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Waste Water Treatment Plant
Elmhurst, Illinois
A Virtual Tour
Finishing Treatment
Text by Dennis Streicher, Assistant Director of
Public Works, Elmhurst, IL
Pictures, Chemistry , and Web Site by Charles Ophardt, Professor
of Chemistry, Elmhurst College, copyright 1999.
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Secondary/Final Clarifier Tanks:
After leaving the aeration tanks, the now treated sewage,
along with the bacteria, enter the secondary clarifiers. The
plant has a total of six secondary clarifiers. These tanks provide
a location where the activated sludge solids can be separated
from the liquid in the mixed liquor coming from the aeration
tanks.
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Chlorination Tanks:
The clear overflow in the final settling tank now goes to
the chlorine contact tanks (three tanks), for disinfection and
a final polishing to remove any solids still present. The chlorination
system is used to provide disinfection of the plant effluent
before final discharge to the receiving stream (Salt Creek).
Disinfection reduces the number of harmful, pathogenic (disease
causing) organisms that may be in the final effluent. After chlorination
a process of dechlorination takes place. Chlorine is a toxic
material and has been shown to be harmful, even in low dosages,
to the stream flora and fauna. In response to this, Illinois
is requiring all wastewater plants who use chlorine to disinfect
to remove that chlorine. Elmhurst is using a chemical compound
called sulfur dioxide. Sulfur dioxide neutralizes the chlorine
so it is not toxic to the stream.
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Final Water Stats:
After all processing has been completed, the final effluent
will contain approximately 3-5 mg/L of solids, or about 250 pounds
of dry solids in eight million gallons of water. This is about
a 97 percent reduction in total solids. In addition, the incoming
raw sewage will contain approximately 50,000 to 100,000 fecal
coliform bacteria, (an indicator of pathogenic organisms), per
milliliter (approximately 10 drops). The final effluent averages
approximately two fecal coliform bacteria per 100 ml. (four ounces).
This is better than a 99.999 percent reduction in bacteria.
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