In which of the water treatment processes additional BOD (Biological Oxygen Demand) is provided to remove the biological material?

The secondary treatment stage uses microorganisms whose added biological oxygen demand breaks down dissolved organic matter.

Here microbes consume the organic material and are then settled out as sludge.

Primary treatment only removes solids by settling and screening, using no biological action.

Tertiary treatment is an advanced polishing stage for nutrients and remaining contaminants.

Advanced treatment goes further still to remove specific dissolved substances.

Secondary treatment is where the bulk of the biological load is actually removed.