Preliminary, Primary and Secondary treatment of sewage

Preliminary treatment of wastewater involves removal of rags, sticks, floating grit and grease that may cause maintenance or operational problems with treatment operations and ancillary systems.
Preliminary treatment consists of the following operations
Screening to remove floating matter
Grit chamber for removal of sand and grit
Communitors for removal of large size suspended solids
Floatation units for removal of oil and grease
Skimming tanks
Flow measuring units such as parshall flume
Pumps and
Pre-aeration units

Primary treatment involves removal of a portion of suspended solids and organic matter from wastewater. This consists of
sedimentation that is carried out in primary settling tanks
Coagulation and flocculation are carried out in secondary settling tanks

Advanced primary treatment involves enhanced removal of suspended solids and organic matter from wastewater that is typically accomplished by chemical addition and filtration known as coagulation and flocculation.


Secondary treatment involves removal of biodegradable matter that is insoluble and in a suspended state. Disinfection of water is included in conventional secondary treatment. Secondary treatment processes for wastewater applications are
Aeration system
Biological treatment systems
Sludge and bio-solids processing systems

Secondary treatment consists of the following processes
Activated sludge process
Oxidation ponds and lagoons
Trickling filters

Sedimentation is a primary treatment process