Waste Water Treatment  Elmhurst College
Secondary Treatment Sludge Treatment  Chemistry Department
Final Treatment    Virtual ChemBook

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Waste Water Treatment Plant
Elmhurst, Illinois
A Virtual Tour
Secondary 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.

Primary Clarifiers

Next, the sewage is directed to one of four primary clarifiers (primary settling tanks). The primary clarifiers remove the larger suspended solids and floating material from the degritted waste water prior to discharge to the aeration tanks. This significantly reduces the load on the aerators and increases efficiency. The clarifiers can effectively remove 50 to 60 percent of the suspended solids and 25 to 40 percent of the BOD (Biochemical Oxygen Demand) from the waste water.

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Secondary Treatment - Aeration Tanks

After leaving the primary clarifiers, the sewage goes to any one of ten aeration tanks. Elmhurst uses a system of sewage treatment called activated sludge. The aeration tanks provide a location where biological treatment of the waste water takes place. In these tanks, microorganisms and waste water in various stages of decomposition are mixed, aerated, and maintained in suspension.

The contents of the aeration tanks, which require a delicate balance of food and oxygen, are commonly referred to as the mixed liquor suspended solids (MLSS) or activated sludge. The activated sludge converts organic substances into oxidized products and a settleable floc which is settled out in the secondary clarifiers. The aeration tanks have a great deal of flexibility built into them. Raw sewage can be introduced in various locations and be aerated and mixed for varying lengths of time and intensity.

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Carbon Cycle - Metabolism Reaction
Bacterial Decomposition of Organic Waster

Metabolism occurs in animals, humans, and in this case Bacteria, after the ingestion of organic plant or animal foods. Organic materials contain at least carbon and hydrogen and may include oxygen. In the cells a series of complex reactions occurs with oxygen to convert for example glucose sugar into the products of carbon dioxide and water and ENERGY. This reaction is also carried out by bacteria in the decomposition/decay of organic waste materials in the water.

The bacterial decay of organic matter occurs naturally to a limited extent in lakes, ponds, and streams. This natural process is designed to decompose dead or decaying plant and animal life naturally present in the water. The amount of organic matter present in water is measured indirectly by by BOD. BOD, Biological Oxygen Demand, measures the oxygen needed by bacteria to metabolize the organic matter during a 5 day period of time. A high BOD level means that there is a high level of organic waste present. In a natural water system the amount of dissolved oxygen in water is limited by solubility and the temperature of the water. A maximum of 10 -12 ppm dissolved oxygen may present in natural water.
If municipal sewage waste is not treated but directly discharged into a stream, the BOD or organic waste level is very high and quickly overwhelms the capacity of the natural bacteria and the available oxygen to decompose the waste. At the sewage treatment plant, the activated sludge process is designed to use natural bacteria in a holding tank with the organic waste that is aerated with air (oxygen) until the organic waste is completely decomposed. The water with a low organic level or low BOD can now be safely discharged to a nearby stream.
An important summary statement is that during combustion/metabolism of organic waste, oxygen is used and carbon dioxide is a product. The whole purpose of the process is to decompose and breakdown organic waste into carbon dioxide, a gas emitted to the atmosphere, and unreacted solids which may be removed by settling and filtration.
See Natural Cycles and Carbon Cycle.