Primary treatment settling removes about half of the solids and a third of the BOD from raw sewage. Secondary treatment is defined as the "removal of biodegradable organic matter (in solution or suspension) and suspended solids. Disinfection is also typically included in the definition of conventional secondary treatment." Biological nutrient removal is regarded by some sanitary engineers as secondary treatment and by others as tertiary treatment.
After this kind of treatment, the wastewater may be called secondary-treated wastewater.