Understanding the Biological Process in Sewage Treatment Plants

Sewage treatment plays a critical role in maintaining public health and environmental safety. Among the various methods used in modern sewage treatment plants (STPs), the biological process stands out as a sustainable and efficient technique for breaking down organic matter in wastewater. But what exactly happens in this process, and why is it so important?



Let’s break it down.

What Is the Biological Treatment Process?

The biological process in sewage treatment involves using microorganisms (mainly bacteria) to consume and break down organic pollutants in sewage. These microbes feed on organic matter, converting it into simpler, harmless substances such as carbon dioxide, water, and biomass. This natural process mimics what happens in nature—but in a controlled, accelerated environment.

Why Biological Treatment Is Necessary

Raw sewage contains a large amount of biodegradable organic material, which, if discharged untreated into water bodies, can cause serious environmental issues like:

  • Depletion of dissolved oxygen in water

  • Growth of harmful pathogens and algae

  • Foul odors and water pollution

  • Threats to aquatic life and human health

Biological treatment helps mitigate these problems by reducing the organic load before the water is discharged or reused.

Key Stages of Biological Treatment

Biological sewage treatment is typically divided into two major stages: Secondary Treatment and Tertiary (Advanced) Biological Treatment.

1. Secondary Treatment: The Core of Biological Processing

Secondary treatment usually includes aerobic or anaerobic biological methods.

a. Activated Sludge Process (Aerobic):
In this process, wastewater is aerated in tanks where bacteria and other microbes break down organic pollutants. The mixture of wastewater and microorganisms (called mixed liquor) is then settled in a clarifier. The clear water is separated, and part of the sludge is recycled back to maintain microbial populations.

b. Trickling Filters:
Wastewater is sprayed over a bed of rocks or plastic media coated with microbial biofilm. As it trickles down, bacteria digest the organic matter.

c. Anaerobic Digesters:
Used mostly for sludge treatment, anaerobic digesters break down organic matter in the absence of oxygen, producing methane gas as a byproduct. This gas can be used as an energy source.

2. Tertiary Treatment (Optional but Beneficial)

Tertiary treatment is an advanced stage that may involve biological nutrient removal (BNR), especially to eliminate nitrogen and phosphorus. It ensures water meets higher quality standards, especially if it's being reused for gardening, flushing, or even industrial purposes.

Microorganisms at Work

The efficiency of biological treatment depends on different types of bacteria, including:

  • Aerobic bacteria (need oxygen) – break down organic matter into CO₂ and water

  • Anaerobic bacteria (work without oxygen) – reduce sludge volume and generate biogas

  • Facultative bacteria (adapt to both environments)

These microscopic organisms are the real “workers” of the treatment process.

Advantages of the Biological Process

  • Eco-friendly and sustainable

  • Cost-effective compared to chemical treatment

  • Efficient in reducing BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand)

  • Produces reusable water and energy (in anaerobic systems)

Conclusion

The biological process in sewage treatment plants is an essential step toward creating a sustainable and safe wastewater management system. By harnessing the natural power of microbes, STPs can treat sewage effectively while protecting the environment and conserving resources. As urbanization continues to rise, understanding and optimizing these biological systems will be key to a cleaner, healthier future.

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