Comprehensive Notes on Primary and Secondary Sewage Treatment

The initial steps of sewage treatment involve the physical removal of both large and small particles from the sewage.
Removals are conducted in stages through two main processes: filtration and sedimentation.
Sequential filtration is used to remove floating debris first.
Following filtration, the grit, which consists of soil and small pebbles, is removed through the process of sedimentation.
The solids that settle during this stage form what is known as primary sludge.
The supernatant, or the overlying liquid remaining after solids have settled, forms the primary effluent.
This primary effluent is then moved from the primary settling tank to the secondary treatment stage.

Secondary treatment is often referred to as biological treatment because it utilizes microbial activity to treat the effluent.
The primary effluent is transferred into large aeration tanks.
In these tanks, the effluent is constantly agitated mechanically, and air is pumped into the mixture.
Mechanical agitation and air infusion facilitate the vigorous growth of useful aerobic microbes.

The aerobic microbes grow into structures called flocs.
Flocs are defined as masses of bacteria associated with fungal filaments that form mesh-like structures.
As these microbes grow, they consume a major part of the organic matter present in the primary effluent.
The consumption of organic matter by these microbes results in a significant reduction of the Biochemical Oxygen Demand (BOD) of the effluent.

BOD refers specifically to the amount of oxygen that would be consumed if all the organic matter in $1 \,\text{liter}$ of water were oxidized by bacteria.
The sewage water undergoes treatment continuously until the BOD level is sufficiently reduced.
A BOD test is used to measure the rate of oxygen uptake by microorganisms in a sample of water.
BOD serves as an indirect measure of the amount of organic matter present in the water.
There is a direct relationship between BOD and pollution: the greater the BOD of the wastewater, the higher its polluting potential.

Once the BOD of the sewage or wastewater is significantly reduced, the effluent is passed into a settling tank.
In the settling tank, the bacterial flocs are allowed to sediment.
This sedimented microbial mass is called activated sludge.
A small portion of the activated sludge is diverted and pumped back into the aeration tank to serve as an inoculum, providing the microbial starter for the next batch of primary effluent.

The remaining major portion of the activated sludge is pumped into large, specialized tanks known as anaerobic sludge digesters.
In these digesters, anaerobic bacteria (bacteria that grow in the absence of oxygen) digest the aerobic bacteria and fungi present in the sludge.
During the digestion process, these bacteria produce a mixture of several gases.
The gas mixture includes:
- Methane
- Hydrogen sulphide
- Carbon dioxide
This mixture of gases is known as biogas.
Biogas is highly inflammable and can be utilized as a source of energy.

After completion of secondary treatment and the reduction of biological pollutants, the effluent is considered treated.
The treated effluent from the secondary treatment plant is generally released into natural water bodies.
Typical release points for treated effluent include rivers and streams.