The Final Settling Tanks

Secondary ClarifiersEffluent Pump

After leaving the aeration tanks, the now treated sewage, along with the bacteria, enter the secondary clarifiers. The plant has a total of six of 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.


The clear overflow in the final settling tank now goes to the chlorine contact tanks (3 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 has required 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.

Excess Flow PumpsFinal Effluent Content

After all processing has been completed, the final effluent will contain approximately three to five mg / L of solids, or about 250 pounds of dry solids in eight million gallons of water. This is about a 97% 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 ten drops).

The final effluent averages approximately two fecal coliform bacteria per 100 ml.( four ounces). This is better than a 99.999% reduction in bacteria.

SolidsDAF Tanks

The settled solids, from the primary clarifiers, are pumped to the digesters where the solids are stabilized. Activated sludge solids from the secondary clarifiers which are not returned to the aerators are wasted. The DAF (dissolved air flotation) thickener tanks receive the wasted solids. Solids enter the DAF tank where they are mixed with water and compressed air.

As the air and water mix, solid particles are lifted to the surface by rising air bubbles in the tank. The floating solids are then collected by a series of tank skimmers while the water is recycled back to the raw sewer to be processed through the plant. The solids from the DAF are pumped to the anaerobic digesters.