What are the Three Stages of Wastewater Treatment?

The three stages of wastewater treatment are known as primary, secondary and tertiary. Each stage purifies water to a higher level. In some applications, only one or two stages are necessary. The level of treatment necessary depends on the water’s intended use case, and what environment it will be discharged into.

Primary treatment (stage 1)

Primary wastewater treatment involves sedimentation. This is when wastewater is temporarily held in large sedimentation tanks to remove settleable solids.

With gravity, heavier solids sink to the bottom while lighter solids rise to the top. Chemicals can also be added as coagulants to remove more solids.  

Sedimentation creates sewage sludge as a by-product at the bottom of the tank. Once this has settled, the treated water is released for secondary treatment. The sludge, meanwhile, may find use in anaerobic digestion or as a liming material.

Secondary treatment (stage 2)

Secondary treatment involves removing soluble organic matter solids that escaped primary treatment. It also deals with smaller suspended solids.  

Secondary treatment methods include:

·       Bioreactors

·       Filter beds

·       Aerated ponds

·       Activated sludge 

·       Rotating biological contactors