CBSE Class 7 Science Exemplar Chapter 18: Wastewater Story NCERT Solutions

Wastewater is defined as water that has been used and contaminated by various activities, making it unsuitable for direct reuse without treatment. Routine household chores like washing clothes, cleaning utensils, and flushing toilets generate wastewater. While water from a damaged tap or a shower might be considered a waste of resources, it is not inherently contaminated in the same way as water from laundry, which contains detergents and dirt. Water flowing in a river is generally a natural water source, not wastewater, unless it is polluted.

Sewage is primarily composed of liquid waste generated from households, industries, and commercial establishments. This liquid waste, also known as wastewater, can contain a variety of dissolved and suspended substances, including human excreta, food waste, detergents, oils, and harmful microorganisms. While it may contain some solid particles and dissolved gases, its fundamental nature is that of a liquid effluent.

During the treatment of wastewater, several byproducts are generated. One significant product is sludge, which consists of the solid organic matter that settles down at the bottom of settling tanks. This sludge can be further processed. Another important product is biogas, which is formed when sludge is decomposed by anaerobic bacteria in separate tanks. Biogas is a combustible gas that can be used as a fuel or to generate electricity. An aerator is a piece of equipment used in the treatment process, not a product.

Open drainage systems, where wastewater flows openly without proper covering or treatment, create highly unhygienic conditions. These stagnant, often foul-smelling environments provide ideal breeding grounds for various pests and disease vectors. Flies are attracted to the decaying organic matter, mosquitoes breed in the standing water, and numerous microorganisms that cause diseases can thrive in such contaminated surroundings.

Water-borne diseases are illnesses caused by pathogenic microorganisms that are transmitted through contaminated drinking water. Cholera, typhoid, and dysentery are classic examples of water-borne diseases. Asthma, on the other hand, is a chronic respiratory condition characterized by inflammation and narrowing of the airways, and it is typically considered an air-borne or environmental condition, not directly transmitted through water consumption.

Disinfection is a crucial step in water treatment to kill or inactivate harmful microorganisms that may be present in the water. Chlorine is a widely used chemical disinfectant due to its effectiveness and relatively low cost. It oxidizes and damages the cellular components of bacteria, viruses, and other pathogens, rendering them harmless. Washing soda, silica, and coal are not primary chemicals used for disinfecting drinking water.