Problems of biology: acidosis and ammonium

The term Acidosis identifies a situation of over-accumulation of volatile fatty acids which leads to a decrease in the pH value in the fermenter below the optimal range between 7.2 and 8.2.

Symptoms (in addition to the decrease in pH):

• decrease in general biogas production;
• decrease in the percentage value of methane due to an imbalance between the hydrolytic phase (more favored in this case) and methane;
• possible significant increase in the value of hydrogen sulphide (H2S);
• excess acetic acid, accompanied by the presence of other volatile fatty acids, such as propionic, butyric, iso-butyric, valeric, iso-valeric, caproic, iso-caproic acid.

Remedies for Acidosis:

• immediately interrupt, in the tank in crisis, any form of solid and liquid feeding, including sewage, which bring a lot of acids and very little buffering power;
• add buffering power to the tank by adding sodium bicarbonate, the quantity of which must be decided in agreement with the plant biologist.

Normally the biological process, and strictly speaking the methanogenic bacteria, require 3-4 days to be able to start growing again once the pH value returns to optimal values. A good thing is to pump hot digestate from a plant that has a similar temperature and power supply: also in this case, the volume to be added is to be agreed with the plant biologist. Finally, the restart of the power supply, always in agreement with the biologist, must take place when the digestion of the volatile fatty acids restarts, easily recognizable by an important increase in the quantity of methane in the biogas produced: the power restart ramp it must "follow" the trend of the methane value, in order to energetically support the resumption of the biological process.

The term Ammoniosis refers to a situation of chemical imbalance between ammoniacal nitrogen and its transformation into ammonia, unbalanced on the latter. The phenomenon is found above all in plants that are manure, for laying hens and for fattening, and large quantities of flour.

Ammonia nitrogen in itself is not an inhibitory substance for the biological process of anaerobic digestion, while ammonia is. In the bibliography the maximum quantity of 3000 mg / l of ammonia nitrogen is reported, but this information is incomplete, as the more or less intense transformation of ammonia nitrogen into ammonia, defined free ammonia, depends not only on the quantity of ammonia nitrogen initial, also from the pH and temperature of the tank.

To give examples, plants powered by silage only but in thermophilicity can have high values of ammonia nitrogen and free ammonia; plants fed with a lot of droppings but with a temperature at the limits of mesophilicity, I can have high values of ammonia nitrogen but not of free ammonia.

Symptomatology:

• the tank digestant, or fermentative liquid, has a pungent odor of ammonia;
• general decrease in biogas production;
• Analyzes reveal an ammonia nitrogen value above 3000 mg / l (data that can be used by the plant biologist to estimate how much free ammonia is produced).

From an analytical point of view, there is also an excess of acetic acid frequently accompanied by propionic acid.

Remedies for ammoniosis:

• decrease of 1-2 ° C in the temperature of the tank, to ensure that the reaction equilibrium shifts towards ammoniacal nitrogen and not towards ammonia;
• dosage of fresh water (the volume of which must always be agreed with the biologist, in order to both cool the tank and dilute the ammonia nitrogen inside the tank itself).

Some plants have good results with the dosage of zeolites, porous aluminosilicates, which, depending on the type, exchange a sodium or potassium ion and bind a molecule of ammonia nitrogen more or less efficiently, thus preventing its conversion into free ammonia. The dosage of this "stone" must be done in agreement with the biologist, taking into account the amount of ammonia nitrogen entering the tank.