The Treatment Process
The Council Bluffs Water Pollution Control Plant treats an average of 7 million gallons of wastewater per day. During storm events, flows to the Plant have reached as high as 30 million gallons per day. The City has implemented an aggressive program to identify and remove storm water inflow sources from the sanitary sewer collection system. Many storm water sources have been removed, and more are being identified.
The wastewater treatment process itself consists of three very distinct steps. The first step involves a mechanical process to screen materials such as paper and plastic from the water. The water then passes through basins where the heavy grit, sand, rock, and other materials are removed. All material removed in this first step is collected and disposed of via County operated landfill.
The second step is to capture and treat the organic solids in the waste stream. This is accomplished by slowing down the velocity of the water in tanks called clarifiers. In the clarifiers, the organic particles are allowed to settle to the bottom of the tanks where they are collected, then pumped to another set of tanks called digesters. In the digesters, organic solids are retained for approximately 30 days. During this time, the temperature in the tanks is maintained at 99 degrees Fahrenheit. This temperature is optimum for a type of bacteria that uses the organic solids as a food source. The digester process not only reduces the volume of the organic mass, but it also serves to stabilize the material and reduce it essentially to nitrogen, phosphorous, and potassium. We call the stabilized material BIOSOLIDS. Farmers call this FERTILIZER, and indeed, it is. Our staff, under the guidance of regulations from the State of Iowa and the United States Environmental Protection Agency (EPA), applies this stabilized material to agricultural land. The City has approximately 3,200 acres of land under agreement with local land owners who wish to participate in our biosolids application program. The City also owns approximately 60 acres near the Plant facility that are included in the program. Beneficial use of biosolids on agricultural land is something that is not only recognized and encouraged by governing officials, but it is also gaining wide acceptance among the public.
The third step in this process is to treat the water remaining in the clarifiers after the heavy organic solids are collected and removed. Bacteria are again put to work. The water is sent to a series of reactor tanks. There are two types of reactor tanks: fixed film and suspended growth. In both types, there exists a colony of bacteria that thrive on the soluble organics in the water. Again, the water is “retained” in these tanks for a period of time to allow the bacteria to absorb all the soluble organic “food” it can. Once these bacteria have done their job, the water quality has improved to a point where it is nearly ready to go. To settle out any remaining solids, the water passes through another set of clarifiers before it leaves the Plant. The remaining solids are returned to the plant headworks, and the clarified water is discharged to the Missouri River.