Every day, wastewater goes down toilets and drains in homes, schools, businesses and factories which ends up at the Madison Metropolitan Sewerage District. The district removes contaminants from wastewater before it is released to local watersheds. How is this done?
At the district plant, physical and biological processes are used to speed up the natural process of purifying water. The process closely mimics how wetlands, streams and lakes naturally purify water. The processes used to purify the water produces solid material called biosolids which is used by farmers as fertilizer.
You may be wondering how the water gets to the plant and does water really get “cleaned”?
After the wastewater leaves a house, school, etc…, it ends up in the sewer system. The sewer system is made up of several thousand miles of sewer pipes that carry the water to a pumping station. The district owns 18 pumping stations, but maintains a total 61 pumping stations that are spread throughout Madison
, and the surrounding municipalities. The pumping stations then transport the wastewater to the district plant where approximately 42 million gallons of wastewater end up daily.
Once wastewater gets to the plant, the treatment process begins. To help explain the treatment process we have 2 friends, Drip and Drop, who will take you through the treatment process at the District.
When Drip and Drop and all of the incoming water, called influent, enter the district plant, they pass through fine screens to remove rags and other large material such as: paper towels, feminine hygiene products and flushable wipes. All of the wastewater passes through ¼” round holes on a screen. Drip and Drop head to the Grit Chambers next.
The wastewater is about 99.9% water. A vortex action is used in the grit chambers to allow sand, gravel, and other inorganic solids to accumulate in the bottom of the Grit Chamber Tanks.
Primary Settling tanks
Primary Settling Tank is the next stop for Drip and Drop, these tanks are about 9 feet deep and anywhere from 86 to 100 feet long.
As Drip and Drop enter the primary settling tanks, also referred to as sedimentation tanks, they are slowed down to allow heavier solids to settle to the bottom of the tanks. The settled solids are called primary sludge. Drip and Drop spend about 2 hours in the primary settling tanks. Next up is Drip’s favorite place!
This stop is sometimes referred to as Secondary Treatment. This is where the biological treatment begins. Air is pumped into large mixing tanks that mix the wastewater with bacteria and microorganisms that will consume the organic matter and nutrients that contaminate the water as food. Drip doesn’t mind the 12 hour bubble bath!
Final Clarifying Tanks
At this stop you can often see some of Drip and Drop friends: ducks, geese and other birds. The aerated wastewater with the bacteria and microorganisms, called “mixed liquor”, flows to the final clarifying tanks. This four hour process allows the microorganisms to be separated from the clean water by settling to the bottom of the tanks. Ninety percent of these microorganisms are sent back to the aeration tank to clean more water. The other ten percent of the microorganisms are pumped to the solids handling process to be made into fertilizer. The water leaving the final clarifiers has completed the secondary treatment.
UV Light Chambers
In the final step of the cleaning process, the treated water, called effluent, passes through disinfection chambers designed to kill disease-causing bacteria with ultraviolet light. The ultraviolet light disinfection system was installed in 1986; at which time it was the world’s largest system.
In less than 24 hours Drop can return to her favorite place, the environment, so she can see friends and start the recycling process all over again.
Earth has a limited amount of water. Through the water cycle, Earth naturally cleans dirty water so that water can be used by plants, animals and humans. It is very important that we protect this natural resource!