Waste Water Treatment Plant - WWTP / WTP
These plants remove contaminants/sewage waste from households, commercial buildings and sometimes even get an inflow of commercial wastewater too. It can also receive rainwater and debris from sewers.
STPs play a critical role in keeping residents healthy and safe by cleaning wastewater with a bunch of chemical, physical and biological procedures before disposing it into the environment.
How does it work?
- The first step in this water treatment plant is that the wastewater drains to the plant with the help of gravity through the main sewer system.
- A mechanical stage then begins called the preliminary treatment or pre-treatment. In this stage, the water moves through the gravel chamber to remove any grit. The gravel is then disposed of at the dump. The water then moves to the bar screens which remove large objects. These are course screens. Next, the fine screens remove smaller objects such as undigested foods, or matches, etc.
- Similar to the gravel chamber, the grit chamber allows any grit in the wastewater to settle at the bottle. This is then removed from the tank and disposed of at the dump. Next comes the sedimentation stage, also known as the primary treatment. In this stage, the water flows to the primary settling tanks, also known as pre-settling basins. These tanks have hoppers which are situated in the base of the tank where water flows through. This hopper moves around the edges which results in the treated water staying at the edges and the particulates in wastewater that contains the highest sedimentation are settled on the bottom of the tank.
- After the primary treatment ends, the secondary treatment begins. This is also known as the biological stage, as it uses natural processes and bacteria that consume the contaminants in the water, such as any biodegradable organic compounds, carbon and phosphorus. This dead bacteria and organic residues then transform into sludge. During this stage, the excess sludge (any excess bacteria in the wastewater) is pumped out and moves to the settling tanks. These settling tanks enable the sludge to settle and then moves to digestion tanks.
- In the digestion tanks, the sludge is heated and mixed. Another important thing that happens here is the production of biogas, which the wastewater treatment plants can reuse, in the production of electrical or thermal energy which is another huge benefit to the environment.
- When the sludge reaches an optimal level in the digestion tank, the next digestion takes place in the storage tanks. This enables the separation of water from the semi-solid sludge, whereas the remaining sludge undergoes the mechanical process again for dewatering, that is taking out as much water from the sludge as possible. The sludge then left behind after the digestion and dewatering process is complete, is finally disposed of in the dump. Another interesting thing is that this sludge in about a months time, can be reused as an fertilizer for industrial crops, if it complies with all agricultural standards.
- The last step in wastewater treatment is inspection. This inspection involves checking the contamination level of the water treated and making sure it complies with the highest standards in order to be released or reused for domestic or industrial purposes.
Applications
Most petroleum refineries or petrochemical as well as chemical industries produce a large amount of wastewater and require on-site wastewater treatment plants. Other industries such as paper and pulp production also give rise to a huge amount of wastewater.
Sewage Treatment Plant - STP
Wastewater is the water that originates from water used in domestic, agricultural, industrial as well as medical or transport activities.
Basically, wastewater can be divided into two:
Sewage Water– This is the wastewater that comes from domestic activities such as toilets, showers, or even sinks.
Industrial Wastewater, on the other hand, comes from manufacturing, industrial and commercial activities carried out and has a completely different composition than sewage water.
How does it work?
When wastewater reaches a sewage treatment plant, it goes through a filtering process that acts as a pre-treatment process. The wastewater here flows through screens and into settlement basins that can take out debris in large quantities. It acts as a pre-treatment as stated above as it takes place before three more aggressive stages- primary, secondary and tertiary treatment.
Primary Treatment
During this phase, the wastewater moves into the clarifiers. The wastewater flows sluggishly in the settlement basins. It is the design of these tanks that result in settling, that is, the organic solid matter collects at the bottom of the tank while the lighter matter floats to the top becoming easier for removal.
The organic matter that settles at the bottom is known as a primary sludge blanket. After a few hours, the sludge that has settled in the clarification tanks moves into aeration basins to carry out another process called the activated sludge process.
Secondary Treatment
This treatment phase consists of aerobic aeration. Aeration basins contain aerators, these have a system of pipes or tubes attached to them. They are made of ceramic or rubber membranes that have small holes in them for air to pass through. When this air flows through the aerators, the small holes present, turn them into bubbles and they get mixed with the water column. This interaction of the oxygen with the bacteria in the sewage results in the bacteria digesting the organic matter that gives wastewater the characteristic appearance and smell.
It is this phase that is also involved in the removal of harmful chemicals. Once aeration is over, the wastewater flows in the next tanks, i.e, the secondary clarification basins. It is in this that the bacteria spend a day or two settling in the bottom and forming a sludge blanket, which the water treatment plant can then pump out.
After the sludge blanket has completely settled, it becomes return activated sludge (RAS). This RAS goes back into the primary clarification tank and the bacteria in it aids in breaking down any organic matter in the sewage.
Once RAS has completely gone through both the primary and secondary clarification basins continuously, i.e several times, it is turned into waste-activated sludge (WAS). The WAS then does not go back to the primary clarification tank but instead moves to the covered tanks, also known as aerobic sludge digesters. In these tanks, the bacteria do not digest the organic matter present in the wastewater, but the activated sludge begins to digest one another, causing most of the sludge to then disappear.
Finally, the remaining sludge moves to the dewatering facility that contains dewatering tanks where the plant uses belt presses to squeeze any remaining water out of the sludge.
Tertiary Treatment
Tertiary treatment follows the process of both primary and secondary processes but also in addition involves mechanical and photochemical processes. This is a more advanced treatment and is very useful in sanitary sewage with microorganism contaminants that need to be disinfected.
In this phase of the treatment, wastewater is passed through sand filters, which remove very fine particulate matter. The photochemical process comes in after this, where the water flows under ultraviolet (UV) lights, which eliminate any bacteria and viruses as well as remove any infections.
Thus, once the wastewater in the sewage water treatment plant has undergone the three mentioned stages above, it is completely safe to flow into the environment as an effluent.
Applications
A great example of STP is the treatment plant you find in big cities. Residential households, commercial buildings, municipal wastewater, etc.
