With reference to "Activated Sludge", consider the following statements: 1. It is completely dry and contains no water. 2. The water from it is removed by sand drying beds. 3. It can be used as manure. Which of the statements given above is/are correct?

The correct option is (c) 2 and 3 only. Explanation Activated sludge is a process used in secondary wastewater treatment. It involves the production of an activated mass of microorganisms capable of stabilizing organic content in waste. The solid material that settles out during this process is known as sludge. Statement-wise Analysis - Statement 1 is Incorrect. Activated sludge is not completely dry; it is a semi-solid slurry. When it settles at the bottom of the tank, it contains a very high percentage of water (approximately 97%). It requires dewatering processes to reduce this moisture content. - Statement 2 is Correct. Before sludge can be disposed of or utilized, the excess water must be removed. Sand drying beds are a widely used, economical method for dewatering sludge, allowing moisture to drain and evaporate. - Statement 3 is Correct. The dried sludge is rich in organic matter and nutrients such as nitrogen and phosphorus. Consequently, it can be utilized as manure or fertilizer to improve soil fertility, provided it is free from hazardous toxins. Key Takeaway: Activated sludge is a byproduct of sewage treatment with high water content (97%). Upon dewatering (e.g., via sand drying beds), the remaining solids are rich in organic matter and can serve as manure.

NCERT Class 7 Environment & Ecology — Chapter 2: Wastewater story

mcqeasy

With reference to "Activated Sludge", consider the following statements:

It is completely dry and contains no water.
The water from it is removed by sand drying beds.
It can be used as manure.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 and 2 only
B2 only
C2 and 3 only
D1, 2 and 3

Did you get this right?

Solution

The correct option is (c) 2 and 3 only.

Explanation

Activated sludge is a process used in secondary wastewater treatment. It involves the production of an activated mass of microorganisms capable of stabilizing organic content in waste. The solid material that settles out during this process is known as sludge.

Statement-wise Analysis

Statement 1 is Incorrect. Activated sludge is not completely dry; it is a semi-solid slurry. When it settles at the bottom of the tank, it contains a very high percentage of water (approximately 97%). It requires dewatering processes to reduce this moisture content.
Statement 2 is Correct. Before sludge can be disposed of or utilized, the excess water must be removed. Sand drying beds are a widely used, economical method for dewatering sludge, allowing moisture to drain and evaporate.
Statement 3 is Correct. The dried sludge is rich in organic matter and nutrients such as nitrogen and phosphorus. Consequently, it can be utilized as manure or fertilizer to improve soil fertility, provided it is free from hazardous toxins.

Key Takeaway: Activated sludge is a byproduct of sewage treatment with high water content (97%). Upon dewatering (e.g., via sand drying beds), the remaining solids are rich in organic matter and can serve as manure.

Your note

More from Chapter 2: Wastewater story

With reference to the composition of sewage, consider the following statements: 1. It is a liquid waste consisting entirely of dissolved impurities. 2. It contains nutrients such as phosphorus and nitrogen. 3. It is devoid of any saprophytic or disease-causing bacteria. Which of the statements given above is/are correct?

medium

Consider the following statements regarding the impurities found in sewage: 1. Urea and pesticides are classified as organic impurities. 2. Nitrates and phosphates are classified as organic impurities. 3. Vegetable waste and animal waste are classified as organic impurities. Which of the statements given above is/are correct?

medium

Consider the following statements regarding the physical treatment of wastewater: 1. Bar screens are used to remove large objects like rags and sticks. 2. The speed of wastewater is increased in the grit and sand removal tank to facilitate filtration. 3. Sedimentation tanks are designed to increase the flow speed to prevent settling of solids. Which of the statements given above is/are correct?

medium

With reference to unit operations used in wastewater treatment, the role of a skimmer is to:

medium

Consider the following statements regarding "Activated Sludge": 1. It consists of suspended microbes that settle at the bottom of the tank after aeration. 2. It contains a very low percentage of water, being mostly solid. 3. A part of it is pumped back into the aeration tank to serve as an inoculum. Which of the statements given above is/are correct?

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The correct option is (c) 2 and 3 only.

Explanation

Statement-wise Analysis

Statement 1 is Incorrect. Activated sludge is not completely dry; it is a semi-solid slurry. When it settles at the bottom of the tank, it contains a very high percentage of water (approximately 97%). It requires dewatering processes to reduce this moisture content.
Statement 2 is Correct. Before sludge can be disposed of or utilized, the excess water must be removed. Sand drying beds are a widely used, economical method for dewatering sludge, allowing moisture to drain and evaporate.
Statement 3 is Correct. The dried sludge is rich in organic matter and nutrients such as nitrogen and phosphorus. Consequently, it can be utilized as manure or fertilizer to improve soil fertility, provided it is free from hazardous toxins.