Renewable energies’ policies in many countries have led to the use of the so-called “energy crops” to boost the production of biogas, which is two orders of magnitude higher than the energy production that can be attained by feeding the digester with only animal dejections. The most diffused energy crops for AD are corn, sorghum, triticale, wheat, barley and oat. The proportions and availability of such substrates depend on the country, being sorghum and triticale silages more common in the dry climates of Southern Europe and corn silage the most diffused feedstock in more humid climates, like Central Europe. All of these crops share a feature: they are cereals, and as such, contain a certain amount of phytic acid, a powerful chelating agent. Chelating agents are molecules capable of immobilizing (chelating) metals. Many metals are necessary as catalyzers of the enzymatic activity of Bacteria and Archaea groups: it is well known that hydrolysis and methanogenesis cannot be carried out efficiently if minerals like Fe, Mo, Mn, Mg, Zn, Co, Ni and Se are missing or not biologically available to the bacteria. Ruminal Bacteria contained in cow manure are usually able to produce phytase, an enzyme capable of breaking phytic acid. In biogas plants fed with cereals and little or no ruminant’s manure, phytic acid will hence cumulate till most minerals present in the digestion slurry will be chelated. At this point the microbial community will suffer a reduction of its metabolic activity, which in extreme cases can reach the total block of the plant. Biogas plants fed with cow manure and cereal silage are less prone to biological collapse, but not totally exempt of potential trouble: intensive cow farming techniques usually employ highly digestible commercial fodder, mainly based on proteins, in order to boost weight gain in short times or milk production. If the animals do not receive a “natural” (rich in cellulose) diet, the bacterial flora in their digestive apparatus does not develop correctly, and hence the dung from such farms has low or null hydrolytic and methanogenic activity and no phytase. Biogas plants fed with such kind of manure and large proportions of silage are hence prone to low bacterial activity and hence low biogas productivity.
This is a short description of an industrial application where AMPTS II Light has been used to find out optimum concentration of micronutrients and give quick restart of an Italian biogas plant.
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