Our Friendly Kingdom
In the secondary treatment process, microorganisms use organic pollutants as their food supply. In trickling filters or biodiscs the microorganisms are attached to a surface and form a biological film. The microorganisms do the purification. Our job is to provide them a friendly environment for their life cycle.
Secondary Biological Treatment: Trickling Filters
Mimicking Nature
There are two stages in the Trickling Filter type of biological secondary treatment. They are modeled after what happens in nature to clear and purify water. In nature, wastewater moves over rocks, which are home to 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.
Facilitating Natural Processes
While these processes can and do take place naturally in streams where there is only a relatively small amount of waste, more people creating more waste 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.
Trickling Filters
"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 put there. 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. They use the oxygen to break down organic matter present in the water. That's why they are called "aerobic bacteria."
Secondary Biological Treatment: Bio Discs
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.
Secondary Physical Treatment: Secondary Clarifiers
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.