3.1 What is aerobic digestion and how does it work?

Aerobic digestion is a treatment process similar to activated sludge; however, it does not involve a continuous inflow of wastewater. Instead, it is a batch process that allows for extended solids retention times.

In the aerobic digestion process, (faecal) sludge is pumped from the holding or thickening tank or pre-treatment unit into an aerobic digester. The aerator (mechanical or diffused air) supplies the required oxygen to the system. Aerobic digestion is a liquid composting process in which microorganisms break down organic material in the presence of oxygen. When the substrate supply is cut off, microorganisms must rely on their energy reserves to survive. This is known as the endogenous phase, during which they break down their biodegradable cell mass (75%-80%). This process leads to the conversion of organic material into carbon dioxide (CO₂), water (H₂O), ammonia (NH₃), nitrogen dioxide (NO₂-), and nitrate (NO₃-) while also stabilising the sludge.

The sludge goes to a solid–liquid separator. The separated solid part is returned to the digestion tank or removed for further processing.

This digestion method can handle various types of sludge, including activated or primary sludge, either separately or as mixtures.

The reliability of the process is high, but its performance depends on the concentration of sludge and the amount of oxygen supplied. The system is resistant to loading, pH, and metal interference changes. However, solid concentrations higher than 3% in typical digesters can reduce oxygen transfer efficiency, making it harder for microorganisms to assimilate oxygen and creating a reducing environment in the bacterial floc. Lower temperatures require longer retention times to achieve a fixed level of volatile solids reduction, but this does not necessarily result in an odorous product. Maintaining a dissolved oxygen concentration between 1-2 mg/L with an adequate retention time results in sludge that is often easier to dewater.

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