During which stage of the purification of the sewage water are microbes used?

Correct Answer: Secondary treatment

Sewage water purification, also known as wastewater treatment, is a multi-step process designed to remove contaminants before the water is discharged back into the environment. It generally consists of three main stages:

1. Primary Treatment (Physical Process):
This stage involves the physical removal of large and small particles from the sewage through filtration and sedimentation. Floating debris is removed by sequential filtration, and grit (soil and small pebbles) is removed by sedimentation. Microbes are not involved in this physical separation phase.

2. Secondary Treatment (Biological Process):
The primary effluent (liquid waste) is passed into large aeration tanks where it is constantly agitated and air is pumped into it. This environment promotes the rapid growth of useful aerobic microbes into "flocs" (mesh-like structures formed by bacteria associated with fungal filaments).
These active microbes consume the major part of the organic matter present in the effluent. As they digest the organic waste, they significantly reduce the Biochemical Oxygen Demand (BOD) of the wastewater. Because this stage relies entirely on the biological activity of microorganisms to clean the water, it is also referred to as biological treatment.

3. Tertiary Treatment (Chemical/Physical Process):
This is the final polishing stage where the water is disinfected using chemical or physical agents such as chlorine, ozone, or ultraviolet (UV) light to kill any remaining pathogens and remove dissolved inorganic nutrients (like nitrogen and phosphorus) before the water is discharged or reused.

Since the biological degradation of organic waste by bacteria and fungi occurs during the second phase, microbes are specifically used in the Secondary treatment stage of sewage purification.