Biology problems: main parameters for monitoring the fermentation process – part 1

In this article, divided into two parts, we will describe the anaerobic digestion processes of agro-zootechnical substrates, which can be classified according to the following parameters:

• thermal regime (i.e., process temperature, and therefore whether mesophilic or thermophilic)
• solids content (both intended as dry matter value and as organic dry matter)
• number of phases foreseen by the design of the plant (i.e., single-stage plants, in which all the fermentation reactions take place in the same tank, or two-stage plants, in which there are two or more distinct tanks, one of which has the main function of hydrolysis and the others have the methane function)

Some chemical-physical parameters can therefore be indicators of process stability or its malfunction. Considering the many factors that govern the anaerobic digestion process, it is appropriate to pay attention to some parameters that regulate it; below are the most important.

Quantity and quality of biogas

The most evident and easiest to understand parameter of the stability of the biological process or its anomaly is the quantitative, qualitative and qualitative-quantitative production of biogas. A biogas deriving from a stable biological process has methane values above 50%, carbon dioxide values equal to 40/45%, oxygen values> 1% and H2S values below the limit of the cogenerator and in any case in line with the supply matrices; the total amount of biogas detected inside the gasometer is an index of the total quantity loaded daily, which also takes into account the modulation threshold with which the cogenerator was set.

A decrease in biogas production can be derived from multiple factors such as matrices with less biogas production, higher humidity of incoming products, etc ..., a condition that can be easily solved by increasing the power supply. A decrease in the quality of the biogas, or a decrease in the value of methane, can be associated with a change in feeding: the passage of a maize and manure feed to one of maize only, for example, displaces the methane from above 54% 51-53% typical of silage. A qualitative-quantitative change of the biogas, i.e. a variation both in the general quantity of biogas and in its composition, with a consequent decrease in the quality of the methane and also in the increase in the concentration of H2S, can be caused by biological imbalance, a greater phase hydrolytic and consequent lower activity of the methanogenic phase, due, for example, to the accumulation of acids, deficiencies of microelements, or the interaction of the bacterial flora with substances that inhibit anaerobic digestion.

Concentration of volatile fatty acids

Fermentation bacteria hydrolyze proteins, carbohydrates and fats into their simpler compounds i.e., polypeptides and amino acids in the case of proteins, simple sugars and alcohols in the case of sugars, and fatty acids with different numbers of carbon. Subsequently, all these compounds are reduced to volatile fatty acids, which, by their definition of "volatile", have a number of carbon atoms between 2 (acetic acid) and 6 carbon atoms (caproic acid). The concentration of these acids can undergo a change as a result of feeding the fermenter, but can also vary in conditions of lack of macro and microelements, or as a result of the interaction of the bacterial consortium with inhibiting substances. Acetic acid is the last volatile fatty acid that is converted into methane and carbon dioxide; this consideration makes us understand how, except for purely hydrolytic tanks, accumulations of acids from propionic to caproic are an indication of a biological anomaly up to a block in the biology of the fermenter, if all these acids accumulate. Optimal values of acetic acid are in the range 0-500 mg / kg, where zero indicates a perfect balance between the hydrolytic and methane phases; values below 500 mg / kg indicate however an "ideal" biological state and above all under control. As previously mentioned, to keep the biological process as stable as possible, the accumulation of other acids must be avoided: in this case, and always in agreement with the plant biologist, we try to understand if this situation is due to a problem. power supply or other and trying to find the right way to be able to give biology time to digest excess acids and transform them into energy.

Buffer system and alkalinity

The buffer system of a biogas plant is determined by the presence of volatile fatty acids, by carbonic acid, produced by carbon dioxide in aqueous solution, and by the concentration of ammoniacal nitrogen. Alkalinity is the analytical expression of the buffer system of a biogas plant: values between 10,000 and 17,000 mg / l, usually measured as mg CaCO3 (calcium carbonate) / l, are optimal for the biological process. Values below 10,000 indicate an insufficient buffer capacity of the tank which is unable to balance the acids present, so the pH value can undergo variations; values above 17,000 mg / l may lead to an increase in the pH value, up to a defined condition of alkalosis, or pH values beyond which the activity of methanogenic bacteria undergoes a drastic decrease.