After passing through the headworks, or pretreatment, facility, flow enters the influent pumping station where it is pumped to the higher elevation of the primary
treatment units of the plant.
The units performing primary treatment at the District’s plant are known as primary
clarifiers. The purpose of the primary clarifiers is to separate solid particles in the wastewater mixture from the liquid.
After passing through the primary clarifiers, the solids and liquid streams are treated by
Primary Clarifiers
Flow Over Primary
Clarifier Weir
entirely different processes.
Primary clarifiers are designed to provide a quiescent volume where solids are allowed
to settle by gravity over a relatively long period of time (usually 2 to 4 hours). Gravity provides a natural
separation of solids and liquid in the clarifier tank. As the solids settle downward,
the liquid flows upward and over a weir. The “clarified” liquid then flows from
the primary clarifier to further treatment called secondary or biological treatment.
Approximately 50% – 60% of the solids contained in the raw wastewater are removed
in the primary clarifiers. Settled solids collect on the floor of the clarifier
to form what is known as a sludge blanket. The clarifiers are equipped with a rotating
arm mechanism called a rake that moves the settled sludge to the center of the clarifier
tank. In the center bottom of the clarifier there is a hopper where the sludge is
concentrated and from which sludge is periodically withdrawn and pumped to the anaerobic digesters
for treatment.
Before implementation of secondary treatment standards by the EPA, many wastewater
plants across the nation were only capable of performing primary treatment before
discharging back to the environment.