Aerobic digesters digest sludge, but can also be used to ___________

Aerobic digesters primarily serve to biologically stabilize sludge by breaking down organic matter through aerobic microbial processes. While their main function is digestion, they can indeed thicken the sludge as a secondary benefit. During aerobic digestion, the microbial activity reduces the volume of the sludge, which effectively increases its solids concentration. This thickening occurs because the solid contents diminish as microbes consume organic material and produce gases like carbon dioxide, thus resulting in a denser final product.

In contrast, disinfecting the sludge typically requires additional processes, such as chemical treatment or thermal methods, to achieve pathogen reduction. Fermentation is not a primary function of aerobic digesters, as it usually involves anaerobic processes that take place without oxygen. Filtering sludge is a mechanical process used to separate solids from liquids and does not occur within the biological and metabolic functions of aerobic digesters.

Thus, the understanding of the aerobic digestion process highlights its role in thickening sludge while clarifying the functions of other options listed.