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

What type of digester configurations should be employed to produce biomethanefrom 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1563

3.3. 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

A R T I C L E I N F O

Article history:

Received 8 December 2008

Accepted 8 February 2010

Keywords:

Grass silage

Biogas

Biomethane

Anaerobic digester

A B S T R A C T

Grass is an excellent energy crop; it may be classified as a high yielding, low energy input, 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 energy crop’’ such as Miscanthus. Of issue therefore is not 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.

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

journa l homepage: www.e lsev ier .com/ locate / rser

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: jerry.murphy@ucc.ie (J.D. Murphy).

1364-0321/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.rser.2010.02.006

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

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 significantlyacross Europe. There is tremendous potential for these on-farmdigesters using grass silage as a bioenergy feedstock, especially inIreland where 91% of agricultural land is under grass [1]. The biogasgenerated may be used on site as a source of combined heat andpower. Alternatively it may be upgraded to biomethane anddistributed via the natural gas grid where better efficiencies andmarkets may be available. For example grass biomethane offersgreat potential as a transport fuel [1]; it may also be used as asource of renewable heat to existing housing stock connected tothe gas grid. The industry in Ireland is in its infancy, howevernumerous technology providers are selling their wares; manyfacilities are at planning stage and numerous farmers areinterested in the industry due to the low farm family incomeassociated in particular with beef farming. The big issue is whattype of digester should be utilized. This paper reviews the presentstate-of-the-art anaerobic digester configurations for high solidcontent feedstocks, and their application to grass silage. This paperhas an ambition of explaining and evaluating various anaerobicdigester configurations.

1.2. Anaerobic digestion

The process of anaerobic digestion has now become a moreattractive source of renewable energy due to reduced technologicalcost and increased process efficiency [2]. A plethora of substratessuch 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], cropresidues [10] and grass silage [11,12] has received considerableattention particularly in Europe [13]. Various researchers havereviewed and compared various digester types suitable fordigesting solid wastes [14,15]. Digesters which are optimizedfor OFMSW may not be ideal for grass silage because the volatilesolid content of grass silage is of the order of 92% where as thevolatile 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 isthe most predominant form of land use providing most of the feedrequirements for ruminants [17] either through grazing or afterconservation as hay or, more recently, silage [18]. Despite itsubiquity in European lands, there is still a considerable risk of itsconversion into surplus land [19] if the land is not usedproductively. Nevertheless usage of grassland as a renewablesource of energy through biogas production will contributesignificantly 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 achievedwithout harming the food industry [22]; this is particularly true forIreland, where land area available to grow grass is 10 times morethan for arable land. Furthermore, to affect a 10% reduction inemissions from the agriculture sector, the National Climate ChangeStrategy for Ireland [23] recommended reductions in the nationalherd. This eventually, in combination with the preservation ofgrassland, will necessitate grass growth as a new source ofrenewable energy in Ireland [1].

1.4. Biomethanation of grass silage

Optimal digestion of grass silage is an area still under activeresearch. Most of the work on optimising grass silage digestion isconducted at laboratory and pilot trials. The interest in using grasssilage as a feedstock for bioenergy and biorefinery systems is due toits high yield potential in terms of methane production per hectare[1]; however its lignin and cellulose content [24] makes it suitable asa multiple source of energy and products [25]. Grass silage is themost important substrate after maize silage for biogas plants inGermany [26] and one of the most used co-substrates in agriculturalbiogas plants between 2002 and 2004 in Germany [27]. Still the useof biomethane from substrates like grass silage in Europe is modestcompared to other raw materials [28]. The high potential of methaneproduction from grass silage has been shown in the studies of 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 (organicloading rate), reduced HRT (hydraulic retention time) and highermethane yields is of great importance [31]. Operational parameterssuch as HRT, mixing, number of tanks and temperature [14] alongwith the properties of the feedstock [32] form the basis of digesterdesign. Moreover, in digesting lignocellulosic substrates such asgrass silage, the dry matter content, the solubility and hydrolysisrates, play a critical role [33]. A detailed description of operationalaspects of various digester types was undertaken by Hobson andWheatley in 1993 [34]. There is a need to examine the digesterconfiguration as applied to grass silage in the 21st Century. Variousdigester configurations are employed which use differentapproaches such as one-stage or two-stage digesters [35], wet ordry/semi-dry digesters [15], batch or continuous digesters [36],attached or non-attached biomass digesters [37], high-rate digesters[38] and digesters with combination of different approaches (Fig. 1).

1.6. Properties of grass silage

Grass silage is wet (less than 20% dry solids content) or dry (20–40% dry solids) depending on whether it is wilted, weatherconditions at time of harvesting and storage conditions (baled orpit) [1]. In Ireland the dry solids content of grass silage is of theorder of 20% for pit silage and 30% for baled silage. The D-value

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 (kg VS m�3 d�1) 0.7–15 10–15 for

second stage

12–15 <5 12–15 0.7–1.4 10–15

Table 2Comparison 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 substrate degradation

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 difficult

Lower 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

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–1568 1561

reflects the digestibility and may be defined as the organic matterdigested by the cow divided by the dry matter digested. The ME(metabolizable energy) value is defined as the energy available forthe cow. In Scotland values for grass silage cut in early June werefound to have a D-value of 67% and an ME value of 10.7 MJ kg�1; forlate cut silage (cut in late June) values of 65% and 10.4 MJ kg�1

were recorded [39]. Thus early cut grass silage is more digestiblethan late cut silage. In Southern Ireland D-values and ME valueswould be considerably higher [39].

2. Potential digester configurations

2.1. One-stage versus two-stage digesters

In one-stage digestion all the microbiological phases of anaerobicdigestion occur in one tank [37]. In two-stage digestion differentmicrobial phases can be separated [14]. Conversely two-stagedigestion may allow both stages to be complete microbial processeswith the second stage incorporating storage of digestate andremedial gas collection [35]. When the microbial phases areseparated the hydrolytic and acidification phases may occur inthe first reactor and acetogenesis and methanogenesis occur in thesecond reactor [9]. The concept of two-stage digestion is driven byoptimization of the digestion process [40], resulting in potentiallyhigher yields of biogas in smaller digesters (Table 1). Parawira et al.

Table 3Performance data of different anaerobic digesters applied for silage/grass digestion.

Studies Digester

characteristics

Operating

temperature

(8C)

Mono/

co-digestion

Prochnow

et al. [11]

Continuous digester,

Laboratory scale

35 Co-digestion

Continuous digester,

Farm scale

Baserga and

Egger [50]a

Batch digester,

Laboratory scale

35 Co-digestion

Baserga [51]a Continuous digester,

Farm scale

35 Co-digestion

Mahnert et al. [52];

Mahnert [53]a

Batch digester,

Laboratory scale

35 Mono-digestion

Semi-continuous

digester,

Laboratory scale

35 Mono-digestion

Amon et al. [54]a Batch digester,

Laboratory scale

37–39 Mono-digestion

Lemmer and

Oechsner [55]a

Semi-continuous

digester,

Laboratory scale

and farm scale

37 Co-digestion

Lehtomaki

et al. [30]

Batch leach

bed-USB reactors

37 Mono-digestion

a These investigations on biogas production from grassland vegetation are tabulated

[43] scrutinized pilot and lab-scale two-stage systems for anaerobicdigestion of MSW, agricultural residues and market waste. The one-stage system is still popular at industrial scale because of thesimplicity in operation, reduced costs and lesser technical problems(Table 2). Process reviews were undertaken by Weiland et al. [47] onone-stage digesters and by Demired and Yenigun [48] on two-stagedigesters. However the scientific literature is relatively sparse onone-stage digestion of grass silage [49], which is the normalapplication for commercial scale. Most of the studies conducted atlaboratory and pilot scale use two-stage digesters (Table 3), whichare 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 arepreferred (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% drymatter, are known as dry anaerobic digesters; those with less than20% dry matter are classified as wet digesters. Therefore,pretreatment (i.e. pulping and slurrying) is required for grasssilage 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

Retention

time

(days)

Characteristics of

substrate

Biogas yield

(m3 /kg VS

added)

Methane yield

(m3 /kg VS

added)

18–36 Extensive grassland cut,

silage and hay

0.5–0.55 Not reported

20 Extensive grassland

cut, silage

0.5–0.55 Not reported

25 Intensive grassland cut,

first cut in June, fresh

and ensiled

0.7–0.72 Not reported

Extensive grassland cut,

first cut in August, fresh,

silage and hay

0.54–0.58 Not reported

Extensive grassland cut,

silage and hay

0.5–0.6 Not reported

20 Extensive grassland cut, silage 0.5–0.55 Not reported

28 Three grass species, first cut

in mid-May, fresh ad ensiled

0.65–0.86 0.31–0.36

28 Three grass species,

second cut, ensiled

0.56–0.61 0.3–0.32

59 Intensive forage mixture of

grassland and clover, ensiled,

Mid-May (before anthesis i.e.

when the flower is ready

for pollination)

0.53 0.37

End of May (anthesis) 0.47 0.32

Mid-June (after anthesis) 0.42 0.29

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

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

by Prochnow et al. [11]

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

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–15681562

expected to continue in the coming years (Table 4). One-stage drybatch systems typically employ a system whereby high solidscontent feedstock is entered into a vessel without initial dilution.Recirculation of water/leachate is employed. Feeding is by frontactor, no mixing takes place, and as such parasitic energy demandsare very low [14]. Vertical CSTR (continuously stirred tank reactor)configuration (Fig. 2) is the most commonly used configuration in90% of the newly erected wet digesters [27]. Parasitic energydemand for wet digesters is higher than for dry digesters due to therequirement to dilute grass silage, pump slurries and mix reactorsfor the total retention time (Table 2).

2.3. Batch versus continuous digesters

In batch digesters (Fig. 4), the reactor vessel is loaded once withraw feedstock for a certain period of time (and inoculated withdigestate from another reactor). It is then sealed and left untilcomplete degradation has occurred [56]. On the contrary, incontinuous digesters (Fig. 2), the substrate is regularly andcontinuously 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, sequentialbatch and hybrid batch digesters are used. According to Bouallaguiet al. [58], about 90% of the industrial scale plants currentlyoperating in Europe are different continuous type digesters inconfigurations such as a continuous one-stage digester [59] used foranaerobic digestion of OFMSW, solid waste and biowaste. However,batch digesters maybe more suitable for grass silage digestion due tothe dry solid contents (bailed silage has a solids content of about

Fig. 3. Various types of one-stage

32%) and fibrous characteristics of grass silage and the reducedparasitic energy demands (Table 2). This is particularly advanta-geous when using more than one batch digester with different 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 throughattachment of biomass to high density carriers and formation ofhighly settleable granules [61]. Upflow anaerobic filters, UASB,anaerobic packed-bed and fluidized bed reactors are utilized ashigh-rate digesters both at lab and industrial scale. The use of UASBamong high-rate digesters has increased and widened in recentyears [62] by taking feed with solid contents less than 4% or up to15% [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, the UASB reactor is suggested by various authors [63,64]to offer benefits over other high-rate digesters when applied tohigh organic loading rates. For digestion of grass silage, high-ratedigesters are applied in connection with leach beds [30,35], or maybe 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 inwet continuous digesters is due to the simplicity of the system indesign and operation and the low capital costs (Table 2). The

dry continuous digesters [14].

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.

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–1568 1563

digestion of grass silage in the CSTR is facilitated by the use of aseparate preprocessing tank with chopper pump, screw-feeder andflushing system (Fig. 5). In addition, solid contents are reduced byrecirculation and mixing of leachate with fresh matter [60]. In alaboratory scale one-stage CSTR, the biogas yield of three fresh

Table 4Five-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%

grass species as mono-substrate was 0.61 and 0.56 m3 kg�1 VSadded at an OLR of 0.7 and 1.4 kg�1 VS added at 35 8C [12]. While,at commercial scale a two-stage CSTR in Eugendorf, Austria, themethane yield from grass silage as mono-substrate is 0.3 m3 kg�1

VS at an OLR of 1.4 kg VS m�3 d�1 [65]. In Eugendorf the biogas was55% methane, thus the biogas yield was 0.55 m3 kg�1 VS at1.4 kg VS m�3 d�1; 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 ananaerobic filter [35]. These leach beds are sequentially batchloaded; leachate is recirculated [67,68] to facilitate a nearcontinuous system in terms of biogas production. The systemrequires a high conversion of volatile solids to COD (chemical

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

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–15681564

oxygen demand) in the leach beds followed by a high rate ofconversion of COD to methane in the UASB to effect stable andrelatively high biogas production (Table 5). Liu et al. [69] reportedthat green waste can be digested within 12 days yielding steadyand high production of biogas. In a study by Lehtomaki et al. [30]using a two-stage process, the total methane yield originating fromthe UASB was 76–98%; the remaining biogas was produced in theleach beds using grass silage as digester substrate. In another studyby Lehtomaki and Bjornsson [70] at pilot scale using batch leachbed digesters coupled with an anaerobic filter, methane yields of0.39 m3 CH4 kg�1 VS added were obtained at 59% VS removal after50 days digestion of grass silage. With the same digester schemeusing grass silage at pilot scale, Yu et al. [35] and Cirne et al. [71]obtained 0.165 m3 CH4 kg�1 VS added and 0.27 m3 CH4 kg�1 VSadded at 67% VS and 60% VS removal respectively. Lehtomaki et al.[30] explained the lower CH4 production at higher volatile solidsdestruction in the following manner. The grass mixtures used assubstrates in these studies had different compositions of lignin andnitrogen. Higher contents of lignin mean lesser contents of volatilesolids, which results in less overall degradation and less methane.Lower contents of nitrogen in grass silage result in a less thanoptimal C:N ratio, which effects microbial degradation, whichultimately affects the methane concentration of biogas.

3.3. Dry continuous digester

DRANCO [72,73], Kompogas and Volarga systems are drycontinuous systems [14]. In DRANCO, the digestate/leachate isrecirculated back vertically, while the Kompogas system workshorizontal (Fig. 3). Both systems may operate in the thermophilictemperature range. Slow moving impellers are used in Kompogas

Table 5Comparison of the optimal anaerobic digesters for grass silage [14,41,42,65,68].

System Example Pretreatment Process Quality of

digestate

Wet continuous

one/two-stage

digester

CSTR Pulping, chopping,

slurry, hydrolyzed

Two-stage

(can be

one-stage)

Juice rich in p

and nutrients,

conditioner

Two-stage sequential

batch digester

connected with

high-rate bioreactor

Leach bed

with UASB

Chopping, pulping Two or

multi-stage

Soil conditione

fertilizer, fibro

materials

One-stage dry

continuous digester

DRANCO Shredding,

Chopping

One-stage Dewatered, go

fibrous materi

One or multi-stage

dry batch digester

BEKON Chopping One-stage Dewatered, go

fibrous materi

to homogenize and re-suspend denser particles [74]. The Valorgasystem operates in the mesophilic temperature range; this systememploys recirculation of biogas at the bottom of the reactorsthrough injection ports to effect mixing [75]. One technicaldrawback with the Valorga digester is clogging of the gas injectionports; maintenance of these systems is difficult [14]. A high level ofOLR is achievable in the DRANCO process (Table 5). The DRANCOplant at Nustedt, Germany treats 12,500 t a�1 of agricultural crops.The feedstock comprises maize (6200 t a�1), sunflowers(2400 t a�1), rye (2000 t a�1) and grass (600 t a�1). The grass addsbiogas at a rate of 90–120 Nm3 t�1. The total biogas production is145 Nm3 t�1 [76].

3.4. Batch digester

Dry batch digesters, such as the BEKON processes (Fig. 4a), areused widely in Europe for dry solids content up to 50% [68]. In thisdigester type, the leachate is recirculated/sprayed back on to thefeedstock. 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]. Inaddition to BEKON, garage type, bag type, immersion liquid storagevat type and wet-dry combination digesters are in the first phase ofcommercialization [77].

In a lab-scale batch digester, the biogas and methane yield offresh and ensiled grass species were examined and reported byMahnert et al. [12]. The observed biogas and methane yield were inthe range of 0.65–0.86 and 0.31–0.36 m3 kg�1 VS respectively.With the same digester scheme at laboratory scale, Baserga andEgger [50] and KTBL [78] reported the values of fresh cut grass inthe range of 0.5–0.6 m3 biogas kg�1 VS added. The values for biogas

HRT

(days)

Solid

contents

(%)

Operating

temperature

(8C)

Cost Destruction

of volatile

solids (%)

OLR

(kg VS m�3 d�1)

rotein

soil

>60 2–14 35–40 Medium 40–70 0.7–1.4

r,

us

12 20–40 35 High 75–98 from

UASB

10–15

od,

als

15–30 20–50 50–58 Low 40–70 12

od,

als

40–70 30–40 35 Low 40–70 12–15

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–1568 1565

production from grass silage in m3 kg�1 VS added range from 0.54by 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 freshand ensiled, Mahnert et al. [12] found the highest biogas yields(0.83 and 0.86 m3 kg�1 VS added) were achieved for perennialryegrass and the lowest (0.72 and 0.65 m3 kg�1 VS added) for freshcocksfoot and silage.

4. Arguments for and against different digester configurationsfor 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 wereanalysed by the authors. The dry solids content of grass silagevaried from 20 to 40%. The volatile dry solid contents for varioustests averaged 92% when expressed as a percentage of dry solids.An ultimate analysis yielded the following stoichiometric equationfor 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 fordigestion [39].

The biogas production per unit of grass silage is an essentialdescriptor of a system. Box 1 highlights the biogas productionassociated with 60% destruction of volatile solids as follows:

� 509 L CH4 kg�1 VS destroyed or 305 L CH4 kg�1 VS added;� 945 L biogas kg�1 VS destroyed or 567 L biogas kg�1 VS added;

It is shown that the CH4 content is of the order of 54% byvolume. Improvements upon these figures necessitate a destruc-tion of volatile solids in excess of 60%. These figures are in line withthe preceding literature; including such data as Mahnert et al. [12]who generated levels of 560 L biogas kg�1 VS added at1.4 kg VS m�3 d�1.

Box 1. 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 + 575

820! 820

300 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 kg

CO2 (60% destruction)

Density of CH4 = 16/22.412 m3 kg�1 = 0.714 kg m�3,

Density of CO2 = 44/22.412 m3 kg�1 = 1.96 kg m�3

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

CO2 = 156 m3 biogas

Thus biogas contains approximately! 53.8% CH4 + 46.2%

CO2 by volume

Energy balance:

1 m3 CH4 � 37.78 MJ

1 m3 biogas @ 53.8% CH4 = 20.3 MJ m�3

1 t VS = 18.77 GJ [77]

84 m3 CH4 = 3.17 GJ; 165 kg VS dest = 3.10 GJ

Biogas production per unit:

84 m3 CH4/165 kg VS dest

= 509 L CH4/kg VS dest = 305 L CH4 kg�1 VS added

156 m3 biogas/165 kg VS dest

= 945 L biogas kg�1 VS dest = 567 L biogas kg�1 VS added.

4.2. The wet continuous two-stage process

Two tanks are employed in series with recirculation of leachateto dilute the grass feedstock [65]. Maceration is required to reducethe potential for the grass silage to get caught in moving parts [39].This type of system is in place in Eugendorf, Austria. The retentiontime for grass silage is relatively high (60 days) and the OLR iscorrespondingly low (ca. 1.4 kg VS m�3 d�1). Gas production of300 L CH4 kg�1 VS added was recorded in operating plants at avolatile solids destruction of 60% [65]. Of concern is the tendency ofgrass silage to float on the liquid surface of the digester; this maybe overcome through good mixing design such as paddle systemthat breaks the liquid surface [39].

4.3. The dry batch process

The dry batch has a significant advantage: simplicity [68]. Thereare few moving parts: little pretreatment is required as the grasssilage does not come into contact with moving parts; the feedstockis not diluted; as a result energy input is low (Table 2). The timebetween loading and unloading is greater than 30 days, but as halfthe substrate is left in place as an inoculum, the actual retentiontimes 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 arelatively constant output [56]. A disadvantage of the process is thelack of facilities actually operating on grass silage. We simply donot know if grass silage is suitable for vertical garage door batchdigester systems. These systems were designed originally fortreatment of OFMSW (in lieu of composting). OFMSW has a lowervolatile solids content [1] and thus a higher quantity of solids in thedigestate. There is a fear that grass digestate, which has a lowersolids content than OFMSW digestate, will flow out the verticaldoor of the digestor on emptying. It is perceived that the batchprocess will not effect the volatile solids reduction of a continuallymixed wet process and will not therefore generate the same levelof gas production.

4.4. The dry continuous process

Again the dry continuous process was designed originally forbiowaste and OFMSW. The low solids content of the digestate fromgrass silage may be problematic, especially for pumping. There islittle recorded evidence of grass silage digestion in these systems.Disadvantages will potentially include: requirement for sizepretreatment; requirement to pump the digestate up the digestera number of times; significant energy input (ca. 80 kWeh t�1

feedstock is documented for OFMSW) [73]. This is extremely highwhen compared to the dry batch process that simply involvesusing a front actor to push substrate into a chamber.

4.5. The leach bed system combined with UASB

This system as outlined in Fig. 4c was shown by Lehtomaki et al.[30] to pull the gas production curve to the left as compared to thedry batch process; in effect this significantly shortens the requiredretention time. This system is different to the others in that thefinal reactor (the UASB) receives a liquid waste high in COD. Thebenefit of the UASB is that it can be loaded to 20 kg COD m�3 d�1

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 kgof COD destroyed produces 350 L CH4. Thus if the UASB effects a90% destruction in volatile solids then each kg of VS destroyed cangenerate 441 L of CH4. Therefore an increase in volatile solidsdestruction is required to obtain the same gas production. Insection 4.1 it is shown that 509 L CH4 is generated per kg volatile

A.-S. Nizami, J.D. Murphy / Renewable and Sustainable Energy Reviews 14 (2010) 1558–15681566

solids destroyed in a CSTR system; to effect the same methaneproduction 69% destruction of volatiles needs to occur in the leachbed. The leach bed may be optimized through thermal orenzymatic treatments [82]. Lehtomaki and Bjornsson [70] usedbatch leach bed digesters coupled with an anaerobic filter to obtainmethane yields of 390 L CH4 kg�1 VS added after 50 days digestionof grass silage. This may be compared with 305 L CH4 kg�1 VSadded 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 toimprove the design of reactors for enhanced biogas productionfrom grass silage. The ubiquitous wet continuous process (CSTR)has been shown to be an effective process for grass silage digestionat loading rates of approximately 1.4 kg VS m�3 d�1 [60]. The UASBreactor offers great potential as it can withstand loading as high as20 kg COD m�3 d�1; thus by converting the feedstock from volatilesolids to soluble COD, loading rates may be increased, retentiontimes may be shortened, and optimistically biogas production maybe increased [82]. The leach bed may be optimized for hydrolysisthrough thermal and enzymatic pretreatment of the grass silagewhile the UASB may be optimized for COD removal.

5.2. Research required

Data from the literature on the best digester configuration usinggrass silage as a feedstock is inconclusive. There is a tendency toutilize data on digester configurations utilizing different feed-stocks (OFMSW, green waste) and to apply the outcome to grasssilage. The feedstock is an important criterion. High solid contentsubstrates may require different reactor configurations; foodwaste and the OFMSW though both having similar dry solidscontents to grass, are very different to grass silage. Grass silage hasa far higher volatile solids content than OFMSW (ca. 92% versus60%) [1] and thus a more complete dry matter removal takes placein the reactor. The digestate from grass silage thus has a lowersolids content than the digestate from OFMSW; it is more liquid innature. This has led to suggestions that dry batch reactors fedthrough a vertical door are more suitable for OFMSW than for grasssilage as the grass silage digestate may require additional handlingdue to the vertical containment of a digestate low in solids content.There is another argument that bailed silage (32% dry solidscontent) is more suited to a dry batch digestion process whileclamp or pit silage (22% dry solids content) is more suited to a wetcontinuous process. These arguments need to be interrogated.

Even digester configurations assessed using grass silage isproblematic. Of significant concern is the comparison of data fromdifferent countries, different institutes, digesting different speciesof grass, at different dry solids content, cut at different times ofyear and times of day [82]. Nizami et al. [82] outlined thesignificant difference in digestibility of grass; for example thewater soluble carbohydrates are higher in the afternoon than themorning resulting in higher biogas production for grass cut in theafternoon. It is suggested that a number of reactor configurationsshould be compared in real time treating similar quantities of grasssilage under similar loading rates to evaluate the optimalconfiguration. Indeed grass cut at different times of the year andfrom different locations can have different D-values, lignincontents and N (nitrogen) values [82]. Thus as shown by Lehotmakiet al. [30] in Section 3.2, incorrect conclusions may be drawn fromdirect comparisons of reactors treating grass silage in differentplants (or labs) from different countries, cut at different times ofthe year and hence with different compositions.

6. Conclusion

Grass digestion offers opportunities to farmers. Instead ofmanaging herds of beef cattle, an industry which offers a low FarmFamily Income, the farmer can continue to draw down single farmpayments while cutting silage two or three times a year as afeedstock for an anaerobic digester. Biogas/biomethane is now theend product rather than beef; the work load is considerably less;greenhouse gas production is eased significantly [1]. This is in linewith the objective of the National Climate Change Strategy forIreland to reduce the herd and to effect an overall reduction ingreenhouse 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 initiallydesigned for grass silage. The authors believe that much workneeds to be undertaken to ascertain optimal digester configura-tions for production of grass biomethane. The CSTR system appearsto be a safe technology if the mixing system is adapted to deal withthe tendency for grass silage to float. However there is a significantpotential to assess the benefits of leach beds followed by a high-rate digester (such as a UASB). The leach beds will permitpretreatment technologies (thermal and enzymatic) to optimizehydrolysis 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 samegrass silage feedstock.

Acknowledgements

Anoop Singh, Nicholas Korres, Beatrice Smyth and ThanasitThamsiriroj for advice, brainstorming sessions, conversations andcritiques.

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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