The Wastewater Treatment Plant Virtual Tour
RAW SEWAGE PUMPS
The Elmhurst Wastewater Treatment Plant (WWTP) has the capacity to fully treat 20 million gallons of wastewater per day (mgd) and can give primary treatment to an additional 40 mgd of wastewater, if necessary, during excess flow conditions. The concentration of suspended solids an average days flow of raw sewage entering the Elmhurst WWTP is approximately 6,540 pounds of dry solids. [Flow Schematic]
The process employed at the Elmhurst WWTP are designed to remove grit, provide primary treatment, extended aeration, final clarification, and disinfection of the wastewater. This treatment is accomplished by the movement of the sewage through a series of treatment tanks.
BAR SCREENS |
PRELIMINARY TREATMENT
The sewage is first pumped using three enclosed screw raw sewage pumps which provide a means of lifting the incoming sewage from the sewer system. The sewage then passes through the bar screens for rag removal. In this section two automatic bar screen cleaners remove large solids (rags, plastics, etc.) from the raw sewage. The collected material is placed in dumpsters to be later taken to the landfill. The action of the bar screen equipment is paced according to the amount of incoming solids and the flow rate. Next,the sewage moves to the grit tanks. These tanks reduce the velocity of the sewage so that heavy particles may fall to the bottom. The solids are pumped to an grit washing system which separates the water from the grit while the water moves onward. The grit (mostly inorganic solids) goes to a dumpster which is taken to a landfill. There are two complete grit washing systems which are rotated in operation for equal hours.
  GRIT TANKS |
 GRIT WASHING SYSTEM |
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THE 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 wastewater prior to discharge to the aeration tanks. This significantly reduces the load on the aerators and increases efficiency. The clarifiers can effectively remove 50 - 60% of the suspended solids and 25 - 40% of the BOD5 from the wastewater.
 AERATION TANKS |
THE 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 wastewater takes place. In these tanks, micro-organisms and wastewater in various stages of decomposition are mixed, aerated, and maintained in suspension.
- History of Activated Sludge
- The Genesis And Evolution Of Activated Sludge Technology
- Biology of the Activated Sludge Process
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.
 EFFLUENT PUMP |
THE FINAL SETTLING 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 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 PUMPS
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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 8 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.
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 form the DAF are pumped to the anaerobic digesters.
DIGESTERS
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 50,000 cubic feet of methane gas is produced per day.
 ENGINE GENERATOR |
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.
DRYING BEDS
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% solids. These solids are applied to agricultural land. If agriculture fields are not available, the solids would be taken to a landfill; however, this is expensive and a waste of valuable soil conditioner. 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 may be toxic to the soil or plants.