Combined energy and feed production by thermophilic digestion of
manure, grass and maize silage.
Sebastiano Falconi, Henrik B. Møller, Alastair J. Ward and Chitra S. Raju
Institute of Agricultural Engineering ,Faculty of Agricultural Sciences, Aarhus University, Denmark.
Introduction.
By anaerobic digestion of crops and manure it is possible to
produce energy, organic fertilizers and good quality feed with
a high content of amino acids. This can be one solution to
solve the problems concerning the competition between food
and energy crops. At the same time, the production of energy
by anaerobic digestion limits the use of fossil fuels.
In this study the productive performance of maize and grass
silage in terms of biogas and CH4 yield was analysed. The
variation of amino acids content of these crops and of cattle
and pig manure by anaerobic digestion was studied also.
As regards the VFA production, the trend was similar for both
the mini-reactors with crops. During the first part of the study,
VFA levels were high. This was attributed to non optimal values
of some process parameters, such as pH and temperature.
Later, VFA fell to a low level indicating that the process was
stable. Towards the end of the experiment VFA increased again,
but to lower levels than was seen during the first phase (fig 2).
9000
8000
Materials and methods.
4
5
6
7
8
9
10
11
12
13
14
15
16
Figure 2. VFA production.per day.
After anaerobic digestion, amino acid concentration increased
for both the maize and grass silage. The increase was more
remarkable for the maize silage, where an 172% increase in
total amino acids was seen. In particular, lysine increased by
388 % and methionine by 108 %. The total amino acid content of
pig and cattle manure did not present a significant variation.
However, there was a pronounced reduction for some amino
acids , and a moderate increase for others (fig.3).
500
400
300
200
Grass
Maize
100
Pig manure
Cattle manure
Methionine
Cystein+Cystine
Arginine
Ornithine
Lysine
Histidine
Phenylalanine
Tyrosine
-100
Leucine
0
-200
100
3
Period (weeks)
Grass
200
2
Isoleucine
300
1
Valine
Maize
0
Alanine
400
1000
Glycine
L CH4/ Kg VS
500
3000
Proline
loading rate change
Grass
Glutamic acid
600
4000
Serine
700
Maize
Threonine
A greater daily biogas and CH4 production in terms of volume
was found for the grass silage in comparison to the maize
silage. On the contrary, the biogas and methane yield per kg
VS presented values higher for the maize silage (fig.1). After
the loading rate increase there was an increase in biogas and
CH4 in terms of digester volume. This increase was more
significant for the grass silage. CH4 content of the biogas was
50 – 60 %.
5000
Aspartic acid
Results and discussion.
6000
2000
% increase on a DM basis
The experiment was conducted at pilot scale using four
reactors of 130 liters each, separately treating maize and
grass silage and pig and cattle manure. Digestion was
conducted under thermophilic conditions, with a temperature
between 50 and 53 ° C.
The experiment lasted almost four months. Water was added
to the silages when feeding and recycled digestate added for
the first ten days of the maize silage experiment and the first
eight weeks of the grass silage experiment.
For the maize the variety NK-Buul was used and for the grass
a mixture of red clover, white clover, rye fescue and two types
of rye grass. The digesters were loaded five days a week. The
loading was increased during the experiment from 1 to 1,5 kg
of fresh silage per day for both substrates.
Concentrations of amino acids were determined on samples of
digested manure and fresh and digested maize and grass
silage.
Total VFA (mg/L)
7000
-300
0
Amino acids
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
Pe riod (we e ks)
Figure 1. CH4 yield in terms of VS added per week. It can be seen that the maize silage presents
values higher than the grass silage for most of the experiment. A temporary decrease in the yield
after the loading rate change is also evident.
Figure 3. Percentage increase of amino acids on a DM basis after anaerobic digestion, showing an
increase for almost all the amino acids in the two different substrates, particularly for the maize silage.
Pig slurry presents a greater variation percentage in comparison to cattle slurry.
Acknowledgements
The study has been funded by the BioConcens project from Ministry of Food Agriculture and Fisheries.
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