What is Secondary Treatment of Sewage?

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Grade Level:

Class 12

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Definition

What is it?

Secondary treatment of sewage is the second main stage in cleaning wastewater. It uses helpful microorganisms (tiny living things like bacteria) to break down organic matter that is still present after primary treatment, making the water much cleaner.

Simple Example

Quick Example

Imagine you have a glass of muddy water. Primary treatment is like letting the big mud particles settle down. Secondary treatment is then adding a special 'filter' of tiny helpful organisms that eat up the smaller, dissolved dirt, making the water much clearer, just like how a water filter at home makes tap water safer to drink.

Worked Example

Step-by-Step

Let's say a sewage treatment plant receives 10,000 litres of wastewater per hour after primary treatment, containing 200 mg/L of organic matter (BOD). We want to reduce this significantly using secondary treatment.

---1. Wastewater enters aeration tanks: The 10,000 litres/hour of water is pumped into large tanks where air is continuously bubbled through it.

---2. Microbes get to work: This air helps good bacteria and other microbes, called 'activated sludge', grow and multiply. These microbes start eating the organic matter (BOD) in the water.

---3. Digestion of organic matter: Over several hours (e.g., 6-8 hours), these microbes break down most of the 200 mg/L organic matter into harmless substances like carbon dioxide and water.

---4. Settling the microbes: After this, the water flows into another settling tank, where the 'activated sludge' (now heavier with the consumed organic matter) settles down at the bottom.

---5. Recirculation and discharge: A part of this settled sludge is sent back to the aeration tanks to treat new incoming wastewater, while the treated water, now with much lower organic matter (e.g., 20 mg/L BOD), is moved to the next stage (tertiary treatment or disinfection).

---Result: The secondary treatment successfully reduced the organic matter (BOD) from 200 mg/L to 20 mg/L, making the water 90% cleaner in terms of organic pollutants.

Why It Matters

Understanding secondary sewage treatment is crucial for environmental engineers designing cleaner cities and for biotechnologists developing new ways to purify water. This process helps prevent water pollution, protecting our rivers and lakes, and is vital for public health. It's also linked to climate science as proper waste management reduces greenhouse gas emissions.

Common Mistakes

MISTAKE: Thinking secondary treatment removes all pollutants including harmful chemicals and heavy metals. | CORRECTION: Secondary treatment primarily focuses on breaking down organic matter using microbes. It does not effectively remove dissolved chemicals, nutrients like phosphorus/nitrogen, or heavy metals.

MISTAKE: Believing primary and secondary treatments use the same methods. | CORRECTION: Primary treatment is a physical process (screening, sedimentation) to remove large solids. Secondary treatment is a biological process using microorganisms to break down dissolved organic matter.

MISTAKE: Confusing secondary treatment with disinfection. | CORRECTION: Secondary treatment cleans water biologically. Disinfection (like chlorination or UV treatment) is a separate step usually after secondary treatment, specifically aimed at killing remaining harmful bacteria and viruses.

Practice Questions

Try It Yourself

QUESTION: What is the main purpose of using microorganisms in secondary sewage treatment? | ANSWER: The main purpose is for microorganisms to break down and consume dissolved organic matter present in the wastewater.

QUESTION: If primary treatment removes 60% of suspended solids and secondary treatment removes 90% of the remaining organic matter (BOD), what percentage of the initial organic matter is removed after secondary treatment? | ANSWER: This question is tricky! Secondary treatment removes 90% of the ORGANIC MATTER, not necessarily 90% of the initial total pollutants. If we assume primary treatment removes negligible BOD, then secondary treatment removes 90% of the initial BOD. If primary treatment removes some BOD, then 90% of the *remaining* BOD is removed. For a simpler interpretation, if 100 units of BOD are present after primary treatment, secondary removes 90 units. So, 90% of the BOD that reaches secondary treatment is removed.

QUESTION: A city's sewage treatment plant uses secondary treatment. If the inflow has a BOD of 250 mg/L and the outflow after secondary treatment has a BOD of 25 mg/L, what is the efficiency of organic matter removal by the secondary treatment process? Show your calculation. | ANSWER: Efficiency = ((Inflow BOD - Outflow BOD) / Inflow BOD) * 100 = ((250 - 25) / 250) * 100 = (225 / 250) * 100 = 0.9 * 100 = 90%. The efficiency is 90%.

MCQ

Quick Quiz

Which of the following is the primary method used in secondary sewage treatment?

Physical screening and settling of large particles

Chemical precipitation of dissolved salts

Biological degradation by microorganisms

Removal of heavy metals using magnets

The Correct Answer Is:

Secondary treatment is primarily a biological process where microorganisms consume and break down organic pollutants. Options A and B describe primary and sometimes tertiary/advanced treatments respectively, while D is incorrect.

Real World Connection

In the Real World

In India, many cities like Delhi and Mumbai have large-scale Sewage Treatment Plants (STPs) that heavily rely on secondary treatment processes like 'Activated Sludge Process' or 'Trickling Filters'. These plants ensure that the water discharged into rivers like Yamuna or Mithi is significantly cleaner, reducing pollution and protecting aquatic life. Engineers and environmental scientists at these STPs continuously monitor the microbial activity to ensure efficient treatment.

Key Vocabulary

Key Terms

ORGANIC MATTER: Substances derived from living organisms, containing carbon, that can pollute water. | MICROORGANISMS: Tiny living things like bacteria, fungi, and protozoa, visible only with a microscope. | AERATION TANKS: Large tanks where air is pumped into wastewater to help microbes grow. | ACTIVATED SLUDGE: A mixture of wastewater and microorganisms used in secondary treatment. | BOD (BIOLOGICAL OXYGEN DEMAND): A measure of the amount of oxygen required by microorganisms to break down organic matter in a water sample.

What's Next

What to Learn Next

Next, you should learn about 'Tertiary Treatment of Sewage'. This stage builds on secondary treatment by removing remaining nutrients and specific pollutants, making the water even cleaner and sometimes safe for reuse, which is super important for water-scarce regions.