Elmhurst, Illinois Waste Water Treatment Plant
A Virtual Tour
Text by Dennis Streicher, Assistant Director of Public Works, Elmhurst,
Pictures, Chemistry , and Web Site by Charles Ophardt, Professor of Chemistry, Elmhurst College, copyright 1999.
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 Wastewater Treatment Plant had a record high flow of over 74 million gallons in one day in February, 1997. An average day's flow of raw sewage entering the Elmhurst WWTP results in approximately 6,540 pounds of dry solids after concentration of suspended solids has been performed.
The Elmhurst WWTP has the capability of providing grit removal, primary treatment, extended aeration, final clarification, and disinfection of wastewater. This treatment is accomplished by the movement of the sewage through a series of treatment tanks.
You'll notice that most of the equipment at the Wastewater Treatment
Plant is provided with an equal back-up. There are more raw sewage pumps
than are necessary as well as grit removal equipment and sludge, pumps,
etc. Illinois EPA design criteria require that there be this redundancy
to ensure reliable operation of the Wastewater Plant in the event of the
largest treatment unit being out of service.
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.
Raw Sewage Pumps
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 taken later 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 auger pump 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 removal systems
which are rotated in operation for equal hours.
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 to 60
percent of the suspended solids and 25 to 40 percent of the BOD (Biochemical
Oxygen Demand) from the wastewater.
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 wastewater takes place. In these tanks, microorganisms and wastewater 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.
Activated Sludge Aeration Tanks
Chemistry/Biology of the Activated Sludge Process
Secondary/Final Clarifer 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.
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.
Excess Flow Pumps
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.
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.
Anaerobic Sludge Digester
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.