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

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Which of the following pairs is correctly matched regarding sewage impurities?

Official previous-year question

Verified 1 Aug 2026.

Options

AUrea : Organic impurity
BAnimal waste : Inorganic impurity
CMetals : Organic impurity
DNitrates : Organic impurity

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Solution

The correct option is

Urea : Organic impurity

Explanation

Sewage is a complex liquid waste containing suspended solids, organic and inorganic impurities, nutrients, saprophytes, and disease-causing bacteria. The classification of these impurities is based on their chemical composition, specifically whether they are carbon-based (organic) or mineral-based (inorganic).

Option (a) is Incorrect: Nitrates ($NO_3^-$) and Phosphates are mineral salts. They are classified as inorganic impurities. These substances often act as nutrients that accelerate algal growth in water bodies.
Option (b) is Incorrect: Animal waste, similar to human faeces, fruit and vegetable waste, and oil, consists of complex carbon compounds. Therefore, it is classified as an organic impurity.
Option (c) is Correct: Urea ($NH_2CONH_2$) is a major nitrogenous waste product found in urine. Since it is a carbon-based compound, it is correctly classified as an organic impurity.
Option (d) is Incorrect: Metals (such as lead, mercury, and arsenic) are elemental substances derived from minerals. They are classified as inorganic impurities.

Key Takeaway: Organic impurities in sewage include human faeces, animal waste, urea, oil, pesticides, and herbicides. Inorganic impurities include nitrates, phosphates, and metals.

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?

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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?

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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?

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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

Urea : Organic impurity

Explanation

Option (a) is Incorrect: Nitrates ($NO_3^-$) and Phosphates are mineral salts. They are classified as inorganic impurities. These substances often act as nutrients that accelerate algal growth in water bodies.
Option (b) is Incorrect: Animal waste, similar to human faeces, fruit and vegetable waste, and oil, consists of complex carbon compounds. Therefore, it is classified as an organic impurity.
Option (c) is Correct: Urea ($NH_2CONH_2$) is a major nitrogenous waste product found in urine. Since it is a carbon-based compound, it is correctly classified as an organic impurity.
Option (d) is Incorrect: Metals (such as lead, mercury, and arsenic) are elemental substances derived from minerals. They are classified as inorganic impurities.

Key Takeaway: Organic impurities in sewage include human faeces, animal waste, urea, oil, pesticides, and herbicides. Inorganic impurities include nitrates, phosphates, and metals.