Microbes in Sewage Treatment — Core Principles

Core Principles

Sewage treatment is the process of removing contaminants from wastewater before its discharge or reuse. It's crucial because untreated sewage, rich in organic matter, nutrients, and pathogens, pollutes water bodies and spreads diseases.

The process typically involves primary (physical), secondary (biological), and sometimes tertiary (advanced) treatment stages. Primary treatment removes large solids and grit through screening and sedimentation, yielding primary sludge and primary effluent.

Secondary treatment is the core biological stage, where aerobic microbes in aeration tanks form 'flocs' to consume dissolved organic matter, significantly reducing the Biological Oxygen Demand (BOD). The treated water then goes to secondary settling tanks, where flocs settle as 'activated sludge.

' A portion of this activated sludge is recycled to maintain microbial populations, while the excess, along with primary sludge, is sent to anaerobic sludge digesters. Here, anaerobic microbes break down organic matter, producing 'biogas' (rich in methane) which can be used as an energy source.

The final treated water, or secondary effluent, is much cleaner and safer for the environment. Microbes are the unsung heroes, transforming harmful waste into less toxic substances and useful energy.

Important Differences

vs Primary Treatment vs. Secondary Treatment of Sewage

Aspect	This Topic	Primary Treatment vs. Secondary Treatment of Sewage
Nature of Process	Primarily a physical process.	Primarily a biological process.
Pollutants Removed	Removes large floating debris, grit, and settleable suspended solids.	Removes dissolved and colloidal organic matter.
Microbial Involvement	Minimal to no direct microbial activity for pollutant removal.	Extensive involvement of aerobic microorganisms (bacteria, fungi, protozoa) in flocs.
BOD Reduction	Reduces BOD by approximately 20-30% (due to removal of settleable organic solids).	Significantly reduces BOD by 85-95% (due to microbial degradation of organic matter).
Key Equipment	Screens, grit chambers, primary settling tanks (clarifiers).	Aeration tanks, secondary settling tanks (clarifiers), activated sludge recycling pumps.
Output	Primary effluent (still high in BOD) and primary sludge.	Secondary effluent (low BOD) and activated sludge.

Primary treatment focuses on the physical separation of larger solid particles from raw sewage, acting as a preliminary step to reduce the load on subsequent stages. It's a mechanical process that doesn't rely on biological agents. In contrast, secondary treatment is the core biological purification stage, where aerobic microorganisms actively break down the dissolved and colloidal organic pollutants that remain after primary treatment. This microbial action is crucial for drastically reducing the Biological Oxygen Demand (BOD) of the wastewater, making it significantly cleaner and safer for discharge into the environment. Understanding this distinction is fundamental for NEET aspirants.