Livestock waste products are the waste products of a farm or, better still, they are the result of the mixture of various materials: zootechnical manure (faeces, urine), washing water, litter, hair, food residues.
Manure, and even more zootechnical manure, therefore have an extremely variable composition, not only according to the animal species that originate them (bovine, swine, poultry), but also according to the methods of breeding and management of the manure in its complex.
From a physical / managerial point of view, zootechnical manure can be found in both palable (manure) and pumpable (sewage) form depending on the dry matter content. Among livestock manure, sewage has a chemical / physical composition on average more suitable for the most common anaerobic digestion processes.
The use of dedicated crops in codigestion has spread over the last few years. Initially available in cases of overproduction, coming from marginal land, partially cultivated or from set-aside land, with the evolution of the supply chain - thanks above all to incentives (green certificates and more) - they are increasingly used in an advantageous way both in large than in small plants. In the first case, in a logic more oriented towards increasing revenues, they are used, in particular, in processes of anaerobic digestion of waste; in the second case, however, they serve to improve the overall efficiency of the process (standardization of the input mixture) and to achieve more appropriate economies of scale.
The by-products that can be conveniently used in codigestion in an anaerobic digestion process are many. There are consolidated experiences of plants for the production of biogas from the organic fraction of the waste inserted in the treatment sites of the same. As regards the agricultural sector, however, the interest is more properly oriented to those plants that use, for different reasons, by-products and / or waste from the agro-industrial sector that can be inserted, more appropriately, within agro-energy supply chains. The definition of “by-product” is of considerable importance due to the repercussions it can have in the overall framework of the energy production activity and the related “production waste”.
In order for it to be possible to classify “by-product”, instead of “waste”, the waste, or residue, sent to another production cycle, (eg production of “biogas” or “methane”) this must comply with the following parameters:
- it must be generated by a production process, even if it is not its main object;
- use in another production process must be certain, right from the stage of its production, and integral. The process in which the waste is reused must be previously identified and defined;
- the by-product must have product characteristics and environmental quality such as to ensure that its use does not generate a qualitative and quantitative environmental impact other than that permitted and authorized in the destination plant;
- the above environmental compatibility characteristics must be possessed by the by-product from the moment of its production; treatments or transformations prior to their re-use for this purpose are not permitted;
- the by-product must have an economic market value.
