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.

Secondary wastewater treatment processes use microorganisms to biologically remove contaminants 
from wastewater. Secondary biological processes can  be aerobic or anaerobic, each process utilizing a different type of bacterial community. Coupled anaerobic–aerobic processes may also be 
employed under certain circumstances.

In an aerobic system, the organic contaminants are converted to carbon dioxide, water, 
additional microorganisms, and other end products.