In this second part we will describe the rest of the anaerobic digestion processes of agro-zootechnical substrates.
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It is defined as the ratio between the concentration of volatile fatty acids and the buffer capacity of the anaerobic digester. Since many biological anomalies of a biogas plant are a direct consequence of the effect of the increase in the concentration of volatile fatty acids, it has been seen that if the endogenous buffering capacities are exceeded, the ratio between alkalinity and volatile fatty acids provides a numerical indication of how much the buffering effect is still able to control the accumulation effect of these acids. Numerically speaking, for a stable biological process, the alkalinity value must be approximately 4 times greater than the value of volatile fatty acids: in this way the acid / alkalinity ratio is 0.25, acceptable up to 0.35 and can also rise up to 1 and beyond, or that condition in which the buffering power of the system is equal to the value of the acids or even the concentration of the acids exceeds that of the buffering power. To keep the acid / alkalinity ratio within the optimal range, the changes in supply, both quantitative and managerial, or recirculation from another tank are decided with the plant biologist.
The methods to establish this ratio are based on an acid / base titration of a known quantity of fermentative liquid filtered to exclude the fiber from its interior, which involves the determination of both the alkalinity of the system, measured as mg / l of CaCO3, and of volatile fatty acids, measured as mg / l of CH3COOH equivalent, with the use of low concentration sulfuric acid.
In the definition of all days, this ratio is indicated with the initials FOS / TAC, from the German wording of the same name of the instrument that is used on the implants.
During the degradation of proteins there is the formation of ammoniacal nitrogen, whose formula is NH4 +. This substance lives in chemical equilibrium with its conversion into ammonia NH3 and a proton H + according to the reaction: NH4 + ↔NH3 + H +; the shift of the reaction equilibrium towards the formation of ammonia is mainly due to three factors: the first, the concentration of ammonia nitrogen present in the digester, the second, the pH value, third, the temperature: it is the biologist who, after having calculated whether this balance is more in favour of ammonia nitrogen or ammonia, to recommend interventions aimed at avoiding a biological crisis or to get out of it. In the bibliography the limit value of 3,000 mg / kg of ammonia nitrogen is reported as the upper limit to be avoided, but it must always be compared to the values previously mentioned and also to the concentration of volatile fatty acids: if this value is within the optimal range for the biological process, with an ammonia nitrogen value greater than 3,000 mg / kg, the process is to be considered stable and the NH4 + / NH3 ratio is in favour of ammonia nitrogen, which in itself is not an inhibitor.
The ammonia concentration is a parameter to be checked very carefully in those biogas plants in which digestate is recirculated as a preserving liquid and have a protein-rich ration
The proteins, hydrolysed in their simplest compounds, i.e. polypeptides and amino acids, provide nitrogen, which in small quantities is used by the microbial consortium of the digester for its growth and is partly released in the form of ammonia nitrogen during anaerobic digestion. Bacteria use carbon 25-30 times faster than nitrogen, and hence the C / N ratio of 25-30 / 1 is obtained: an insufficient content of nitrogen compared to carbon (C> 30) slows down the rate of microbial growth and all biogas production reactions; a very high nitrogen content of nitrogen (C / N 1/1) leads to an increase in the production of ammoniacal nitrogen, which, in conditions of high temperatures and high pH values, leads the digester to inhibit ammonia stress.
The temperatures currently recorded in the tanks of biogas plants indicate two types of regime: the mesophilic regime, with temperatures between 38 ° C and 45 ° C, and the thermophilic regime, with temperatures between 45 ° C and 50 ° C. below 25 ° C we speak of a psychrophilic or "cold" regime, while above 55 ° C we speak of hyperthermophilia. The mesophilic regime is, by definition, the most stable regime in which anaerobic digestion reactions take place, as the bacterial microflora seems to be less sensitive to environmental variations, on the one hand, but on the other, it could be slower in the production of biogas and the exploitation of the organic substance of the feed products. On the other hand, making the anaerobic digestion reactions take place at higher temperatures means having faster kinetics, greater biogas production (with the same process stability), shorter hydraulic retention times for the same degraded organic substance, and therefore smaller volumes of the fermenters, but this type of regime cannot always be applied: the discriminating factor is the matrices used in feeding, the plant biologist will be the person in charge of managing the feeding available with the temperature of the system, advising sometimes even the decrease or increase of the temperature or of the whole system of some specific tanks.