3.4 Aerobic digestion and pathogen reduction

Aerobic digestion, when properly designed and operated, can contribute to the reduction of pathogens in the sludge. The process relies on the activity of aerobic (oxygen-loving) microorganisms to break down organic matter, including pathogens, through biological processes. Conditions that favour pathogen reduction are:

• Increased temperatures
• Extended retention times
• Sufficient oxygen availability

Mesophilic (20 – 45°C) digestion is a process that can reduce pathogens by up to 85% within 15 to 30 days. However, to achieve sludge that does not contain detectable levels of pathogens, the retention time should be more than 40 days. In case adequate retention times cannot be guaranteed and no further treatment follows, the sludge can only be applied with site restrictions and management practices in place to ensure the protection of public health and the environment.

On the other hand, thermophilic digestion (45 – 70°C) can remove volatile organics more completely and disinfect pathogens at temperatures above 50°C within less than 8 days of retention. This requires a much shorter retention time than aerobic digestion operated at ambient temperatures or anaerobic digestion.

In regard to oxygen, the continuous supply creates an environment that is less favourable for the survival of certain anaerobic pathogens.

It is important to note that while aerobic digestion contributes to pathogen reduction, complete pathogen elimination may not be guaranteed.