8 Factors inhibiting the anaerobic digestion process

The factors that can inhibit the biological process of anaerobic digestion are summarized below.

1) Volatile Fatty Acids (AGV)

A stable biological process is an indispensable condition for the production of biogas to be constant over time. The stability of the biological process depends on its ability to convert the substrates in feed into acetic acid and carbon dioxide, passing through the formation of other fatty acids (propionic, butyric, isobutyric, valeric, isovaleric, caproic and isocaproic), which if they are not converted can lead to an inhibition of the biological process, especially in the methane phase, which fails to adapt to the production of these acids from the previous 3 steps. Furthermore, an accumulation of volatile fatty acids can lead to a decrease in the pH value, also due to a probable blockage of the biological process.
It is therefore essential that, if one or more of the aforementioned acids are present in the biological analyzes, immediately consult the plant biologist who will be able to recommend the right intervention to stabilize the biological process again, allowing the digestion of bothersome acids.

2) Ammonia nitrogen and ammonia

In an anaerobic digestion process there is a balance of ammonia and ammonium ion. The presence of high concentrations of ammonia, which originates from the degradation of nitrogen-based compounds such as proteins, can cause inhibitory and toxic effects on microorganisms, as well as total concentrations of ammonia and ammonium ion can inhibit fermentation if very high. The balance between the ammonia concentration and that of the ammonium ion depends on the pH value and the digester temperature, and increases as these two values increase. Ammonia inhibition can be compensated for by adding carbonaceous substrates or decreasing the digester temperature, always in agreement with the company biologist.

3) Relationship between carbon, nitrogen, phosphorus, sulfur

Is a parameter that indicates the optimal supply of nutrients for the development of the bacterial growth process. The demand for these nutrients is very low as not much biomass develops in the anaerobic digestion process. If the amount of carbon is much higher than the other elements, not all organic carbon is degraded and therefore the potential of methane is not exploited to the maximum. Furthermore, nitrogen deficiency has negative effects on the formation of proteins and therefore on the metabolism of microorganisms. On the contrary, if nitrogen predominates excessively, the formation of ammonia can inhibit the process and therefore the production of biogas.

4) Microelements

They are the nutrients needed in very small quantities for the stability of anaerobic digestion reactions. Problems of lack of microelements occur above all in digesters fed with a single type of substrate and in the case of the presence of a high concentration of sulfur., Which by reacting with the microelements forming metal sulphides, steal them from the microorganisms. The lack of microelements can inhibit fermentation and cause acidification of the fermentative liquid.

5) Hydrogen sulfide (H2S)

This compound, also called hydrogen sulphide, is produced by the degradation of proteins and an excessive concentration inhibits microbial activity even at low concentrations. Its presence in biogas, in addition to precipitating the microelements making them not bioavailable for bacteria, also causes corrosion to equipment and cogeneration units. The concentration of H2S increases with increasing temperature and decreasing and the pH value. An unexpected increase in the H2S value can be treated by reducing the protein fraction entering the digester, or by dosing specific desulphurizing products such as iron salts or caustic soda, however in agreement with the plant biologist.

6) Temperature

A sudden temperature variation in the digester implies a thermal shock of the process, especially in thermophilic digestion, where bacteria are more sensitive to temperature variations, and affects the other process parameters. In particular, as the temperature increases, the concentrations of ammonia and hydrogen sulphide also increase, which can inhibit fermentation.

7) Partial pressure of hydrogen

It can be an inhibiting element in the event that, in correspondence with a high input of rapidly biodegradable organic substance, the acetogenesis phase, which produces hydrogen, is faster than the methanogenic one, which consumes hydrogen, thus causing an accumulation of H2. At a certain concentration, acetogenic bacteria are themselves inhibited by hydrogen, resulting in an accumulation of volatile fatty acids, namely propionic and butyric acids, resulting from the previous acid-forming phase, resulting in a drop in pH.

8) Sediments and floating layers

The presence of significant floating layers, which if wet take the name of cap and if instead they begin to harden they become crusts, or sediments on the bottom of the tanks, can decrease the useful volume of the digester and constitute an obstacle to the productivity of the system, which is overloaded. It is therefore advisable not to mix frequently on the bottom of the tanks, also for a hydraulic question to keep the pump suction clean or in any case to move the sediment or sludge present, and in the case of the floating layer, not to use substrates that are too fibrous and a cut or size that would favor the ascent of the fiber from the fermentation liquid.