The goal of primary treatment is to physically remove the suspended solids from the wastewater. From 40 to 60 percent of the solids are removed from the wastewater stream as sludge and scum during the primary phase of treatment.
Removal of Suspended Solids
Sedimentation and Floatation: 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. The “setteable solids” are collected and pumped to the digesters for further treatment.
Particles less dense than water (scum), grease and oil float to the surface in a process called floatation. Rotating arms scrape the surface of the water to collect the scum, which is also sent to the digesters for further treatment.
The water that is left is called the primary effluent. It flows by gravity to the next step in the process of its purification.