liq

Mari Seppl a,, Ville Pyykknen a,1, Ari Visnen b, Jukka Rintala a,2aUniversity of Jyvskyl, Department of Biological and EbUniversity of Jyvskyl, Department of Chemistry, P.O.

h i g h l i g h t s

e was sed, whwhen migestat

/kg VS). The post-methanation potential of the digestate was determined inbatch assays. The minimum value (maize 40%, 75 1 Nl CH4/kg VSfeed) occurred when the methane yield

renewable energy from energy crops and other organic materials.The benets of anaerobic digestion include the production of twousable products: biogas and digestate. The biogas can be used inheat and power production or upgraded to biomethane and useda vehicle fuel or injected into the gas grid. Agricultural wastes like

ic wastes is useful. Manures provide good buffering capacity and awide range of nutrients, while the addition of energy crops in-creases the energy yield of the process. The anaerobic digestionis also recycling the nutrients as it captures nutrients from agricul-tural wastes and energy crops and recycles them to the land in theform of organic fertilizer, which is a valuable fertilizer for crops.

Maize (Zea mays) is considered a chill-sensitive species with arelative high temperature required for germination, development,and dry matter accumulation. Improvements in temperature toler-ance would assist maize cultivation in northern Europe. Maize cul-tivation for biogas production has recently gained interest also in

Corresponding author. Tel.: +358 40 8053904; fax: +358 14 617 239.E-mail address: mari.p.seppala@jyu. (M. Seppl).

1 Present address: MTT Agrifood Research Finland, Animal Production Research, FI-71750 Maaninka, Finland.

2 Present address: Tampere University of Technology, Department of Chemistry

Fuel 107 (2013) 209216

Contents lists available at

Fue

.eand Bioengineering, P.O. Box 541, FI-33101 Tampere, Finland.Anaerobic digestionTrace elementsManureMethane productionMaize

in the reactor was at a maximum and when the methane yield was at a minimum (maize 67%,153 46 Nl CH4/kg VS) the potential at a maximum (140 Nl CH4/kg VSfeed). Plant nutrients and trace ele-ments were determined in the feedstock and the digestate. As maize contained fewer nutrients thanmanure the concentration of the nutrients and trace elements in the digestate decreased when the shareof maize in the feedstock increased. Thus it seems that even though the CSTR co-digesting maize andmanure can be operated with high organic loading rate (OLR) and short hydraulic retention time (HRT)a signicant part of the methane yields of the feedstock may be lost, if the post-methane potential isnot considered. Based on the yield results of the reactor trials a biogas plant (270 kW) could produceenergy (methane) almost 2400 MW h, when the reactor operates at OLR 2.5kg VS/m3/d and feedstockconsists of 60% VS of maize and 40% VS of liquid cow manure.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Biogas production is one of the most promising ways to produce

cow and pig manures are commonly available and are used feed-stock in the biogas production. Manures have a low biogas yieldpotential and that is why co-digestion with crop material or organ-Keywords:stock was 40% (VS) and thwas 60% (VS) (234 Nl CH4" Co-digestion of cow manure and maiz" The highest methane yield was achiev" Post-methanation potential was low," Trace elements content decreased in d

a r t i c l e i n f o

Article history:Received 23 April 2012Received in revised form 14 December 2012Accepted 18 December 2012Available online 8 January 20130016-2361/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.12.069nvironmental Science, P.O. Box 35, FI-40014 University of Jyvskyl, FinlandBox 35, FI-40014 University of Jyvskyl, Finland

tudied in different share of maize.en share of maize was 40% VS of feed.ethane yield from reactor was high.e when the share of maize increased.

a b s t r a c t

This study investigates the co-digestion of liquid cow manure and maize with different share of maize incontinuously stirred tank reactors (CSTRs). The objective was to determine the methane yield of reactorand the post-methanation potential of the digestate from different reactor trials. The highest specicmethane yield (259 Nl CH4/kg volatile solids (VSs)) was obtained when the share of maize in the feed-

e second highest specic methane yield was when the proportion of maizeof maize, post-methanation potential and digestate characteristics

Biomethane production from maize and

journal homepage: wwwll rights reserved.uid cow manure Effect of share

SciVerse ScienceDirect

l

l sevier .com/locate / fuel

low. After thawing the maize particles were further reduced usingscissors to particle size about 0.5 cm to assist syringe feeding. Anal-

ard

(g/l)

ays

el 1Finland. In previous studies maize cultivation in southern Finlandhave been promising with yields of 16 t total solids (TSs)/ha and60009000 Nm3 CH4/ha for biogas production [1]. In comparisonaverage grass cultivation yields have been on average 10 t TS/haand energy yields of 12003600 Nm3 CH4/ha [2]. Despite this sig-nicant difference in yield, the total energy balance of the supplyand production chain (LCA) should be considered to make a fullcomparison.

Nowadays, maize breeders have developed new biogas maizevarieties that are late maturing, and the TS yields per hectare ofthese are over 30 t TS/ha in Central Europe [3]. Silage maize is akey component of ruminant diets in Central Europe due to its highyield and energy content and that is why maize has characterizedby good digestibility for biogas production. The digestibility andnutritional value of the maize (cell wall concentration, compositionand degradability) has been studied, improved and developed forenergy use in biogas production [4,5]. Maize has been the mostpopular feedstock for biogas plants in Germany and Austria, be-cause it produces the highest net energy yield per hectare in com-parison with other energy crops like sunower, grasses and wholecrop grain silage [6,7]. Mono-digestion of energy crops (i.e. maizeor grass) have been of interest, still the co-digestion of energycrops and manure is the most stable way to produce methane.

Biogas production has moved towards more and more econom-ically viable activity, which relies on maximizing prot for the saleof renewable energy. The efcient and reliable process technologyand energy production per unit forces to use a high organic loadingrate (OLR) and short hydraulic retention time (HRT). There hasbeen little research into the co-digestion of feedstock with a highshare of the energy crops. One research has been made with energycrops, when share of crop has been 4060% of volatile solids (VSs)of the feedstock [8]. Co-digestion with maize and manure has beencarried out differing shares of maize and using various HRT andOLR [9,10] but the post-methanation potential of the digestatewas not measured in any of these studies with maize [810]. Thepost-methanation potential is of importance because it describesthe methane potential of digestate and even some energy yield

Table 1The characteristics of inoculum, liquid cow manure, and maize, variety valdes, stand

TS (%ww) VS (%ww) Ntot (g/l) NH4N

Inoculum 5.0 3.9 2.5 Maizea 16.4 15.2 15b 0.15b

Cow manure Ia 5.7 4.7 2.3 0.76Cow manure II 5.0 4.3 1.5 0.45

Cow manure I was used in feedstock during days 166, and cow manure II during da Methane production of inoculum subtracted.b mg/g TS.

210 M. Seppl et al. / Fuwould be lost from the methane yield of the digestate. In orderto maximize the yield of produced methane, necessary nutrientsmust be present in the reactor, which allow using higher OLR, low-er HRT and to get higher energy production. To achieve high ratesof methane production, trace nutrients are needed so that micro-organisms are supplied with essential nutrients [11,12]. A lack oftrace nutrient have been demonstrated research papers and it isgenerally problem in biogas plants which use only energycrops or some other single substrate material, such as organicwaste [11].

The objective of the study was to investigate the methane yieldof co-digestion of maize and liquid cow manure with increasingshare of maize in the feedstock. The aim was also to measure thepost-methanation potential of the digestate from different shareof maize in the feedstock. The trace elements of the feedstockand digestate were determined also.yses of the maize (Table 1) were performed on melt maize materialafter size reduction (to 0.5 cm). The feed mixtures for reactor stud-ies were prepared daily.

Liquid cow manure was obtained from a dairy farm, Kalmari, inLaukaa Central Finland. The shipment of manure (Cow manure Iand II) was obtained twice from the farm during the reactor runsand was stored at 4 C. Inoculum was obtained from a mesophilicfarm digester (Kalmari, in Laukaa) processing dairy manure, energycrops and industrial by-products from a candy factory.

2.2. Experimental set-up

The reactor experiments were carried out in three parallel con-tinuously stirred tank reactors (CSTRs). The reactors (referred asR1, R2 and R3) were constructed of glass, each with a total volumeof 5 l and a liquid volume of 4 l, stirred continuously at 300 rpmand incubated at 35 1 C. The reactors were inoculated on day 0with 4 l of inoculum and when the methane concentration of theproduced biogas rose to 50% feeding of the feedstock was started.R1 was fed with manure and R2 and R3 with the mixture of man-ure andmaize, using different share of maize andmanure (Table 2).The reactors were fed with a syringe once a day, 5 d per week and2. Material and methods

2.1. Origin of materials

The maize, variety Valdez (middle late ripening, FAO 290), usedin this study was cultivated at Agrifood Research Finland (MTT) inPiikki in southern Finland. The cultivation data can be found fromSeppl et al. [1]. After harvesting the maize material was cut usingchopper (SD 180 E, Wolf Garten, Germany) to a particle size ofapproximately 3 cm. Subsequently, maize material was then fro-zen and stored at 20 C in plastic bags in portions equivalent tothree to four days worth of feedstock for the reactors described be-

deviation.

SCOD (g/l) Specic methane yield

Nl CH4/kg VS Nl CH4/kg TS Nl CH4/kg ww

12.0 81 104 4134b 360 8 334 7 55 113.1 212 18 172 16 10 28.6

67136.

07 (2013) 209216the equivalent volume of the digestate was removed from the reac-tor. In the reactor R3 the liquid fraction of the digestate was recir-culated back to the reactor along with the daily feed, consequentlythat the HRT will get the long enough. The liquid fraction was ob-tained by centrifuging (10 min, 350 rpm) the removed digestate.The biogas was collected in aluminium gas bags via silicone tubes.The gas in the tubes was sampled through glass septa.

The assays used to measure specic methane yields were per-formed in triplicate 1 l glass bottles at 35 1 C. Firstly, 300 ml ofinoculum was added to each bottle followed by the addition ofsubstrate in a VSsubstrate/VSinoculumratio of 1 and using methodsdescribed by Seppl et al. [2]. The batch assays were incubatedfor 80100 d.

The post-methanation potentials of the digestates were mea-sured in batch experiments in triplicate 120 ml serum vials incu-bated at 35 1 C. The digestate (40 g) was added to the vials

Table 2Operational conditions, feedstock and digestate characteristic, and methane production in CSTRs standard deviation.

Substrate

Cow manure Cow manure and maize (low feed) Cow manure and maize (high feed)

Reactor R1 R2 R3Feeding regime (FR) I I II III I II III

Share of maize(% VS) 0 20 30 40 50 60 67(% ww) 0 8 11 17 18 26 35

Share of recycled digestate(% ww) 29 15 6

OLR (kg VS/m3 /d) 2 2 2 2 2 2.5 3HRT (d) 23 27 28 30 25 25 25

Duration(d) 042 042 4391 98140 042 4383 8491, 98126d/HRT 1.8 1.6 1.5 1.4 1.7 1.6 1.4

FeedstockTS (%) 5.7 6.4 6.2 6.7 5.7 6.9 8.3VS (%) 4.7 5.4 5.4 5.9 5 6.3 7.5SCOD (g/l) 13.6 14.5 11.7 11.7 14.8 13.9 14.9NH4N (g/l) 0.72 0.67 0.45 0.33 0.63 0.41 0.32Ntot (g/l) 2.3 2.4 1.6 1.7 2.2 1.8 1.9

DigestateTS (%) 4.8 0.2 5.0 0.4 4.5 0.1 4.0 0.1 4.7 0.0 4.7 0.2 4.9 0.1VS (%) 3.7 0.3 3.9 0.4 3.4 0.1 3.1 0.1 3.7 0.1 3.7 0.2 4.0 0.1SCOD (g/l) 10.0 1.0 10.4 0.4 12.2 1.0 10.3 1.0 10.5 0.8 9.1 0.7 12.7 1.0NH4N (g/l) 0.87 0.04 0.81 0.05 0.84 0.03 0.66 0.03 0.79 0.04 0.65 0.05 0.53 0.05Ntot (g/l) 2.5 0.1 2.5 0.1 2.3 0.1 1.9 0.1 2.4 0.0 2.2 0.1 1.9 0.1NO3N (g/l) 0 0 0 0 0 0 0pH 7.5 0.1 7.5 0.1 7.4 0.1 7.4 0.1 7.47 0.1 7.4 0.1 6.9 0.3VFAtot (mg/l) 60 104 69 119 1918 568 55 22 112 165 1079 129 4854 798Soluble P (g/l) 0.11 0.16 0.15 0.18 0.13 0.15 0.21Ptot (g/l) 0.51 0.51 0.46 0.37 0.44 0.38 0.35Ktot (g/l) 3.38 3.47 3.22 3.05 3.42 3.55 3.4

TS removal (%) 16 22 28 41 18 32 41VS removal (%) 21 27 36 48 26 41 47CH4 content (%) 57 2 52 1 44 6 52 2 51 3 48 2 41 9

Calculated specic CH4 yieldof feedstock

(Nl/kg VS) 212 241 256 271 286 301 311

Specic CH4 yield of reactor(Nl/kg VS) 193 2 198 7 194 84 259 6 221 14 234 16 153 46(Nl/kg ww) 9.0 0.1 10.7 0.4 10.5 4.5 15.3 0.3 11.0 0.7 14.6 1.0 11.5 3.5

% Of total CH4 potentialin substrates

91 82 76 95 77 78 49

M.Seppl

etal./Fuel

107(2013)

209216

211

two unfed days during weekends should be considered. Values forthe gas production, feedstock and digestate characteristics are pre-sented either as weekly averages or as averages over the last

el 12 weeks feeding period. The specic methane yield is given in nor-mal litres per kg VS (Nl CH4/kg VS) i.e. the volume of methane isbased on normal conditions: 273 K and 101.3 kPa. The specicmethane yields from methane potential assays were calculatedas Nl CH4/kg VS added, Nl CH4/kg TS added and Nl CH4/kg wetweight (WW) added with CH4 content of inoculum subtracted.and they were sealed with butyl rubber stoppers and aluminiumcrimps. The contents of the vials were ushed with nitrogen gasfor 3 min to remove residual oxygen. The post-methanation poten-tials of the digestates were measured from R2 and R3, when theshare of maize in the feedstock was 30%, 40%, 60%, and 67%. Thepost-methanation assays were incubated for a total of 125136 d.

2.3. Analysis and calculations

The methane content of the biogas was measured using a gaschromatograph (Perkin Elmer Arnel Clarus 500, Alumina column30 m 0.53 mm) with ame-ionization detectors. Operating condi-tions were: oven temperature 100 C, detector 225 C and injectionport 250 C. Argon was used as the carrier gas (14 ml/min). Thevolume of the biogas produced was measured by use of a waterdisplacement. Volatile fatty acids (VFAs) were measured chro-matographically (Perkin Elmer Autosystem XL, HP-INNOWax-col-umn, 30 m 0.32 mm) with ame ionization detector. Theoperating conditions were: oven temperature 100160 C (25 C/min), injection port and detector 225 C. Helium was used as thecarrier gas.

The TS and VS were determined according to Finnish standardmethods [13] and pH was measured with a Metrohm 774 pH-meter. Total nitrogen (Ntot) and ammonium nitrogen (NH4N)were determined according to the Tecator application note(Perstorp Analytical/Tecator AB, 1995) with a Kjeltec system1002 distilling unit (Tecator AB). The soluble chemical oxygendemand (SCOD) and NH4N from crop samples were analysedafter extraction and ltered with GF50-glass bre lter papers(Schleicher and Schuell). SCOD was analysed according to SFS5504 [14].

Water soluble phosphorus (P) was measured (from the dige-states) to take 30 g of non-dried slurry and shake with 150 ml ofwater for 1 h. The sample was extracted with lter paper (approx-imately 8 lm pore size) [15]. P content of the extract was mea-sured after peroxodisulphatedigestion [16,17] with a Lachatautoanalyser. NitrateN was measured from 1:5 water extract(EN 13652) with a Lachat autoanalyser. Total phosphorus (Ptot)and total potassium (Ktot) were determined from 0.51.0 g of driedmatter (105 C) with dry ashing (450 C) and dissolved with 100 mlof 0.2 M HCl. P was determined colourimetrically with the modi-ed ammoniumvanadatemolybdate method [18].

The digestate and feedstock samples for trace element analysiswere rst dried (24 h, 105 C), and then milled in a rotor mill (Pul-verisette 14, Fritsch, Germany) and stored at 22 C until analyzed.The concentration of trace elements was measured using a PerkinElmer (Norwalk, CT, USA) Optima 4300 DV ICP-OES using the fol-lowing default parameters of the instrument: nebulizer ow 0.51.0 l/min, auxiliary gas ow 0.2 l/min, plasma gas ow 15 l/minand plasma power of 1300 or 1400W [19].

HRT and OLR in the reactor experiments were calculated for theve feeding days per week while for practical (weekly) values the

212 M. Seppl et al. / FuThe methane yields of post-methanation potential (Nl CH4/kg VSfeed, Nl CH4/kg TSfeed) assays has been calculated per VS andTS per feed of CSTR reactor.3. Results and discussion

3.1. Specic methane yield of maize

In the batch assays the specic methane yields of the maize was360 8 Nl CH4/kg VS (Table 1), which is in the same range as previ-ously reported for various maize species. In earlier studies the spe-cic methane yield of early, middle and late ripening varieties wasfound to be between 350 and 400 Nl CH4/kg VS in Denmark [20]and in Germany the specic methane yields of silage maize hybrids(energy maize prototypes) were 282419 Nl CH4/kg VS [21]. The TSyield of the studied maize was 16.7 t TS/ha and methane yield perhectare was 5600 m3/ha [1], while the TS yield of maize species(FAO 300400) have been 2429 t TS/ha, corresponding 70009000 Nm3 CH4/ha in Slovenia [22]. The specic methane yield ofthe cow manure was 212 18 Nl CH4/kg VS (Table 1), which is inthe upper range of previously reported values, 131230 Nl CH4/kg VS [8,9,23]. The different specic methane yields of manureshave been a consequence of different feeding intensities and foragecomposition (grass/maize silage) for dairy cows [23].

3.2. Co-digestion of maize and liquid cow manure

To evaluate the methane yields and process performance of theco-digestion of liquid cow manure and maize three parallel CSTRreactors were operated simultaneously for 136 d. Two reactors(R2 and R3) were at rst fed with the feedstock containing 20%(R2) and 50% (R3) of VS maize and one reactor (R1) was run as acontrol and fed with manure only. OLR in all reactors was2 kg VS/m3/d. After 42 d, control reactor (R1) was stopped whilein R2 the share of maize was rst increased to 30% and subse-quently to 40% on day 98 of feedstock VS while maintaining OLRof 2 kg VS/m3/d, HRT 2730 d. In R3 the share of maize was in-creased from 50% to 60% of feedstock VS and OLR was 2.5 kg VS/m3/d. On day 84 the share of maize was increased at 67% of thefeedstock VS and the OLR to 3 kg VS/m3/d and HRT was 25 d. Therewas a break for feeding the reactors in the days 9297.

The share of maize in the feedstock increased the methane yieldin the reactors. With stable process performance the highest spe-cic methane yield (259 Nl CH4/kg VS) was obtained when theshare of maize in the feedstock VS was 40% (R2) and the secondhighest specic methane yield was when the proportion of maizein the feedstock VS was 60% (234 Nl CH4/kg VS) (Fig. 1, Table 2).The specic methane yield of manure (R1) was 193 2 Nl CH4/kg VS and the lowest methane yield was obtained in R3 whenthe proportion of maize in the feedstock was 67% (153 46 Nl CH4/kg VS). The highest specic methane yield (share of maize 40%)was 95% of the calculated specic methane yield of the feedstockdetermined in the batch assays (Table 2). The lowest specic meth-ane yield was due to apparent overloading of the R3, when theshare of maize was 67% of the feedstock VS. In that case, also themethane content of biogas decreased to 41 9% CH4 and furtherto 21% CH4.

The HRT of this study was 23d in cow manure (R1) and 2530 din two maize reactors (R2 and R3). Thus the present study as wellas other recent studies [6,8] suggests that mesophilic biogas reac-tors can be operated with a HRT of 30 d, which is shorter than thatused in full-scale biogas plants. The typical HRT of biogas plantswhich treat energy crops together with manure is between 60and 90 d [24]. The OLR of the reactors in this study was 23 kg VS/m3/d and share of maize used in the feedstock was similarto or lower than that found in previous studies of the same nature(Table 3) [9,10,25]. The highest single OLR (6 g VS/l/d) has been re-

07 (2013) 209216ported in laboratory scale in CSTR with maize share 50% and gave amethane yield of 210 l CH4/kg VS. This OLR however was not opti-mal because of a decrease in methane yield [10]. Furthermore, in

el 150

100

150

200

250

300

Spec

ific

met

hane

yie

ld (

Nl C

H4/k

gVS)

M. Seppl et al. / Fufull scale digester the OLR was doubled from 2.11 to 4.25 kg VS/m3/d and stable operation was still achieved at OLR 5.5 kg VS/m3/dwith maize and pig manure [25]. In this study the overloading ofreactor (R3) occurred when the share of maize in the feedstockwas increased to 67% and OLR was 3 kg VS/m3 d. In co-digestionof crop silage and cow manure the overloading occurred the OLR7.78 g VS/l/d and the share of crop silage in the feedstock was81% VS [6], while the process performed well at OLR of 5.15 g VS/l/d and the cow manure VS:crop silage VS was 29:71 [6]. In thepresent study the HRT was 25 d while in the study by Comino itwas ca 42 d [6]. In this study the HRT was too short for efcientdegradation, as the amount of non-degraded matter in the dige-state increased leading to an increase in the post-methanation po-tential (Table 4).

The post-methanation potential of the digestate was deter-mined for digestates when the share of maize was 30%, 40%, 60%

00 10 20 30 40 50 60 70

d

Fig. 1. Specic methane yields as weekly averages in digestion of manure alone (R1) and50%, 60% and 67% of feedstock VS (R3). The vertical dashed line indicates the change of

Table 3The co-digestion of the animal manure and plant material in CSTRs operated within the m

Feedstock (ratio on VS basis) Reactor volume (l) OLR (kg VS/m3/d) HRT (

Liquid cow manure, maize (80:20) 5 2 27Liquid cow manure, maize (70:30) 5 2 28Liquid cow manure, maize (60:40) 5 2 30Liquid cow manure, maize (50:50) 5 2 25Liquid cow manure, maize (40:60) 5 2.5 25Liquid cow manure, maize (33:67) 5 3 25Pig manure, corn stover (75:25) 30 3.84 16Cattle slurry, maize (100:0) 5 2 33Cattle slurry, maize (67:33) 5 3 30Cattle slurry, maize (50:50) 5 4 28Cattle slurry, maize (40:60) 5 5 26Cattle slurry, maize (50:50) 5 3 29Cattle slurry, maize (50:50) 5 4 22Cattle slurry, maize (50:50) 5 5 18Cattle slurry, maize (50:50) 5 6 15

n.r. = not reported.

Table 4The post-methanation potential of the digestate at 35 C calculated per feedstock (TS or V

Share of maize (OLR) TS (%) VS (%) P

N

30% (2 kg VS/m3/d) 6.2 5.440% (2 kg VS/m3/d) 6.7 5.960% (2.5 kg VS/m3/d) 6.9 6.3 167% (3 kg VS/m3/d) 8.3 7.5 1R1 R2 R3

R3

R2

07 (2013) 209216 213and 67% of feedstock (Table 4). The post-methanation potentialwas the lowest (maize 40%, 75 1 Nl CH4/kg VSfeed) when themethane production in the reactor was the highest(259 6 Nl CH4/kg VS) and when the methane production in thereactor was low (maize 67%, 153 46 Nl CH4/kg VS) the post-methanation potential was high (140 Nl CH4/kg VSfeed). In thisstudy the post-methanation potential increased (from 75 to140 Nl CH4/kg VSfeed) when the OLR increased from 2 to 3 kg VS/m3/d. Also in previous study the post-methanation potential hasincreased when the OLR have been risen in the reactor [8] andwhen the share of energy crops in the feedstock increased [26].The measured methane yields of the reactor experiments andpost-methanation experiments were about 1020% higherthan the calculated methane yields from the feedstock. Thepost-methanation potentials were 2348% of the total methaneyields (methane yields of the reactor plus post-methanation exper-

80 90 100 110 120 130 140 150ays (d)co-digestion of cow manure with maize 20%, 30% and 40% of feedstock VS (R2) andthe share of maize in the feedstock.

esophilic temperature range as reported in literature.

d) VS removal (%) Specic CH4 yield (Nl CH4/kg VS) CH4 (%) Reference

27 198 52 This study36 194 44 This study48 259 52 This study26 221 51 This study41 234 48 This study47 153 41 This study46 210 67 [28]n.r. 171 n.r. [9]n.r. 263 n.r. [9]n.r. 304 n.r. [9]n.r. 300 n.r. [9]n.r. 240 n.r. [10]n.r. 220 n.r. [10]n.r. 210 n.r. [10]n.r. 210 n.r. [10]

S) of CSTR.

ost-methanation potential

l CH4/kg/VSfeed Nl CH4/kg/TSfeed Nl CH4/kg/ww

99 86 575 66 401 92 640 127 11

70

2

el 1789

101112131415

0 10 20 30 40 50 60

SCO

D (g

/l)

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5000

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214 M. Seppl et al. / Fuiments). Thus it seems that even though the CSTR co-digestingmaize and manure can be operated with higher OLRs and shorterHRTs a signicant part of the methane potentials of the feedstockmay be lost, and in the worst case even emitted to the atmosphere.The relatively high post-methanation potentials also suggest thatdegradation of the feedstock was apparently not limited by lackof nutrients but rather with the retention time and the microbialpopulation in the reactor. For energy crops digestion with manure,two-stage digester systems are preferred because of high gas yieldand reduced post-methanation potential. In two stage systems therst reactor is high-loaded main reactor and the second reactor islow-loaded, which treats the digestate from the rst reactor [7,27].Also the storage tank should be covered to recovery the methane.The studies [9,10,28] presented in Table 3 did not report thepost-methanation potentials of the digestates.

The recirculation of the digestate back into the reactor (R3) didnot increase the specic methane yield. The recycling of the dige-state has been shown to improve the gas production marginally,

0.4

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0 10 20 30 40 50 60 70

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4-N

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0 10 20 30 40 50 60 70

TS (%

)

Tim

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0 10 20 30 40 50 60 70

VFA

tot

Fig. 2. The characteristics (SCOD, VFA, NH4-N, TS% of digestate) of the digestates in dige20%, 30% and 40% of the feedstock VS (R2) and 50%, 60% and 67% of the feedstock VS (R3)80 90 100 110 120 130 140 150

R3

R3

R2

R3

07 (2013) 209216because the microbes have been returned to the digester, providingan additional microbial population [27,29]. Increasing the recircu-lation of the digestate (only 6% ww of feedstock) the overloading ofR3 might be prevented. Also too short HRT (only 25 d) of the reac-tor might be one reason for overloading. In the previous studieswith maize the HRT has been 26 d when the share of maize inthe feedstock was 60% VS [9]. Cornell [10] has also used a shortHRT only 1529 d when the reactors were fed OLR 36 g VS/l/d(50% of VS maize and 50% of VS cattle slurry), but then the methaneyields were only 210240 Nl CH4/kg VS. The HRT was 3343 dayswhen crop silage was fed as 5171% of the feedstock VS [6]. In bothstudies there was no indication of overloading or processinhibition.

The pH in the reactors was about 7.5 except in the R3 duringoverloading (maize 67%) when it dropped to 6.9. SCOD ranged from9.1 to 12.7 g/l while VFAtot concentrations were generally less than1100 mg/l except with maize 30% (R2) when the VFAtot concentra-tion was higher (1918 mg/l) as there were some feeding problems

80 90 100 110 120 130 140 150

R3

R2

80 90 100 110 120 130 140 150

e (d)

R2

R3

80 90 100 110 120 130 140 150

R2

stion of the manure alone (R1) and the co-digestion of the cow manure with maize. The vertical dashed line indicates the change of the share of maize in the feedstock.

anu

P(m

23876

9998777

14

el 1Table 5The trace element (plant nutrient and heavy metal) content of the maize, liquid cow msilage and digestate 100% maize) [30].

Plant nutrients S(mg/kg TS)

Mg(mg/kg TS)

K(mg/kg TS)

Na(mg/kg TS)

Maize 534 1428 19327 90Maize silagea 990 1250 8000 50Inoculum 3728 6164 47360 2751Cow manure I 3706 5595 40753 1616Cow manure II 2855 4181 33207 2496

Reactor Share ofmaize

1 0 3659 7200 50673 24132 20 3840 7488 57327 26802 30 3597 6704 50600 25992 40 3195 5875 49273 27053 50 3336 6254 62617 27643 60 3169 5427 49370 20093 67 2915 4375 43377 1722Digestate 100%

maizeb3610 4120 28020 680

M. Seppl et al. / Fuwith syringe during operating the reactor (Fig. 2). Also during theoverloading the VFAtot concentration was high, 4854 798 mg/l,when 67% feedstock VS was maize (Fig. 2, Table 2), consistingmainly of acetate (3950 mg/l) and propionate (980 mg/l). TheVFAtot concentrations were about 1% of SCOD concentrations inreactors, except when the share of maize was 30%, 60% and 67%.In that case the VFA contributed for 16%, 12% and 38% of the SCOD,respectively. During the trials in the three reactors, the ammoniumconcentration of the digestate varied from 0.53 to 0.87 g/l (Fig. 2),Ntot from 1.9 to 2.5 g/l indicating decreasing trends during the tri-als due to the lower nitrogen content of maize as compared tomanure and inoculum (Table 1, Fig. 2). TS and VS removal were cal-culated for all operational conditions in the reactors. TS removalwas 16% and VS removal 21% of liquid cow manure reactor (R1).When the share of maize was increased (share of maize 30% inthe feedstock) the TS and VS removals increased being 28% and36%, respectively and 41% (TS) and 48% (VS), when the share ofmaize was 40% in the feedstock (Table 2). The TS and VS removalswere at low level, which is due to short HRT. This can be noticedalso in the relatively high post-methanation potentials (Table 4).

Based on the results of these reactor experiments a biogas plant(270 kW) could produce about 240,000 Nm3 of methane annually ifthe reactor (liquid volume 1000 m3) operates with OLR 2.5 kg VS/

Heavy metals Cd Cu Cr0.82.7c

(mg/kg TS)1.24.3c

(mg/kg TS)1.13.7(mg/kg

Maize 0.0 4.3 0.0Maize silagea n.a. 4.9

el 167% VS (R3) (Table 5). In previous studies the deciency of thetrace elements was clear from the relatively low OLR, when themaize silage has been the sole substrate. The lack of the trace ele-ments does not occur immediately but only after a certain opera-tion time [11]. The lack of the trace elements has been reportedin mono-digestion of grass-clover [31], but especially in case ofthe mono-digestion of maize the lack of the trace elements is re-ported to be a challenge with many biogas plants [11]. Addingthe trace elements to the reactor should attain the long-term pro-cess stability, high methane production and also operation at high-er OLR [11]. The co-digestion of maize with liquid manure helps tomaintain the trace elements concentration in a sufcient level ofthe methane production as manures typically contain higher nutri-ent and trace elements concentration [32]. On the other hand, ascrops contains different concentrations of trace elements, whichdepend on the concentration of soil nutrients and regional varia-tion of soil [33], the need of the trace nutrients in biogas plantsshould be assessed case dependently.

In previous studies the most limiting trace elements in biogasproduction have been reported to be cobalt (Co), molybdenum(Mo), selenium (Se) and nickel (Ni) [11,34]. The Co, Mo and Se werenot analyzed in the digestate in this study, but Ni concentration ofthe digestate were 216.6 mg/kg TS (Table 5) corresponding to0.090.66 g/m3. Ni addition stimulated both the biogas productionand the methane content of the biogas in batch study, where cattledung was used as a substrate [35]. The optimum concentration ofNi for batch cultures of methanogens was reported to range between0.012 and 5 g/m3 [36]. Also in previous studies the appropriate addi-tion of Ni and Co of CSTR experiment (model substrate maize silage)gives an opportunity to increase the OLR in the reactor [32].

4. Conclusions

The highest methane yield 259 Nl CH4/kg VS was achievedwhen the share of maize was 40% of the feedstock VS and theOLR was 2 kg VS/m3/d.

If the biogas process has operated with a short HRT and highOLR, the post-methanation potential has to be taken intoaccount. When planning biogas plants have to be ensured thatthe process runs at stable conditions and that the producedmethane can be used efciently.

Maize contains fewer trace elements than liquid cow manure.Anaerobic digestion of maize and manure operates effectivelywithout the addition of trace element.

Acknowledgements

This study was supported by grants from the University ofJyvskyl and the Fortum Foundation. The authors wish to thankthe Kalmari biogas farm for providing the liquid cow manure andinoculum, and Agrifood Research Finland (MTT) in Piikki for pro-viding the maize material. Furthermore, Mervi Koistinen is kindlyacknowledged for her help in the laboratory.

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Rev Biotecnol 1990;19:46579.

Biomethane production from maize and liquid cow manure Effect of share of maize, post-methanation potential and digestate characteristics1 Introduction2 Material and methods2.1 Origin of materials2.2 Experimental set-up2.3 Analysis and calculations

3 Results and discussion3.1 Specific methane yield of maize3.2 Co-digestion of maize and liquid cow manure3.3 Trace elements

4 ConclusionsAcknowledgementsReferences