Secondary & Tertiary Treatment

During secondary treatment, organic material is removed through biological treatment. The most widely used biological treatment method is the activated sludge process. The activated sludge process requires an aerated tank containing bacteria that break down the organic materials. The bacteria use the organic material in the liquid and clump together to form a microbial floc, which is also known as activated sludge.

This liquid flows into the secondary clarifiers where the activated sludge is allowed to settle. In some wastewater treatment plants, ferric chloride is added after biological treatment to cause precipitation of phosphate materials remaining in the liquid. Flow enters the clarifiers from the bottom of the tank through a pipe located at the center of the tank. The clarifiers are designed to direct the flow from the center of the clarifier in a downward direction to encourage the solids to settle. The activated sludge settles at the bottom of the secondary clarifier. Some of the settled activated sludge is collected and is returned to the aeration tank to insure sufficient bacteria and organic waste supply to maintain the biological process.

This material is called Return Activated Sludge (RAS). The activated sludge not needed for the biological process is called Waste Activated Sludge (WAS) and will be pumped to the sludge conditioning stage for further processing. The clarified liquid, with over 95% of the organic materials removed, flows to the tertiary treatment stage. Scum, formed on the top of secondary clarifiers is sent to a thickener for solids processing.

The tertiary treatment stage normally starts with the filtering of the clarified liquid that flows from the secondary clarifiers.  The liquid is processed through a bed of sand or other filtering device that removes additional pollutants from the liquid.   

The water then moves to the disinfection tank to kill the microorganisms remaining in the wastewater that may be harmful to fish life. 

The water or effluent can now be discharged into the ecosystem.