Secondary treatment uses biological processes in which aerobic microorganisms break down organic material in the remaining wastewater (after separation from grit and solids in preliminary and primary treatment). These microorganisms use organic pollutants as their food supply.
Facilitating Natural Processes
These processes can and do take place naturally in streams and rivers. In nature, wastewater moves over rocks, which are home to protozoa, fungi, bacteria and algae. The dissolved organic material in the wastewater is food to them. Each type of organism performs a different function in the cleaning of the water as it moves through the streambed.
Cities and urban areas produce too much wastewater for the organisms in a small body of water like the Kishwaukee River to handle. At the DeKalb Sanitary District plant, we create a larger community of microorganisms and provide them optimal conditions in which to do their work to mimic nature’s process faster and more efficiently.
The wastewater treatment plant at 303 Hollister Avenue uses both an older Trickling Filter (installed circa 1950) and BioDisc Facility (installed 1980) and a newer Activated Sludge Facility (installed 1998) for accomplishing secondary biological treatment. The “secondary splitter” divides the wastewater flow between the two.
Old Secondary Treatment Process
“Trickling Filters” do the same thing as the “trickling through rocks” process. The circular tanks contain rocks on which bacteria, protozoa and other organisms grow. The slimy coating of living organisms on the “media” (rocks, plastic, or other coarse material) doesn’t have to be provided. It develops naturally. The wastewater is sprinkled onto the surface of the filter media from a rotating arm above the tank.
As the sewage is sprayed into the air, it absorbs the oxygen that the organisms need to do their work. The organisms use the oxygen to break down organic matter present in the water. That’s why they are called “aerobic bacteria.”
At the DeKalb plant, wastewater moves from the trickling filters into the biodiscs. The “bio discs” do the same thing as the Trickling Filters, except that huge discs on which the organisms grow rotate through the wastewater instead of the wastewater passing over them.
The organisms on the “rotating biological contactors” (RBC’s) are somewhat different than those on the trickling filter beds, but they function in a similar manner. Bio Discs remove ammonia from the wastewater by bringing aerobic bacteria into contact with it and with oxygen.
In each of the stages above, the water is allowed to settle after the biological processes have been successful. The clarified wastewater moves on to the next process, leaving the beneficial organisms behind to treat another batch of wastewater.
By the time this stage of treatment has been completed up to 95% of the original pollutants will have been removed from the wastewater.