Problems of biology: foaming

The formation of foams is an undesirable effect for the operators of "wet" biogas plants which can lead to a decrease in electricity production (biogas trapped and consequently not sent to the cogenerator), to obstruct pipes and ducts, and, in cases more serious occurred, to an overflow of the digestion tank.

The term foam defines a two-phase system consisting of a part of biogas over a very thin layer of liquid, called the lamellar layer.

Causes of foaming

The formation of foams derives mainly from the formatting substrate, or, more in detail, from the composition of the substrate in fermentation; in the lamellar phase, some macromolecules may be present deriving from the digestion of the matrices used in nutrition, such as proteins, saponins (partially soluble glycosides in water and oil) and some sugars, such as pectins.

Some plant by-products containing saponins are for example:

• alfalfa
• peanuts
• oil sludge (i.e., the dregs of oil obtained from olives)

NB: pomace is a completely different by-product which, on the contrary, helps in the abatement of foam thanks to their oily content.

Some vegetable by-products with high protein content are:

• lupine
• pea protein
• fava beans
• beans

Even if the highest protein content is certainly found in the By-products of Animal Origin, with peaks, for some types such as flours, higher than 40% on as it is.

Some plant products containing pectins are:

• biogas or sugar beets
• surpressed pulps (as a by-product of sugar beet processing)
• marcomela and other fruit processing by-products
• beer threshers

The formation of foam from imbalance of the C / N ratio (carbon / nitrogen) caused by sewage or high dosages of manure is also interesting.

Remedies for foaming

In case you notice the formation of foam in the digester it is necessary:

• activate the mixing manually, as a mechanical stirring assists the breaking of the surface tension and the abatement of the foam
• use an antifoam agent, possibly vegetable in the case of agricultural biogas plants, which can break the gas phase above the lamellar phase: in this case seed oil is very useful, in the amount that the plant biologist deems appropriate
• if yeasts are present in the feeding ration, it is advisable to bring the temperature of the tank above 40°C to inactivate them

To reduce the possibility of foaming, it is not recommended to abruptly change the substrates in feeding; a gradual change allows the bacterial flora to adapt, keeping gas production constant.

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