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).

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).

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.

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Sebastiano Falconi, Henrik B. Møller, Alastair J. Ward and Chitra S. Raju Institute of Agricultural Engineering ,Faculty of Agricultural Sciences, Aarhus University, Denmark.

Combined energy and feed production by thermophilic digestion of manure, grass and maize silage.

Materials and methods.

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.

Results and discussion.

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 %.

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.

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 2. VFA production.per day.

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Period (weeks)

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Acknowledgements The study has been funded by the BioConcens project from Ministry of Food Agriculture and Fisheries.