Which one of the following is known as the heart of sewage treatment?

Secondary treatment is called the heart of sewage treatment because microbes here break down most of the dissolved organic matter.

It relies on biological action to sharply cut the organic load measured as BOD.

Primary treatment only settles out solids physically and removes far less organic matter.

Tertiary treatment is a later polishing step for nutrients and pathogens, not the core stage.

Advanced treatment goes beyond tertiary for specific contaminants and is not the central step.

The activated sludge process is a common secondary-treatment method.