4.4.12 (HL)—Sewage treatment stages

Primary treatment physically removes suspended solids; secondary treatment uses aerobic microorganisms to biodegrade organic matter; tertiary treatment uses additional chemical or physical processes to remove nutrients, remaining particles and pathogens before safe release or reuse.

Settling separates sludge in primary treatment. Aeration and activated-sludge bacteria lower organic load in secondary treatment. Tertiary processes may precipitate phosphorus with alum, filter remaining solids, disinfect with chlorine and then dechlorinate to protect aquatic life. Access, sewers, energy, operators and cost determine whether treatment is implemented equitably.

A nutrient-rich effluent needs more than screening: tertiary nutrient removal is required before discharge to a sensitive lake.

Disinfection targets pathogens during tertiary treatment; solids, biodegradable organic matter and nutrients require the matching earlier or additional processes.

A treatment plant only helps if sewer connections, maintenance and equitable access keep pollutants out of the environment.