Sewage Treatment Plants (STP)

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?

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