25–60 Grass from intensively used

sites, 4 cuts per year ensiled

0.39 Not reported

Grass from extensively used

sites, 2 cuts per year, ensiled

0.22 Not reported

Grass from landscape

management

0.08 Not reported

Lehtomaki

et al. [30]

Batch leach

bed-USB reactors

37 Mono-digestion 55 Grass silage (two-stage

leach bed process without pH)

0.27–0.39 0.197

Grass silage (two-stage

leach bed process with pH)

0.16 0.1

One-stage leach bed 0.2 0.06

a

These investigations on biogas production from grassland vegetation are tabulated by Prochnow et al. [11]




expected to continue in the coming years (Table 4). One-stage dry

batch systems typically employ a system whereby high solids

content feedstock is entered into a vessel without initial dilution.

Recirculation of water/leachate is employed. Feeding is by front

actor, no mixing takes place, and as such parasitic energy demands

are very low [14]. Vertical CSTR (continuously stirred tank reactor)configuration (Fig. 2) is the most commonly used configuration in

90% of the newly erected wet digesters [27]. Parasitic energy

demandfor wetdigesters is higher than fordry digesters dueto the

requirement to dilute grass silage, pump slurries and mix reactors

for the total retention time (Table 2).

2.3. Batch versus continuous digesters

In batch digesters (Fig. 4), the reactor vessel is loaded once with

raw feedstock for a certain period of time (and inoculated with

digestate from another reactor). It is then sealed and left until

complete degradation has occurred [56]. On the contrary, in

continuous digesters (Fig. 2), the substrate is regularly and

continuously fed either mechanically or by force of the newlyentered substrate [57]. In continuous digesters, plug flow, CSTR,

anaerobic filters and UASB (upflow anaerobic sludge blanket)

systems are used, while in batch digesters, one-stage, sequential

batch and hybrid batch digesters are used. According to Bouallagui

et al. [58], about 90% of the industrial scale plants currently

operating in Europe are different continuous type digesters in

configurations such as a continuous one-stage digester[59] usedfor

anaerobic digestion of OFMSW, solid waste and biowaste. However,

batch digesters maybe more suitablefor grass silage digestion dueto

the dry solid contents (bailed silage has a solids content of about

32%) and fibrous characteristics of grass silage and the reduced

parasitic energy demands (Table 2). This is particularly advanta-

geous whenusing more thanone batch digester withdifferent start-

up times to guarantee a continuous yield of biogas [60].

2.4. High-rate digesters

In these digesters high solid retention time is achieved through

attachment of biomass to high density carriers and formation of

highly settleable granules [61]. Upflow anaerobic filters, UASB,

anaerobic packed-bed and fluidized bed reactors are utilized as

high-ratedigesters both at laband industrial scale.The use of UASB

among high-rate digesters has increased and widened in recent

years [62] by taking feed with solid contents less than 4% or up to

15% [41,42] at retention times of 0.5–12 days [14]. Marchaim [41]

suggests solids content of less than 4% in UASB, while Barnett et al.

[42] allow for solids content of up to 15% in UASB (Table 1).

Moreover, theUASB reactor is suggestedby various authors [63,64]

to offer benefits over other high-rate digesters when applied to

high organic loading rates. For digestion of grass silage, high-ratedigesters are applied in connection with leach beds [30,35], ormay

be used with CSTR in two or multi-stage fashion.

3. Digester configurations suitable for grass silage

3.1. Wet continuous digester

The popularity of the one-stage and two-stage CSTR systems in

wet continuous digesters is due to the simplicity of the system in

design and operation and the low capital costs (Table 2). The

Fig. 2. Design variations in one-stage and two-stage digesters.

Fig. 3. Various types of one-stage dry continuous digesters [14].




digestion of grass silage in the CSTR is facilitated by the use of a

separate preprocessing tank with chopper pump, screw-feeder andflushing system (Fig. 5). In addition, solid contents are reduced by

recirculation and mixing of leachate with fresh matter [60]. In a

laboratory scale one-stage CSTR, the biogas yield of three fresh

grass species as mono-substrate was 0.61 and 0.56 m3 kg1 VS

added at an OLR of 0.7 and 1.4 kg1

VS added at 35 8C [12]. While,at commercial scale a two-stage CSTR in Eugendorf, Austria, the

methane yield from grass silage as mono-substrate is 0.3 m3 kg1

VSat an OLR of 1.4 kg VS m3 d1 [65]. In Eugendorfthe biogaswas

55% methane, thus the biogas yield was 0.55 m3 kg1 VS at

1.4 kg VS m3 d1; almost the same result as Mahnert et al. [12].

3.2. Leach bed system connected with high-rate digesters

In this system one or several solid-bed reactors (leach beds)

[66] are connected with high-rate digesters such as a UASB or an

anaerobic filter [35]. These leach beds are sequentially batch

loaded; leachate is recirculated [67,68] to facilitate a near

continuous system in terms of biogas production. The system

requires a high conversion of volatile solids to COD (chemical

Fig. 4. (a) One-stage dry batch digester [14]. (b) Two-stage dry batch digesters [14]. (c) Sequencing fed leach bed digesters coupled with UASB.

Table 4

Five-year development in different digester types [15].

Period One-stage versus

two-stage digesters

Wet versus dry

digesters

One-stage Two-stage Wet Dry

1991–1995 85% 15% 37% 63%

1996–2000 91% 9% 38% 62%

2001–2005 92% 8% 59% 41%

2006–2010

(estimated)

98% 2% 29% 71%




oxygen demand) in the leach beds followed by a high rate of

conversion of COD to methane in the UASB to effect stable and

relatively high biogas production (Table 5). Liu et al. [69] reported

that green waste can be digested within 12 days yielding steady

and high production of biogas. In a study by Lehtoma ki et al. [30]

using a two-stage process, thetotalmethane yield originating fromthe UASB was 76–98%; the remaining biogas was produced in the

leach beds using grass silage as digester substrate. In another study

by Lehtomaki and Bjornsson [70] at pilot scale using batch leach

bed digesters coupled with an anaerobic filter, methane yields of

0.39 m3 CH4 kg1 VS added were obtained at 59% VS removal after

50 days digestion of grass silage. With the same digester scheme

using grass silage at pilot scale, Yu et al. [35] and Cirne et al. [71]

obtained 0.165 m3 CH4 kg1 VS added and 0.27 m3 CH4 kg1 VS

added at 67% VS and 60% VS removal respectively. Lehtomaki et al.

[30] explained the lower CH4 production at higher volatile solids

destruction in the following manner. The grass mixtures used as

substrates in these studies haddifferent compositions of lignin and

nitrogen. Higher contents of lignin mean lesser contents of volatile

solids, which results in less overall degradation and less methane.Lower contents of nitrogen in grass silage result in a less than

optimal C:N ratio, which effects microbial degradation, which

ultimately affects the methane concentration of biogas.

3.3. Dry continuous digester

DRANCO [72,73], Kompogas and Volarga systems are dry

continuous systems [14]. In DRANCO, the digestate/leachate is

recirculated back vertically, while the Kompogas system works

horizontal (Fig. 3). Both systems may operate in the thermophilic

temperature range. Slow moving impellers are used in Kompogas

to homogenize and re-suspend denser particles [74]. The Valorga

system operates in the mesophilic temperature range; this system

employs recirculation of biogas at the bottom of the reactors

through injection ports to effect mixing [75]. One technical

drawback with the Valorga digester is clogging of the gas injection

ports; maintenance of these systems is difficult [14]. A highlevelof OLR is achievable in the DRANCO process (Table 5). The DRANCO

plant at Nustedt, Germany treats 12,500 t a1 of agricultural crops.

The feedstock comprises maize (6200 t a1), sunflowers

(2400 t a1), rye (2000 t a1) and grass (600 t a1). The grass adds

biogas at a rate of 90–120 Nm3 t1. The total biogas production is

145 Nm3 t1 [76].

3.4. Batch digester

Dry batch digesters, such as the BEKON processes (Fig. 4a), are

used widely in Europe for dry solids content up to 50% [68]. In this

digester type, the leachate is recirculated/sprayed back on to the

feedstock. After completion of digestion, the digester is reopened,

half unloaded and half of the feedstock is left as inoculum; it isrefilled with fresh feedstock and the cycle continues [56]. In

addition to BEKON,garage type, bagtype,immersion liquidstorage

vattype and wet-dry combination digesters arein the first phase of

commercialization [77].

In a lab-scale batch digester, the biogas and methane yield of

fresh and ensiled grass species were examined and reported by

Mahnert et al. [12]. Theobserved biogas andmethane yield were in

the range of 0.65–0.86 and 0.31–0.36 m3 kg1 VS respectively.

With the same digester scheme at laboratory scale, Baserga and

Egger [50] and KTBL [78] reported the values of fresh cut grass in

therangeof 0.5–0.6 m3 biogas kg1 VS added.The valuesfor biogas

Fig. 5. Preprocessing systems for high solid substrates [62].

Table 5

Comparison of the optimal anaerobic digesters for grass silage [14,41,42,65,68].

System Example Pretreatment Process Quality of

digestate

HRT

(days)

Solid

contents

(%)

Operating

temperature

(8C)

Cost Destruction

of volatile

solids (%)

OLR

(kgVSm3d1)

Wet continuous

one/two-stage

digester

CSTR Pulping, chopping,

slurry, hydrolyzed

Two-stage

(can be

one-stage)

Juice rich in protein

and nutrients, soil

conditioner

>60 2–14 35–40 Medium 40–70 0.7–1.4

Two-stage sequential

batch digester

connected with

high-rate bioreactor

Leach bed

with UASB

Chopping, pulping Two or

multi-stage

Soil conditioner,

fertilizer, fibrous

materials

12 20–40 35 High 75–98 from

UASB

10–15

One-stage dry

continuous digester

DRANCO Shredding,

Chopping

One-stage Dewatered, good,

fibrous materials

15–30 20–50 50–58 Low 40–70 12

One or multi-stage

dry batch digester

BEKON Chopping One-stage Dewatered, good,

fibrous materials

40–70 30–40 35 Low 40–70 12–15




production from grass silage in m3 kg1 VS added range from 0.54

by Linke et al. [79], 0.58 by Niebaum and Dohler [80], 0.63 by KTBL

[78] to 0.81 by Jakel [81]. Among different grass species both fresh

and ensiled, Mahnert et al. [12] found the highest biogas yields

(0.83 and 0.86 m3 kg1 VS added) were achieved for perennial

ryegrass and the lowest (0.72 and 0.65 m3 kg1 VS added) for fresh

cocksfoot and silage.

4. Arguments for and against different digester configurations

for grass silage digestion

4.1. Biogas production per unit of grass silage based on volatile

solids destruction

Various grass silage samples from a farm in Cork, Ireland were

analysed by the authors. The dry solids content of grass silage

varied from 20 to 40%. The volatile dry solid contents for various

tests averaged 92% when expressed as a percentage of dry solids.

An ultimate analysis yielded the following stoichiometric equation

for dry grass: C28.4H44.5O17.7N. The ratio of carbon to nitrogen (C:N)

in grass silage was found to be 25:1 which is very suitable for

digestion [39].

The biogas production per unit of grass silage is an essential

descriptor of a system. Box 1 highlights the biogas productionassociated with 60% destruction of volatile solids as follows:

509 L CH4 kg1 VS destroyed or 305 L CH4 kg1 VS added;

945 L biogas kg1 VS destroyed or 567 L biogas kg1 VS added;

It is shown that the CH4 content is of the order of 54% by

volume. Improvements upon these figures necessitate a destruc-

tion of volatile solids in excess of 60%. These figures are in line with

the preceding literature; including such data as Mahnert et al. [12]

who generated levels of 560 L biogas kg1 VS added at

1.4 kg VS m3 d1.

4.2. The wet continuous two-stage process

Two tanks are employed in series with recirculation of leachate

to dilute the grass feedstock [65]. Maceration is required to reduce

the potential forthe grass silage to get caught in moving parts [39].

This type of system is in place in Eugendorf, Austria. The retention

time for grass silage is relatively high (60 days) and the OLR is

correspondingly low (ca. 1.4 kg VS m3 d1). Gas production of

300 L CH4 kg1 VS added was recorded in operating plants at a

volatile solids destruction of 60% [65]. Ofconcern isthe tendencyof

grass silage to float on the liquid surface of the digester; this may

be overcome through good mixing design such as paddle system

that breaks the liquid surface [39].

4.3. The dry batch process

Thedry batch has a significant advantage: simplicity [68]. There

are few moving parts: little pretreatment is required as the grass

silage does not come into contact with moving parts; the feedstock

is not diluted; as a result energy input is low ( Table 2). The time

between loading and unloading is greater than 30 days, but as half

the substrate is left in place as an inoculum, the actual retention

times is of the order of 45 days. Gas production starts from zero,

increases, peaks and decreases; thus a series of batch digesters arerequired which are fed sequentially to generate a gas curve with a

relatively constant output [56]. A disadvantage ofthe process is the

lack of facilities actually operating on grass silage. We simply do

not know if grass silage is suitable for vertical garage door batch

digester systems. These systems were designed originally for

treatment of OFMSW (in lieu of composting). OFMSW has a lower

volatile solids content [1] and thus a higherquantityof solids in the

digestate. There is a fear that grass digestate, which has a lower

solids content than OFMSW digestate, will flow out the vertical

door of the digestor on emptying. It is perceived that the batch

process will not effect the volatile solids reduction of a continually

mixed wet process and will not therefore generate the same level

of gas production.

4.4. The dry continuous process

Again the dry continuous process was designed originally for

biowaste andOFMSW. The low solids content of the digestate from

grass silage may be problematic, especially for pumping. There is

little recorded evidence of grass silage digestion in these systems.

Disadvantages will potentially include: requirement for size

pretreatment; requirement to pump the digestate up the digester

a number of times; significant energy input (ca. 80 kWeh t1

feedstock is documented for OFMSW) [73]. This is extremely high

when compared to the dry batch process that simply involves

using a front actor to push substrate into a chamber.

4.5. The leach bed system combined with UASB

This systemas outlined in Fig. 4c was shown by Lehtomaki et al.

[30] to pull the gas production curve to the left as compared to the

dry batch process; in effect this significantly shortens the required

retention time. This system is different to the others in that the

final reactor (the UASB) receives a liquid waste high in COD. The

benefit of the UASB is that it can be loaded to 20 kg COD m3 d1

while effecting a 90% destruction of COD [82]. Nizami et al. [82]

showed that 1 kg of VS destroyed generates 1.4 kg of COD and 1 kg

of COD destroyed produces 350 L CH4. Thus if the UASB effects a

90% destruction in volatile solids then each kg of VS destroyed can

generate 441 L of CH4. Therefore an increase in volatile solids

destruction is required to obtain the same gas production. In

section 4.1 it is shown that 509 L CH4 is generated per kg volatile

Box1. Biogas production per unit of grass from first principles

Stoichiometry :C28.4H44.5O17.7N + 8.425H2O ! 15.335

CH4 + 13.065CO2

668.5 + 151.6 ! 245 + 575820! 820300 kg solids + 68 kg water ! 110 kg CH4 + 258 kg CO2

(30% dry solids)276 kg VS + 62.5 kg water ! 101 kg CH4 + 237 kg CO2

(92% volatiles)165 kg VS dest + 37.5 kg water ! 60.6 kg CH4 + 142 kgCO2 (60% destruction)Density of CH4 = 16/22.412 m3 kg1 = 0.714 kg m3,

Density of CO2 = 44/22.412 m3 kg1 = 1.96 kg m3

Thus the proportion of gas by volume ! 84 m3 CH4 + 72 m3

CO2 = 156 m3 biogasThus biogas contains approximately! 53.8% CH4 + 46.2%CO2 by volumeEnergy balance :1 m3 CH4 37.78 MJ1 m3 biogas @ 53.8% CH4 = 20.3 MJ m3

1 t VS = 18.77 GJ [77]84 m3 CH4 = 3.17 GJ; 165 kg VS dest = 3.10 GJBiogas production per unit :84 m3 CH4 /165 kg VS dest= 509 L CH4 /kg VS dest = 305 L CH4 kg1 VS added156 m3 biogas/165 kg VS dest= 945 L biogas kg1 VS dest = 567 L biogas kg1 VS added.




solids destroyed in a CSTR system; to effect the same methane

production 69% destruction of volatiles needs to occur in the leach

bed. The leach bed may be optimized through thermal or

enzymatic treatments [82]. Lehtomaki and Bjornsson [70] used

batch leach beddigesters coupled with an anaerobic filter to obtain

methane yields of 390 L CH4 kg1 VS added after 50 days digestion

of grass silage. This may be compared with 305 L CH4 kg1 VS

added for 60% destruction of volatiles in Section 4.1.

5. Discussion

5.1. Potential improvements in anaerobic digesters

So far limited and inconclusive work has been undertaken to

improve the design of reactors for enhanced biogas production

from grass silage. The ubiquitous wet continuous process (CSTR)

has been shown to be an effective process for grass silage digestion

at loading rates of approximately 1.4 kg VS m3 d1 [60]. The UASB

reactor offers great potential as it can withstand loading as high as

20 kg COD m3 d1; thus by converting the feedstock from volatile

solids to soluble COD, loading rates may be increased, retention

times may be shortened, and optimistically biogas production may

be increased [82]. The leach bed may be optimized for hydrolysis

through thermal and enzymatic pretreatment of the grass silagewhile the UASB may be optimized for COD removal.

5.2. Research required

Data from theliterature on thebest digester configurationusing

grass silage as a feedstock is inconclusive. There is a tendency to

utilize data on digester configurations utilizing different feed-

stocks (OFMSW, green waste) and to apply the outcome to grass

silage. The feedstock is an important criterion. High solid content

substrates may require different reactor configurations; food

waste and the OFMSW though both having similar dry solids

contents to grass, are very different to grass silage. Grass silage has

a far higher volatile solids content than OFMSW (ca. 92% versus

60%) [1] and thus a more complete dry matter removal takes placein the reactor. The digestate from grass silage thus has a lower

solids content than the digestate from OFMSW; it is more liquid in

nature. This has led to suggestions that dry batch reactors fed

through a vertical door aremore suitable forOFMSWthan for grass

silage as the grass silage digestate may require additional handling

dueto thevertical containment of a digestatelow in solids content.

There is another argument that bailed silage (32% dry solids

content) is more suited to a dry batch digestion process while

clamp or pit silage (22% dry solids content) is more suited to a wet

continuous process. These arguments need to be interrogated.

Even digester configurations assessed using grass silage is

problematic. Of significant concern is the comparison of data from

different countries, different institutes, digesting different species

of grass, at different dry solids content, cut at different times of year and times of day [82]. Nizami et al. [82] outlined the

significant difference in digestibility of grass; for example the

water soluble carbohydrates are higher in the afternoon than the

morning resulting in higher biogas production for grass cut in the

afternoon. It is suggested that a number of reactor configurations

should be compared in real time treating similar quantities of grass

silage under similar loading rates to evaluate the optimal

configuration. Indeed grass cut at different times of the year and

from different locations can have different D-values, lignin

contents and N (nitrogen) values [82]. Thus as shown by Lehotmaki

et al. [30] in Section 3.2, incorrect conclusions may be drawn from

direct comparisons of reactors treating grass silage in different

plants (or labs) from different countries, cut at different times of

the year and hence with different compositions.

6. Conclusion

Grass digestion offers opportunities to farmers. Instead of

managing herds of beef cattle, an industry which offers a low Farm

Family Income, the farmer can continue to draw down single farm

payments while cutting silage two or three times a year as a

feedstock for an anaerobic digester. Biogas/biomethane is now the

end product rather than beef; the work load is considerably less;

greenhouse gas production is eased significantly [1]. This is in line

with the objective of the National Climate Change Strategy for

Ireland to reduce the herd and to effect an overall reduction in

greenhouse gas emissions from the agriculture sector by 10% [23].

Many digester systems are available for the farmer to choose.

Technology providers are selling digesters that were not initially

designed for grass silage. The authors believe that much work

needs to be undertaken to ascertain optimal digester configura-

tions forproductionof grass biomethane. The CSTR systemappears

to be a safe technology if the mixing system is adapted to deal with

the tendency forgrass silage to float. However there is a significant

potential to assess the benefits of leach beds followed by a high-

rate digester (such as a UASB). The leach beds will permit

pretreatment technologies (thermal and enzymatic) to optimize

hydrolysis while the UASB may be optimized for methanogensis.

Optimal configurations can only be established by operating thedifferent configurations in parallel, in real time, digesting the same

grass silage feedstock.

Acknowledgements

Anoop Singh, Nicholas Korres, Beatrice Smyth and Thanasit

Thamsiriroj for advice, brainstorming sessions, conversations and

critiques.

Funding sources:

Department of Agriculture and Food (DAFF) Research Stimulus:

‘‘GreenGrass.’’

Environmental Protection Agency (EPA) Strive Programme:

‘‘Compressed biomethane generated from grass used as atransport fuel.’’

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