8.10 Sewage Treatment

Primary treatment: physical removal

Sewage treatment first removes solids physically. Wastewater flows through screens to catch large debris, then into a sedimentation tank where gravity settles out sludge (solid waste) while grease and oil float and are skimmed off. This stage removes about 30-40% of BOD (biochemical oxygen demand) but does nothing to dissolved pollutants or pathogens.

Secondary treatment: biological breakdown

Secondary treatment uses aerobic bacteria to decompose remaining organic matter. In the activated sludge process, aerated tanks let microbes consume dissolved organic material, converting it into more bacteria and CO2. This dramatically lowers BOD before water moves to final disinfection and discharge.

Disinfection and sludge disposal

Before release, treated water is disinfected using chlorine, ozone, or UV light to kill remaining pathogens. The leftover solid sludge from earlier stages can be treated in an anaerobic digester, producing biogas (methane) for energy, and the resulting material can sometimes be used as fertilizer (biosolids) if free of contaminants.

Key terms

Primary treatment

Physical removal of solids from wastewater via screening and sedimentation.

Secondary treatment

Biological process where aerobic bacteria break down dissolved organic waste (e.g., activated sludge process).

BOD (Biochemical Oxygen Demand)

A measure of the oxygen required by microbes to decompose organic matter in water; high BOD signals heavy pollution.

Anaerobic digester

A tank where anaerobic bacteria break down sludge without oxygen, producing methane biogas as a byproduct.

Disinfection (final step)

Treatment using chlorine, ozone, or UV light to kill remaining pathogens before water is discharged.

Try an AP-style question

What happens during primary sewage treatment?

1. Nitrogen is removed chemically
2. Solids settle and are screened out
3. Bacteria digest dissolved waste
4. Chlorine kills all pathogens