TOC > Ch. 11 > 11.6 Secondary treatment

Chapter 11 Wastewater Treatment

James R. Mihelcic, David W. Hand and Martin T. Auer


Secondary treatment

Activated Sludge

Microorganisms such as bacteria and protozoa can use the small particles and dissolved organic matter, not removed in primary treatment, as food. Secondary or biological treatment is performed in a tank containing a "soup" of starved microbes called activated sludge. Like us, these microbes require air to live (they are aerobic organisms) and thus air is pumped into the tank. Microorganisms in this aeration tank use the dissolved and particulate organic matter as food, producing more microorganisms. Thus the waste materials that left the primary clarifier are changed into microorganisms which can be collected and separated from the water in the next step.

Empty Aeration Tank

Empty aeration tank showing air diffusers

Operating Aeration Tank

Operating aeration tank

Secondary Clarifier

It then remains to separate out the microorganisms (activated sludge) so that just clean water is left. This is done in a secondary clarifier which operates in the same manner as the primary clarifier described previously. Some of the solids collected in the secondary clarifier (return activated sludge) are sent back to the aeration tank to treat more wastewater and the excess (waste activated sludge) is pumped to another location in the plant for further treatment. The clean water that flows out the top of the clarifier is sent along for disinfection.

Activated Sludge Tank

Wastewater leaving the activated sludge aeration tank on its way to the secondary clarifier

Secondary Clarifier

Secondary clarifier covered to prevent icing


Wastewater leaving the secondary clarifier

Secondary Clarifier Operation

The effluent from the activated sludge aeration basin flows to the secondary clarifier where the mixed liquor suspended solids are separated from the liquid (final effluent). The 4 minute 40 second video presented here offers a time-lapse presentation of the settling process.

This video is close captioned.


Empty Aeration Tank photo: Andy Whaley (Michigan Tech).
Operating Aeration Tank photo: Andy Whaley.
Wastewater Aeration Tank animation: © 2007 State of Michigan.
Wastewater Leaving Aeration Tank photo: Andy Whaley.
Secondary Clarifier Tank photo: Andy Whaley.
Secondary Clarifier Weir photo: Andy Whaley.
Secondary Clarifier Video: © 2010 Michigan Technological University.

Thanks to Dr. Ernest R. "Chip" Blatchley III, Professor of Civil Engineering at Purdue University for providing the idea that resulted in the secondary clarifier video.

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