Biogas Production from Maize Grains and Maize Silage

Abstract

The main objective of this work was anaerobic digestion of maize grains and maize silage and biogas production from these crops. Maize grains were treated using one-stage and two-stage anaerobic techniques; using hydrolysis and acidification as the first stage and methanogenesis as the second stage. Processing nonacidified maize grains in an anaerobic reactor is more stable, though the anaerobic degradation start-up period is longer, specific production of biogas is lower and excess sludge production is higher as from acidified maize grains. Maximum specific biogas production was 0.72 m 3· kg -1 of volatile suspended solids - VSS (nonacidified maize) (at 35 °C) and 0.770 m 3·kg -1 VSS (acidified maize) during anaerobic digestion of maize grains. At average yield of 9 t·ha -1 of dry maize 5,450 Nm 3·ha -1 of methane can be generated from nonacidified maize and 5,828 Nm 3·ha -1 methane from acidified maize grains. Due to low nitrogen content in maize silage, anaerobic digestion of maize silage is rather unstable. Alkali or complementary substrates with higher nitrogen content (e.g. excess sludge from wastewater treatment plant or manure) can be used for anaerobic process stabilization. Maximum measured biogas specific production from maize silage reached 0.655 m 3·kg -1 VSS. At average yield of 30 t·ha -1 of the dry maize silage 9,058 Nm 3·ha -1 of methane can be generated.

Full text

Introduction
Energy crops are one alternative for how to diversify
agricultural production and enhance the business of a farm.
Biogas energy can be used to improve the energy balance
of a farm itself, or the excess energy can be offered for sale
(e.g. to an electricity network). Maize, which in a form of
silage offers interesting yields (about 30 tons of total solids
- TS per hectare [1-3]), was selected as an energetic crop in
this paper. The possibility of maize grain treatment was also
studied. It is obvious that maize grains, both from econom-
ic and nutritional points of view, are not the most suitable
material for biogas production, since a large portion of pro-
duction costs was used to obtain dry and pure grains.
Despite this, attention has been paid to this material, as
well. Experience shows that there are periods in an agricul-
tural commodities market when maize grains cannot be
sold or are sold deeply below its production costs. Then,
maize grains that are too inefficient to be sold, can be used
as a supplementary material for biogas production.
In general, there is only little information on maize grain
anaerobic digestion available in literature. Pouech et al. [4],
Polish J. of Environ. Stud. Vol. 19, No. 2 (2010), 323-329
Original Research
Biogas Production from Maize Grains
and Maize Silage
Miroslav Hutňan*, Viera Špalková, Igor Bodík,
Nina Kolesárová, Michal Lazor
Department of Environmental Engineering, Institute of Chemical and Environmental Engineering,
Faculty of Chemical and Food Technology, Slovak University of Technology,
Radlinského 9, 812 37 Bratislava, Slovak Republic
Received: 24 April 2009
Accepted: 24 September 2009
Abstract
The main objective of this work was anaerobic digestion of maize grains and maize silage and biogas
production from these crops. Maize grains were treated using one-stage and two-stage anaerobic techniques;
using hydrolysis and acidification as the first stage and methanogenesis as the second stage. Processing
nonacidified maize grains in an anaerobic reactor is more stable, though the anaerobic degradation start-up
period is longer, specific production of biogas is lower and excess sludge production is higher as from acidi-
fied maize grains. Maximum specific biogas production was 0.72 m3·kg-1 of volatile suspended solids – VSS
(nonacidified maize) (at 35ºC) and 0.770 m3·kg-1 VSS (acidified maize) during anaerobic digestion of maize
grains. At average yield of 9 t·ha-1 of dry maize 5,450 Nm3·ha-1 of methane can be generated from nonacidified
maize and 5,828 Nm3·ha-1 methane from acidified maize grains.
Due to low nitrogen content in maize silage, anaerobic digestion of maize silage is rather unstable. Alkali
or complementary substrates with higher nitrogen content (e.g. excess sludge from wastewater treatment plant
or manure) can be used for anaerobic process stabilization. Maximum measured biogas specific production
from maize silage reached 0.655 m3·kg-1 VSS. At average yield of 30 t·ha-1 of the dry maize silage 9,058
Nm3·ha-1 of methane can be generated.
Keywords:anaerobic digestion, biogas production, maize grains, maize silage
*e-mail: miroslav.hutnan@stuba.sk

deal with anaerobic digestion of maize grains. In a batch
laboratory reactor under mesophilic conditions, specific
production of methane of 0.397 m3kg-1 of the dry maize
grains was achieved.
Similarly, there is only a very little information on
anaerobic digestion of maize silage as an only substrate.
Generally, it may be said that studies focusing on anaerobic
digestion of fresh or ensiled materials did not show signifi-
cant differences in biogas production, which is discussed,
e.g. in Zubr [5]. Conservation qualities are an advantage
when using silage, i.e. it may be used year round regardless
to the season. Negligible differences in biogas production
from fresh or silage material also are presented in the work
of Gunaseelan [6].
Anaerobic digestion of maize silage is mentioned by
Zauner and Küntzel [5]. In batch laboratory reactors, they
achieved specific methane production of 0.270 – 0.289
m3·kg-1 of the TS. In a laboratory flow reactor, specific
methane production was a little bit lower – 0.181-0.184
m3·kg-1 of the TS.
Amon et al. [2], dealt with biogas production from ener-
getic crops – maize and clover grass, in more detail. In their
work, the team focused on biogas production from various
varieties of maize in various stages of ripeness (milk
ripeness, wax ripeness and full ripeness). Different varieties
reached the harvesting optimum in different stages of
ripeness. Specific methane production ranged from 0.206-
0.286 Nm3·kg-1 VSS and methane yield ranged from 5,300
to 8,530 Nm3·ha-1. These results were achieved in
mesophilic (40ºC) batch tests of anaerobic degradation,
which lasted for 60 days. Some varieties showed minimal
difference in the methane production, depending on the
stage of harvest. Some varieties showed a difference of
more than 25% (Saxxo variety, vax ripeness, Amon et al.
[2]).
Specific methane yields of 0.282-0.419 Nm3·kg-1 VSS
was obtained in the study of Schittenhelm [8], which dealt
with the effect of maize composition and its stage of matu-
rity on the methane yields. These results are comparable
with other information in the literature [3, 9-12]. In the
work [8], specific methane yields of the late maturity
hybrids largely increased with sampling date, whereas the
climatically adapted medium-early hybrids reach their
maximum methane production at an earlier date. The same
tendency was observed by Schumacher et al. [12] in a har-
vest date experiment with a broad maturity spectrum of
maize hybrids.
In our work, we focused on determining conditions for
anaerobic digestion of maize grains and maize silage, long-
term operation of maize grains and maize silage anaerobic
digestion laboratory models and on obtaining technological
parameters of the process.
Experimental
In the experimental part of our work, laboratory tests of
maize grain (hereinafter referred to as the “maize”) and
maize silage anaerobic degradation were carried out and
maize hydrolysis and acidification tests were carried out, as
well. Long-term operation of laboratory models for anaero-
bic treatment of nonacidified and acidified maize and maize
silage was also monitored.
Prior to its processing, maize grains were milled to the
size of approximately 2 mm. The size of particles in silage
was not adjusted in the lab tests, i.e. it was of a size to which
it was adjusted by the harvesting machine. Most of the
silage particle sizes were ranging several cm. Average TS of
the maize used was 90.6%, volatile suspended solids (VSS)
of TS 92.8%. Average TS of silage was 35%, VSS 95.8%.
Value of pH of maize water leachate (mixture of 100 g
maize topped up to 400 ml with tap water) was 5.9; pH of
water leachate of the silage (in the same ratio with water)
was 2.7.
At the beginning, batch tests of anaerobic degradation
were carried out. The same anaerobically stabilized sludge
was used for these tests as for the laboratory models inocu-
lation. Total volume of the mixture during the tests was 1 l,
therefore the volume of used anaerobically stabilized
sludge was 0.5 l, 3 g of maize or 6 g of silage (dry matter),
topped up to 1 l with tap water. Blank tests to measure bio-
gas production by the anaerobic sludge itself were also car-
ried out.
The maize was treated in a one- and two-stage anaer-
obic semi-continuous laboratory model. The two-stage
model consisted of hydrolysis and acidification in the first
stage and methanization in the second stage. To first stage,
the maize was dosed in a mixture of 25 g of maize filled
to 100 ml with tap water. Retention time of the mixture in
this stage was 4 d. This retention time was selected as a
result of the hydrolysis and acidification test. Their results
are presented below. Acidification mixture volume
increased with methanogenic reactor loading rate growth.
324 Hutňan M., et al.
0
400
800
1,200
1,600
0 50 100 150 200
Time (hrs)
Methane production (ml)
maize grains
maize silage
blank test
Fig. 1. Tests of anaerobic degradation of maize grains and
maize silage (amount of dry maize grains at test – 3 g, amount
of dry silage at test – 6 g, amount of anaerobically stabilised
sludge at all tests – 0.5 l, with concetration of TS 37.23 g·l-1 and
VSS 20.74 g·l-1).

Mixed methanogenic reactor volume was 4 l. In the one-
stage system, the maize was treated without acidification.
Silage was processed without prior acidification and was
fed into the methanogenic reactor directly, having the same
quality as that delivered from silage pits. All methanogenic
reactors (both for treatment of maize and silage) were
filled by anaerobically stabilized sludge from the
Wastewater Treatment Plant Bratislava-Vrakuňa to half of
their volume; sludge concentration of 37.2 g·l-1 TS and
VSS of 55.7%, and were topped up with tap water to 4 L.
Laboratory models were fed once a day and worked as
chemostates. All experiments were carried out at tempera-
ture of 35ºC. Concentrations of chemical oxygen demand
(COD), volatile fatty acids (VFA), ammonia nitrogen and
pH value were monitored in filtered samples of sludge
water from methanogenic reactors. In the reactors, concen-
trations of suspended solids and production of biogas were
monitored. Standard methods [13] were used to carry out
all analysis. Analysis of VFA was made according to Kapp
[14]. GA 2000 Plus (Geotechnical Instruments, UK) appa-
ratus was used to measure the content of biogas (methane,
CO2, H2and H2S).
Results and Discussion
Tests of Anaerobic Degradation,
Hydrolysis and Acidification
Fig. 1 shows the results of anaerobic degradation tests;
about 470 ml of methane were obtained from one gram of
the TS (maize) and about 233 ml of methane from one gram
of dry silage. These quantities are in compliance with data
stated by Amon et al. [2].
Then the maize acidification test was carried out (Table
1). COD of filtered and unfiltered sample, VFA and pH,
were monitored in the mixture of maize and water (100 g of
maize topped up to 400 ml with tap water). An acidification
test was carried out since the maize contains a significant
amount of polysaccharides and proteins, and their hydroly-
sis and acidification separate from the methanogenic phase
can accelerate anaerobic degradation. Acidification test
results show that a sufficient acidification period is 4 days.
The technology of maize silage production when it is
stored in silage pits for several weeks or months shows that
acidification will not be necessary in this case and it is pos-
sible to feed it directly into a methanogenic reactor.
Laboratory Models Operation
Anaerobic Digestion of Nonacidified Maize
A gradual increase of organic loading rate (OLR) in the
methanogenic reactor is obvious from Table 2. In the peri-
od of nonacidified maize treatment, the initial OLR of the
methanogenic reactor was 1.05 kg·m-3·d-1 (VSS), the maxi-
mum achieved OLR was 6.3 kg·m-3·d-1. According to Table
2, specific biogas production ranged from 0.420 m3to 0.720
m3per kilogram of the VSS (maize). Maximum achieved
specific biogas production per unit volume of the reactor
was 4.5 m3·m-3·d-1 for a day. Maximum specific biogas pro-
duction was 0.720 m3per kilogram of VSS at OLR 5.25
kg·m-3·d-1. Fig. 2 shows a cumulative biogas production
from nonacidified maize during the methanogenic reactor
operation.
Biogas Production from Maize Grains... 325
Day pH CODfil.
[mg·l-1]
CODunfil.
[mg·l-1]
VFA
[mg·l-1]
05.9 5,860 18,830 332
13.8 10,100 20,470 2,040
23.7 14,350 31,380 2,500
33.7 16,440 36,300 3,620
43.6 23,200 38,500 8,520
63.5 28,700 42,000 10,500
73.3 33,480 46,010 11,400
Table 1. Acidification test of the maize grains.
fil. – filtered sample;
unfil. – unfiltered sample.
Dose of maize
(raw material)
Dose of maize
(VSS)
Organic loading
rate (VSS)
Nonacidified maize Acidified maize
Day of operation Specific biogas
production (VSS) Day of operation Specific biogas
production (VSS)
[g·d-1][g·d-1][kg·m-3·d-1][m3·kg-1][m3·kg-1]
54.2 1.05 0-10 0.420 0-10 0.510
10 8.41 2.1 11-50 0.510 11-20 0.590
12.5 10.51 2.63 51-80 0.595 21-50 0.630
20 16.82 4.2 81-210 0.660 51-100 0.715
25 21.03 5.25 211-260 0.720 101-200 0.770
30 25.23 6.3 261-300 0.710 201-300 0.680
Table 2. Operational parameters of the methanogenic reactors during the anaerobic digestion of nonacidified and acidified maize
grains.

Figs. 3-5 show the course of monitored parameters in the
sludge water from the methanogenic reactor. Operation of
methanogenic reactor was stable during the whole period of
anaerobic digestion of nonacidified maize. COD concentra-
tion is shown in Fig. 3; VFA concentration in Fig. 4 and
ammonia nitrogen concentration is shown in Fig. 5. pH val-
ues were very stable and varied in the range 7-7.2. Measured
average concentrations of monitored parameters in the peri-
od of maximum specific biogas production (between day
211 and 260): COD 1406 mg·l-1, VFA 612 mg·l-1 and ammo-
nia nitrogen 610 mg·l-1. After increasing organic load to 6.3
kg·m-3·d-1, COD and VFA (Figs. 3 and 4), concentrations
increased and specific biogas production decreased.
Therefore, we consider organic load of 5 kg·m-3·d-1 as the
optimal value.
Concentration of suspended solids in the methanogenic
reactor gradually increased, being 41.5 g·l-1 at the end of the
operation. Specific excess sludge production was calculat-
ed on the basis of the suspended solids balance; its value
was 0.15 g·g-1 of maize TS.
Average composition of biogas produced from nonacid-
ified maize is shown in Table 3.
Anaerobic Digestion of Acidified Maize
As already mentioned, retention time of the maize and
water mixture in the acidification stage was 4 days. Dosage
of acidified maize in individual periods of methanogenic
326 Hutňan M., et al.
0
500
1,000
1,500
2,000
2,500
0 100 200 300
Time (days)
Cumulative production of biogas (l)
nonacidi ficated maize
acidificated maize
Fig. 2. Cumulative production of biogas from nonacidified and
acidified maize grains.
0
1,000
2,000
3,000
4,000
5,000
0 50 100 150 200 250 300
Time (days)
Concentration of COD (mg/l)
nonacidi ficated mai ze
acidificated mai ze
Fig. 3. Concentration of COD in sludge water in the
methanogenic reactor during the treatment of nonacidified and
acidified maize grains (filtered sample).
0
1,000
2,000
3,000
4,000
5,000
0 50 100 150 200 250 300
Time (days)
Concentration of VFA (mg/l)
nonacidi ficated ma ize
acidificated maize
Fig. 4. Concentration of VFA in sludge water in the
methanogenic reactor during the treatment of nonacidified and
acidified maize grains (filtered sample).
0
500
1,000
1,500
0 50 100 150 200 250 300
Time (days)
Concentration of NH
4
-N (mg/l)
nonacidificat ed maize
acidificated maize
Fig. 5. Concentration of NH4-N in sludge water in the
methanogenic reactor during the treatment of nonacidified and
acidified maize grains (filtered sample).

reactor operation is shown in Table 2. Since the start of the
methanogenic reactor operation, biogas production was
higher than for the nonacidified maize. Therefore, it was
possible to increase organic load faster. According to Table
2, the specific biogas production was ranging from 0.510
m3to 0.770 m3per kilogram of the VSS (maize). Maximum
achieved specific biogas production per unit volume of the
reactor was 4.3 m3·m-3·d-1 for a day. Maximum specific pro-
duction of biogas was 0.770 m3per kilogram of VSS at
OLR 5.25 kg·m-3·d-1. Fig. 2 shows a cumulative production
of biogas from acidified maize during the methanogenic
reactor operation. The comparison of cumulative biogas
production from nonacidified and acidified maize showed
that a high rate of biogas production was reached faster
(after 100 days of operation) from acidified maize than
from nonacidified maize (after more than 200 days of oper-
ation). It is obvious from Figs. 3 and 4 that the operation of
the methanogenic reactor treating acidified maize was not
as stable as in the case of nonacidified maize. After increas-
ing organic loading rate to 5.25 kg·m-3·d-1, COD and VFA
concentrations increased significantly and gradually stabi-
lized at levels below 2,000 mg·l-1 (COD) and 1,700 mg·l-1
(VFA). ORL increase to 6.3 kg·m-3·d-1 caused a permanent
increase of COD and VFA concentrations and the specific
biogas production was also reduced. Optimal OLR for
anaerobic digestion of acidified maize was 5.25 kg·m-3·d-1.
Values of pH were in the range 6.5-7.8.
Ammonia nitrogen concentrations (Fig. 5) show a very
interesting development. Nitrogen from acidified and
hydrolyzed maize was immediately released to the sludge
water. After approximately 130 days, ammonia nitrogen
concentration in the sludge water from nonacidified maize
treatment reached the values of acidified maize.
Suspended solids concentration in the methanogenic
reactor gradually increased and reached 35.6 g·l-1 at the end
of the operation. Specific excess sludge production was cal-
culated on the basis of the suspended solids balance; its
value was 0.13 g·g-1 of maize TS.
Composition of biogas from acidified maize is shown in
Table 3.
Comparison of anaerobic digestion of nonacidified and
acidified maize showed that:
• anaerobic digestion of nonacidified maize was more
stable;
• methanogenic reactor start-up period was significantly
shorter in the case of acidified maize;
• specific biogas production was higher by 7% in the case
of acidified maize;
• specific excess sludge production was lower by 15% in
the case of acidified maize;
• biogas from acidified maize had a slightly higher con-
tent of methane.
After anaerobic biomass adaptation, the rate of maize
acidification and hydrolysis were sufficient in the
methanogenic reactor. It is obvious that if the maize is used
in a biogas plant as a supplementary material alongside
other substrates, it is not necessary to acidify it before it is
fed into the methanogenic reactor.
Anaerobic Digestion of the Maize Silage
The silage treated in the anaerobic reactor had the same
quality as when delivered from silage pits. Reactor opera-
tion parameters are shown in Table 4.
Biogas Production from Maize Grains... 327
Component Biogas from
nonacidified maize
Biogas from
acidified maize
Biogas
from silage
CH4[%] 54.8 55.5 54.5
CO2[%] 44.9 44.3 45.4
H2[ppm] 10 90 215
H2S [ppm] 170 170 5
Table 3. Composition of biogas produced from maize grains
and maize silage.
Day of
operation
Dose of
silage (raw
material)
Dose of
silage
(VSS)
Organic
loading rate
(VSS)
Specific
biogas
production
(VSS)
[g·d-1][g·d-1][kg·m-3·d-1][m3·kg-1]
0-20 20 6.71 1.68 0.195
21-40 30 10.06 2.52 0.230
41-80 40 13.42 3.36 0.430
81-120 50 16.77 4.19 0.530
121-220 60 20.12 5.03 0.655
220-300 80 26.83 6.71 0.420
Table 4. Operational parameters of the methanogenic reactor
during the anaerobic digestion of maize silage.
0
2
4
6
8
10
12
14
16
18
20
0 50 100 150 200 250 300
Time (days)
Biogas production per dose (l/d)
0
500
1,000
1,500
2,000
2,500
3,000
Cumulative biogas production (l)
Biogas production per dose
Cumulative biogas production
Fig. 6. Production of biogas per dose of maize silage and cumu-
lative production of biogas in the methanogenic reactor.

Fig. 6 shows biogas production per dose of the maize
silage, and cumulative biogas production during gradual
load of the methanogenic reactor. Organic loading rate in the
reactor increased from 1.68 to 6.71 kg·m-3·d-1 (Table 4).
Specific biogas production varied from 0.195 to 0.655 kg
per kg of silage VSS. Maximum specific production of bio-
gas was reached at OLR 5.03 kg·m-3·d-1. It is obvious from
Figs. 6-8 that the methanogenic reactor operation was not as
stable as in the case of grain maize treatment. COD and VFA
concentrations were rising rapidly after each increase of
OLR (Fig. 7). Stabilized COD and VFA values lasted sever-
al days or weeks, after a dose increase. Duration of stabi-
lization period depended on the depth of destabilization after
a dose increase. At higher doses of silage, response on OLR
increase was stronger and the stabilization period was longer.
In these periods, pH values dropped below 6.5 – Fig. 8.
Sodium bicarbonate was used to adjust pH values. pH was
not stable even after COD and VFA concentrations stabi-
lized; therefore, pH was adjusted during the whole operation
of methanogenic reactor. The reactor was fed with 100 g of
NaHCO3during 300 days of its operation, which corre-
sponds to a sodium bicarbonate dose of about 0.33 g.d-1 into
a 4 L reactor, or 0.08 kg per m3of reactor per day.
In comparison with the course of the pH values in the
reactor for the treatment of acidified and nonacidified
maize, the pH values during treatment of silage have been
more unstable. This stability in the case of maize treatment
can be explained by a higher concentration of ammonia
ions in the sludge water, as maize is richer in proteins than
silage (Fig. 5 vs. Fig. 8). Alongside a carbonate buffer sys-
tem (CO2/CO3
2-/HCO2
¯), ammonium buffer system
(NH3/NH4
+) also plays an important role in anaerobic
processes. The study shows that when maize silage is the
sole substrate processed in a biogas plant, doses of alkali or
complementary substrates with higher nitrogen content
(e.g. excess sludge from wastewater treatment plant or
manure) need to be added.
At the OLR of 6.71 kg·m-3·d-1, COD and VFA concen-
trations exceeded 18,000 mg·l-1 and 11,000 mg·l-1, respec-
tively (Fig. 7). It is obvious that the anaerobic reactor was
328 Hutňan M., et al.
Parameter Dimension Maize Silage
nonacidified acidified
ORL (VSS) kg·m-3·d-1 5.25 5.25 5.03
Suspended solids in reactor g·l-1 41.5 35.6 79
Specific biogas production (35ºC) m3·kg-1 0.720 0.770 0.655
Specific methane production Nm3·kg-1 0,35 0.380 0.316
Specific excess sludge production g·g-1 0.15 0.13 0.17
Degradation of TS %85 87 83.0
Table 5. Achieved parameters of the anaerobic digestion of maize grains and maize silage.
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
0 50 100 150 200 250 300
Time (days)
Concentration (mg/l)
COD
VFA
0
200
400
600
800
1,000
0 50 100 150 200 250 300
Time (days)
Concentration of NH
4
-N (mg/l)
5
5,5
6
6,5
7
7,5
8
pH
NH -N
pH
4
Fig. 7. Concentrations of COD and VFA in the sludge water in
the methanogenic reactor during the treatment of maize silage
(filtered sample).
Fig. 8. Concentrations of NH4-N and pH values in the sludge
water in the methanogenic reactor during the treatment of
maize silage (filtered sample).

overloaded when silage dose increased. High VFA concen-
trations caused a significant methanization inhibition,
which resulted in a temporary reduction in biogas produc-
tion. COD and VFA concentrations were stabilized at
around 6,000 mg·l-1 and 2,800 mg·l-1, respectively. At this
OLR, the specific biogas production amounted 0.420 kg·kg-
1of VSS, which is significantly less that with OLR values
of 5.03 kg·m-3·d-1 (0.655 kg·kg-1 of VSS). Therefore, 5.03
kg·m-3·d-1 is considered to be the optimum OLR value.
Average concentration of suspended solids in the anaer-
obic reactor was 79 g·l-1 during the stable period of opera-
tion (between days 121-220). The daily amount of excess
sludge was 3.57 g. For a dose of 21 g TS (60 g of raw silage,
dry matter 35%), this represents the excess sludge produc-
tion of 0.17 g per gram of silage TS. Therefore, the degree
of anaerobic degradation of silage material was 83.0%.
Content of individual components of biogas from maize
silage is shown in Table 3.
Table 5 summarizes the results from anaerobic diges-
tion of nonacidified maize, acidified maize and maize
silage.
Assuming that 1 ha of arable land produces 9 t of grain
maize (TS), then 5,450 Nm3·ha-1 methane can be obtained
from nonacidified maize, or 5,828 Nm3·ha-1 methane from
acidified maize. If 30 t of maize silage (TS) is obtained from
1 ha, then the production of methane is 9,058 Nm3·ha-1. For
a biogas plant with electrical output of 1 MW burning bio-
gas in a cogeneration unit, maize from about 1.1 ha or
maize silage from 0.67 ha is needed for its daily operation;
taking 90% efficiency of cogeneration unit and the 1:1.5
ratio of produced electrical and thermal energy as a base for
calculations.
Conclusions
Anaerobic digestion of maize in laboratory conditions
shows that the operation of an anaerobic reactor is more sta-
ble when nonacidified maize is processed, though the start-
up period of anaerobic degradation is longer, specific pro-
duction of biogas is lower and production of excess sludge
is higher compared to acidified maize. It would depend on
the decision of a biogas plant designer whether the 7%
higher production of biogas and consequent electricity pro-
duction will cover the investments and operational costs of
a more complex technology needed for the acidification of
maize.
Anaerobic digestion of maize silage produced interest-
ing yields of biogas per unit of processed material.
However, due to the low nitrogen content in maize silage
the operation of an anaerobic reactor is rather unstable.
Alkali or complementary substrates with higher nitrogen
content (e.g. excess sludge from wastewater treatment plant
or manure) can be used for the stabilization of anaerobic
processes.
For a biogas plant with electrical output of 1 MW burn-
ing biogas in a cogeneration unit, maize from about 1.1 ha
or maize silage from 0.67 ha is needed for its daily opera-
tion.
Acknowledgement
This work was supported by the Slovak Grant Agency
for Science VEGA (grant No. 1/0145/08).
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