Foaming can be a common occurrence in the AD process. Although it happens regularly, this is not necessarily a good thing. Foam can prohibit biogas yield due to its structure, meaning it can be detrimental to the process. It can be the difference between making or losing money from an Anaerobic Digestion system.
What is Foaming?
The foam is formed on top of the slurry, inside of the digester. It is often dark brown in colour and can be extremely potent in smell. In some cases, the foam itself can comprise of up to 95% gas (Methane and Carbon Dioxide). There are many permutations of why the foam can occur, and it largely depends on the particular process itself.
Why does Foaming occur?
In a fully functioning system, as the bacteria deteriorates and eats at the feedstock, it goes through the four stages of AD. Gas bubbles form and then collapse, allowing the gas to escape and be given off. This is where the biogas is then stored at the top of the digester, ready for extraction.
Foaming can occur when the bubbles collapse slower than the gas is trying to escape, or the bubbles form a skin due to the surfactants not allowing the bubble to collapse.
There a many other ideas as to variable that can causing foaming. These include:
- High organic loading rates (OLR)
- Poor/insufficient mixing systems
- Unstable pH
- Temperature fluctuations
Consequences
In extreme cases it can block the pressure relief valve and the outlet, meaning that ultimately it could blow the roof off of the digester. Although, this is a rare occurrence. More common side effects are poor biogas yields and damaged process equipment e.g. pumps and valves.
What can be done?
The solids loading variation should be kept to a minimum as best as possible. Ideally, there should be not much more than a 5-10% variation in the OLR of the feedstock. To keep the process running at optimal levels it should be fed continuously, especially if it is fats, oils and grease (FOG), food and high-strength organic wastes.
Inside the digester, the mixing system should be suitable for the digester, ensuring that the mixing should not be too fast. Opposing that, the mixing should not be too slow, either. Ensuring that the mixer is not creating dead zones inside the digester.
At the design stage of the system, the digester should be correctly specified with additional headspace and overflow capacity, to allow for any foaming inside the digester, ensuring that it can be dealt with effectively before damage to the process equipment occurs.
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