Primary
From the preliminary treatment, the wastewater travels to the primary treatment clarifiers, also called primary settling tanks,which removes about half of the pollutants found in the wastewater. About 50-60% of Total Suspended Solids and up to 50% of pathogens and toxic contaminants are removed during this treatment process.
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| Clarifiers To double the settling capacity of the tanks, the clarifiers have a "stacked" settling surface at mid-depth. The Deer Island Treatment Plant has 48 primary clarifiers. Image credit: MWRA |
During primary treatment, gravity separates the sludge and scum from
the wastewater. The sludge settles to the bottom and the scum floats
to the top. The scum is skimmed off and sent to a landfill. The sludge
is sent for further sludge processing.
The remaining wastewater is called primary effluent.
| scum:
Grease,
oil, and plastics that float to the top of wastewater and are skimmed
off. sludge: Solids that settle to the bottom of wastewater. Examples include human waste, food scraps, and other debris. |