Microbes in Sewage Treatment — Core Principles

Core Principles

Sewage treatment is the process of removing contaminants from wastewater before it's discharged into the environment. It's crucial because raw sewage contains harmful organic matter and pathogens that can pollute water bodies and spread diseases.

The process typically involves primary (physical) and secondary (biological) treatment stages. In primary treatment, large solids are removed through screening and sedimentation. The real purification happens in secondary treatment, where aerobic microbes, forming 'flocs', consume dissolved organic matter in aeration tanks, significantly reducing the Biological Oxygen Demand (BOD).

The settled microbial masses are called 'activated sludge', a portion of which is recycled to maintain microbial activity. The excess sludge undergoes anaerobic digestion, where anaerobic microbes produce biogas (methane, carbon dioxide).

The treated water is then disinfected and safely released, protecting aquatic life and human health.

Important Differences

vs Primary Treatment vs. Secondary Treatment in Sewage Treatment

Aspect	This Topic	Primary Treatment vs. Secondary Treatment in Sewage Treatment
Nature of Process	Primarily a physical process.	Primarily a biological (microbial) process.
Main Goal	Removal of large floating debris, grit, and settleable suspended solids.	Degradation of dissolved and fine suspended organic matter, significant reduction of BOD.
Microbial Role	Minimal direct microbial involvement in the main removal steps; microbes are present but not actively harnessed for degradation.	Central role of aerobic microbes (bacteria, fungi) in forming flocs and consuming organic pollutants.
BOD Reduction	Modest reduction (typically 25-35%) of BOD.	Significant reduction (typically 85-95%) of BOD.
Key Components	Bar screens, grit chambers, primary settling tanks (clarifiers).	Aeration tanks, secondary settling tanks (clarifiers), activated sludge recycling system.
Effluent Quality	Primary effluent still contains high levels of dissolved organic matter and pathogens.	Secondary effluent is significantly cleaner, with low BOD and suspended solids, but may still require disinfection.

Primary treatment focuses on the physical removal of large and settleable solids from sewage, offering a preliminary reduction in pollution load. It's a mechanical process that prepares the wastewater for subsequent biological treatment. In contrast, secondary treatment is the core biological stage where microorganisms actively break down dissolved and fine suspended organic matter. This microbial action is responsible for the substantial reduction in Biological Oxygen Demand (BOD), making the effluent much cleaner and less harmful to the environment. Both stages are sequential and indispensable for effective sewage purification.