Sludge Sinks and Scum Floats
The goal of primary treatment is to remove as much of the suspended solids as possible. From 40 to 60 percent of the solids are removed from the wastewater stream as sludge and scum during the primary phase of treatment.
Primary Physical Treatment: Clarifiers
The wastewater is pumped into one of three primary "clarifiers." These are large round settling tanks. Here the water that has been flowing rapidly is slowed down.
Organic particles denser than water (sludge) sink to the bottom of the tank in a process called sedimentation. Particles less dense than water (scum) float to the surface in a process called floatation. Rotating arms scrape the surface of the water to collect the scum. The scum and the sludge are sent to the digesters for further treatment.
Primary Effluent
The water that is left is called the primary effluent. It flows by gravity to the next step in the process of its purification.