Wastewater Treatment Process

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226 West 4th Street
Davenport, IA 52801
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Wastewater Treatment Process

Raw sewage, or wastewater, is delivered to the Water Pollution Control Plant by gravity, through a network of hundreds of miles of sewer pipe. The incoming wastewater, which is usually gray in color, looks like dirty dish water. It is approximately 99.5% water and 0.5% pollutants.

The Federal Clean Water Act gives authority to the Iowa Department of Natural Resources to regulate wastewater treatment discharge into natural waterways. The IDNR in turn issues a National Pollutant Discharge Elimination System (NPDES) permit for each treatment plant. This permit sets measurable parameters and compliance limitations on the amounts of pollutants that a treatment plant can discharge. The permit issued to the City of Davenport allows this plant to discharge treated wastewater to the Mississippi River. It states that there must be at least an 85% removal of the Total Suspended Solids (TSS) and Carbonaceous Biochemical Oxygen Demand (CBOD) that is contained in the plant influent.

The goal of this plant is to see that our effluent meets the permit requirements. Being a continuous operation plant, flow and treatment are monitored 24 hours per day, seven days per week.

The average wastewater flow through the plant is 22 million gallons per day. (During heavy rains or flooding situations the flow may reach 60 MGD.) The incoming wastewater contains an average of 150 mg/l of CBOD and 200 mg/l of TSS. The outgoing effluent averages below 15 mg/l of CBOD and TSS. The removal rate for CBOD and TSS is above 90%.

The cities' network of sewers terminates in a 96 inch diameter pipe that is big enough to drive a small car through. Forty-five feet below ground level, the pipe delivers wastewater to the Wet Well, which is the start of this plant. The initial treatment, the removal of solids, begins here. A bar screen traps the larger solids that flow in with the sewage (plastics, sticks, cans, etc.). A giant rake cleans the bar screen and the solids are deposited in hoppers for removal by truck to the landfill.

After the grit tanks, the wastewater enters one of the 4 round primary clarifier tanks. Each of these tanks holds approximately 1.3 million gallons. Depending on the rate of flow and the operating conditions, the wastewater will stay in these tanks for 2 to 4 hours. This allows heavier solids to settle and be removed from the cone shaped bottom of the tank. It also allows grease and other floatable materials to rise to the top of the tank where they can be skimmed off. About 65% of the organic solids are removed here. 

 From the primary tanks, the flow moves to the activated sludge area. Activated sludge is a biological treatment process. In eight large rectangular tanks, which hold 750,000 gallons each, the nutrient-rich wastewater is mixed with air to provide oxygen for a previously developed, very dense population of microorganisms.
Activated sludge is made up primarily of bacteria, fungi, and protozoa. In this oxygen- and nutrient-rich environment, the microorganisms carry on life functions at an accelerated rate. In approximately 6 hours, they consume the pollutants, reproduce, and generate a waste by-product of their own.
The wastewater is then sent to four secondary clarifiers. These tanks are similar to the primary clarifiers. It takes about 5 hours for the flow to make its way through the secondary clarifiers. Movement is again slowed and the activated sludge is allowed to settle and is then returned to the aeration tanks. (A relatively small amount of this settled activated sludge is "wasted" to a thickener tank and then to 1 of 4 anaerobic digesters. A skimmer also removes any floating material from the tanks. The treated effluent water, which is now clear, flows through a tunnel to the Mississippi River. Many wastewater treatment plants are required to chlorinate their effluent, but because of the high quality of our effluent and the size of our receiving stream, we are not required to do so.