Let's begin by learning how Johnson County collects wastewater and transfers it to a treatment facility:
1. Collecting wastewater
What's the first step in treating wastewater?
Johnson County Wastewater and other wastewater treatment organizations clean used water in almost the same way that nature would, only we speed up the process. If the water was left alone in a stream or lake, it would naturally clean itself, but that could take a long time. The first thing we have to do, then, is collect the wastewater and transport it to a wastewater treatment plant.
The transfer system
Wastewater leaves your home through a service line that collects waste from sources like toilets, sinks, and dishwashers. It flows from the service line to a larger "main" sewer line that transports the flow to one of our wastewater treatment plants. In Johnson County's system there are slightly more than 2,200 miles of sewer line. They are usually made of plastic, iron, or clay. Sewer manholes allow us to access the sewer lines to inspect and clean them. Manholes can be very dangerous. Only Johnson County Wastewater personnel should open and enter them!
Sanitary sewers are not designed to carry rainwater, only used water from homes and businesses. Rainwater goes into storm sewers and creeks. You can see storm sewer inlets along the curb on many streets. Johnson County Wastewater uses pump or lift stations to help carry the wastewater to one of our plants. Lift stations are needed to pump the sewage to a geographic high point where gravity takes over and sewage flows to a treatment plant. After the wastewater reaches the plant, the treatment process begins! The first step is "Preliminary Treatment."
2. Preliminary Treatment
Usually, the first thing that happens at the treatment plant is the used water goes through a bar screen. The screen catches and removes large things from the water (such as paper cups, leaves, and sticks). The water goes through a grit removal chamber which removes heavy things like gravel, seeds, and coffee grounds (grit). When the water enters the chamber, the “grit” settles to the bottom and is removed. The material removed by the screen and grit chamber is usually disposed of in a landfill. The water then flows to the next stage of treatment - "Primary Treatment."
3. Primary Treatment
After grit and large solid materials are removed from the wastewater, many smaller solid particles still remain. To remove these particles, the wastewater is sent through large tanks called "clarifiers". In the clarifier, the flow moves very slowly and small solid particles settle to the bottom and are removed. This process is called "sedimentation". The material that is removed is called sludge. The sludge is treated separately. The remaining wastewater contains mostly dissolved wastes and goes to the next stage - "Secondary Treatment."
4. Secondary Treatment
The work of Secondary Treatment is a biological process accomplished by living organisms. Bacteria and other microscopic organisms grow by using the waste for food. This is a common process which occurs in nature. At the treatment plant, we just speed up the process. Wastewater treatment facilities create a utopia for microorganisms: optimal food, optimal oxygen, and plenty of space to grow. As the microorganisms feed on the waste we’ve flushed away, they grow and divide - very happily! And as the organisms feed and grow, the water is being cleaned. Microorganisms that feed on the waste in the water are "bacteria" and "protozoa." We sometimes call them the “bugs.” The wastewater has lots of food to help the microorganisms grow, but they also need lots of oxygen. The wastewater itself doesn’t have very much oxygen, so we have to make sure the environment has plenty of oxygen for them. Johnson County Wastewater uses two methods of making a good environment for the microorganisms to eat and grow, in turn, cleaning the water.
Trickling Filter Methods
At some of our treatment plants, we use "trickling filters" which pour water over tanks filled with rocks. The microorganisms live on the rocks which are surrounded by air. As the wastewater trickles past them, they eat the waste and grow. This trickling of the water also provides oxygen so the microorganisms can thrive.
Activated Sludge Method
Some of our treatment plants grow microorganisms in large tanks. Air is blown into the tanks full of wastewater and microorganisms to add oxygen to the environment. The air is bubbled in the water and mixes food and oxygen together for the “bugs.” When wastewater is treated in this way, we are using the "Activated Sludge Method." With all of this food and air, the microorganisms grow and multiply rapidly. It's not too long before the population of "bugs" is too large, and some must be removed in order to make room for new "bugs" to grow. The excess "bugs" are removed by sedimentation in the same kind of tanks used for Primary Treatment. In the tank, the "bugs" sink to the bottom and they are removed. The settled "bugs," along with settlement in the primary stage are both referred to as "Activated Sludge." The sludge is treated separately. The remaining wastewater is now much cleaner. In fact, after Primary and Secondary treatments, 85 percent or more of all pollutants in the wastewater have been removed. The remaining wastewater moves on to "Tertiary (meaning third) Treatment."
5. Tertiary Treatment
Before we release the treated water back to a stream or river, we kill any microorganisms that might cause disease. At some of our facilities, we use a solution that contains chlorine to kill them. Extra chlorine left in the wastewater is then removed by another chemical. At other facilities, we use ultraviolet (UV) light to disable and kill the microorganisms.
Where does wastewater go when we’re done? After the wastewater is cleaned, we put it back into a stream or river where it’s safe for fish to live and animals to drink. Wastewater that is cleaned and sent back to the streams is called "Effluent."