STAGES OF SEWAGE TREATMENT

Preliminary

▪ Screening to remove large solid materials, which can be
composted or shipped to landfills (wood, rocks, trash,
dead animals, etc.)
Primary

PRIMARY

▪ Settlement and removal of suspended solids (removes up
to 60% of suspended solids)

SECONDARY

▪ Promotes microbial digestion of organic material that
remains in the sewage
▪ 1ST – the activated sludge process – sewage is aerated
and agitated
▪ 2ND – sewage is trickled over filter beds
▪ Microorganisms that are present naturally in sewage or
that may be added to enhance microbial action aid in the
digestion during aeration.

TERTIARY

▪ Directed at removal of remaining solids and
microorganisms from the liquid portion of sewage
▪ Filtration through sand and charcoal filters
▪ Deactivation of microorganisms (disinfection) by
using chlorine or UV radiation