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
Sludge 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.

Sludge/Solids Treatment

Now, let's backtrack a little and see what happens to the solids which have been settled out of the liquid sewage. 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.

The City of Elmhurst is modifying its anaerobic digester design to separate the two phases of anaerobic digestion. New acid faced tanks have been installed on the east side of the existing north digester. These tanks will hold the raw and waste activated sludge for a short period of time, oftentimes less than 24 hours. The separation of these two processes will allow the different groups of bacteria that carry out the anaerobic digestion to perform in an optimum environment for each group. The acid producing bacteria will have their individual stage and the gas producing bacteria will have theirs. Experience has shown at other wastewater plants around the country that when these two processes are separated, the quality of the digestion is improved and the production methane gas is increased. The anaerobic digestion process will dissolve the solids to their basic components.

In the anaerobic digesters another group of bacteria begin to digest and dissolve the solids to their basic components. This process uses bacteria which do not need atmospheric oxygen to survive, so therefore, no air is bubbled into the tanks. In fact, air mixed with the gasses may be explosive, so we strive to keep all air out. The anaerobic digesters produce a stable sludge which is readily dewatered. The process is also a source of methane gas, which is used as a fuel source for heating the digesters, heating several buildings, and fueling the engine generator to produce electricity. The digester is kept at an optimum temperature of between 90-95 degrees F. About 40,000 cubic feet of methane gas is produced per day.

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Sludge Dewatering and Drying:

The engine generator runs on digester or natural gas. The generator supplies electrical power to essential pieces of treatment plant equipment. In the event of a complete power outage, important equipment will be powered by the engine generator. Waste heat is used to help heat surrounding buildings.

After most of the organic solids have been digested, the sludge is pumped to sand drying beds or to the belt filter presses. The belt filter presses use a chemical flocculent to separate the water from the solids. The dewatered solids are then squeezed between two belts to further dewater them. The resulting solids are in the range of 18-20 percent solids. These solids are applied to agricultural land. The solids can also be taken to a landfill. The sludge drying beds also provide a means of drying the sludge treated by the anaerobic digesters. As an alternative, the digested sludge may be pumped to the truck loading station to be hauled to other locations for drying or for use as fertilizer. Sludge is a good soil conditioner as well as fertilizer.

Elmhurst sludge has been approved by the Illinois Environmental Protection Agency for application on agricultural soils. It has very low concentrations of elements which by be toxic to the soil or plants. Dewatered sludge is stored on a sludge storage pad located south of the plant until there is a quantity great enough to take to local farms.