The Collection System
Pre-Treatment
Primary Treatment
Secondary Treatment
Disinfection
Sludge Handling
Dewatering
Summary
Interesting Facts
In general, most people don't like to think about wastewater treatment. Once water is used and goes down the drain, it's "Out of sight, Out of mind." But many fascinating things occur as the wastewater is collected and treated before returning it to the environment.
Used water from our homes enters a vast underground network of pipes called sewers. These pipes convey the wastewater from our homes and businesses to a central location where it can be cleaned. In the collection system, the lines are laid with a slight downward slope so that the wastewater will flow naturally. We call this gravity flow.
By laying the sewers with a downhill slope, eventually the sewers would become too deep for construction to be practical. That is when a lift station is built. The lift station pumps the wastewater from a low elevation back up to a higher elevation where gravity flow can resume.
Manholes are located at specific intervals along the sewer line to allow workers access to the sewers for maintenance and repairs.
The wastewater eventually arrives at the treatment plant where several process are employed to remove the pollutants from the water. The various steps are outlined below.
Otherwise known as Pre-Treatment. This is where large objects and inorganic particles are removed. Surprisingly, there are a lot of different things in the wastewater stream that you may not think of. Things like: Toys, golf balls, sticks, tennis balls, cans, bottles........Well you get the picture.
This junk can cause severe problems if it is allowed to enter pumps and other equipment, so we screen it and remove it.
In addition to the trash that can be found in wastewater, there is a lot of inorganic particles such as gravel, sand, metal and metal particles. These items, called grit, can cause excessive wear to pumps and other equipment, thus the grit is removed before the wastewater flows to the main Treatment Plant.
The next step in the process is called Primary Treatment or Clarification. Throughout all of the previous mentioned treatment processes, the flow of the water has not been allowed to drop below 2.5 feet per second. This velocity has been determined to keep the solid particles suspended in the wastewater stream.
At the Primary Clarifier, the velocity is allowed to drop below 2.5 feet per second. A Primary Clarifier is a large tank that has skimmers that skim the surface of the water and skim the bottom of the tank. As the wastewater enters the tank, the velocity drops to about 1/2 a foot per second. This causes the larger particles to settle to the bottom of the tank as sludge and the grease and scum float to the top. The skimmers collect the scum at one end of the tank and the sludge at the other end. The sludge and scum are then removed to other portions of the plant for treatment.
The Primary Clarifier removes around 25% of the pollutants in the wastewater. The Weirton Wastewater Treatment Plant has 2 Primary Clarifiers that hold 424,000 gallons of wastewater.
The process that follows Primary Treatment is, of course, called Secondary Treatment. While the Primary Treatment process was mostly a physical process, Secondary Treatment is a combination of physical and biological processes. The wastewater that leaves the Primary Clarifiers has had nearly all of the larger heavier solids settle out. However, there are some solids that do not settle well and will pass through the Primary Treatment process. This is where the biological treatment process comes in.
The solids that are still in the wastewater are exposed to microorganisms that consume the wastes in the water. These microorganisms such as ciliates, rotifers, flagellates, and bacteria are naturally occurring organisms that can be found in nearly all streams, rivers, and lakes. People in the wastewater field refer to these microscopic creature as "bugs". The bugs see the pollutants in the wastewater as "food" and begin to "chow down" in a tank called an aeration basin.
The mixture of the bugs and wastewater is supplied with air from large compressors that give the bugs oxygen and provide mixing. After a period of time, the mixture of bugs and food flow to another section of the Secondary Treatment process called the Secondary Clarifiers. Weirton has 2 Secondary Clarifiers that hold approximately 486,000 gallons. Here, the mixture is allowed to settle. The now heavier solids and bugs settle to the bottom of the clarifiers and clean water exits the top.
This Secondary Treatment Process remove approximately 60% of the pollutants that were in the wastewater. Combined with the 25% that was removed in the Primary Clarifiers, gives us a total removal of about 85%.
The cleaned wastewater leaving the Secondary Clarifiers is almost ready for disposal in the Ohio River. While the wastewater has been substantially cleaned, there are still many types of bacteria present in the water. Some of these bacteria may be harmful to life. Before the water can be disposed of in the River, it must be disinfected.
The wastewater leaves the Secondary Clarifiers and flows to the newly installed Ultra-Violet (UV) disinfection unit. Here the wastewater passes in close proximity to UV lamps. The UV rays penetrate the wastewater, killing the harmful microorganisms that are present. The wastewater is then ready for disposal in the Ohio River.
So far, we've looked at the treatment of the wastewater as it passes through the treatment plant. But what about the stuff that's left behind? The stuff that settles out in the various tanks is called sludge. This sludge must be treated so that it can be safely disposed of. Weirton uses a new state-of-the-art process for treating sludge called Autothermal Thermophilic Aerobic Digestion of ATAD for short. The ATAD system uses large amounts of atmospheric air to create thermal conditions in the digester Reactors. The high heat produced in the process helps to break down the organic substances in the sludge. The remaining sludge is free from most disease causing organisms and can be used as a soil amendment much as you would use commercial fertilizer.
The last step in the sludge handling process is called dewatering. Dewatering means removing the water from the sludge. Weirton uses a Belt Filter Press to accomplish this.
The sludge is mixed with a chemical called Polymer which aids in separating the water from the sludge particles. This mixture is then pumped to the belt filter press which squeezes the water from the sludge leaving a fairly dry substance called Bio-Solids. The Bio-Solids are disposed of at a landfill. The water that is squeezed from the sludge is sent back to the beginning of the treatment plant for further treatment.
This has been a rather brief and simplified look at what happens to our wastewater as it leaves our homes and businesses and is treated to be safely returned to the environment. The actual treatment process is much more complicated than what has been described above. However, we hope that this overview gives you a little bit of understanding as to what is happening at The Weirton Wastewater Treatment plant.
Tours of the Weirton Wastewater Treatment Plant are available for groups of 5 or more. If your group is interested in touring our facility, please contact the plant.
The Weirton Wastewater Treatment Plant is designed to treat 4 million gallons of wastewater a day. However, during wet weather, it can treat a maximum of 20 million gallons in a day.
To put this in perspective, a typical bathtub holds around 40 gallons of water. The Weirton Plant can effectively treat 100,000 bathtub's full of water every day. That's equivalent to about 69 bathtub's of water every minute.
At a peak flow of 20 million gallons a day, the plant is processing about 347 bathtub's of water every minute.
People in the wastewater field measure the pollutants in the water in units called P.P.M. or parts per million. The wastewater that is received at the treatment plant has about 200 P.P.M. suspended solids in it. After it has been treated by the various plant processes, the water that leaves the plant has only about 20 P.P.M. suspended solids.
