3.3　pH Value

The optimum pH value for biogas digestion is 6.5～7.5. In normal digestion process for rural small digesters, the pH balance can be kept automatically and needn't to be adjusted. Because of feedstock in an unsuitable proportion of composition and poor operation, the pH value may not be in a constant state. In case it is below 5 or above 8, the fermentation process may be inhibited or even stopped. In the initial period of fermentation, a large amount of organic acid in digesters has been produced by the activity of the acid-forming bacteria and the lower pH value appeared. Afterwards, while the fermentation goes on, the ammonia concentration rises gradually due to the action of protein-splitting bacteria. The ammonia can neutralize organic acid in the fermenting slurry and turn pH to the normal value.

The following measures have been taken to adjust pH in practice.

①A small amount of effluent is removed frequently and fresh raw material is added in the same quantity simultaneously.

②Low pH value can be remedied by addition of ash fertilizer, diluted ammonia water or mixture of them and fermented liquor. In case lime is used, the quantity of lime should be controlled strictly to avoid doing harm to the bacteria.

It is necessary to adjust pH which is below 6 or above 8 when some industrial wastes are used as feedstock.

pH value can usually be determined by pH detector, pH test paper and pH indicating agent. Within a certain pH range, pH test paper and pH indicating agent would change in color. Hence the pH value can be roughly judged by the change of color.