CBSE Class 7 Science Chapter 18: Wastewater Story NCERT Solutions

The blanks are filled as follows:

1. (a) Cleaning of water is a process of removing pollutants. These are harmful substances that contaminate the water.
2. (b) Wastewater released by houses is called sewage. This includes water from toilets, sinks, and washing machines.
3. (c) Dried sludge is used as manure. Sludge is the solid waste collected during wastewater treatment.
4. (d) Drains get blocked by oils and fats. These substances can solidify and accumulate, preventing water flow.

Sewage is a liquid waste that has water as its largest component, along with various types of impurities. It is the wastewater generated from houses, offices, factories, and hospitals. Sewage contains a complex mixture of suspended solids, organic and inorganic impurities, nutrients, and harmful microorganisms like saprotrophic and disease-causing bacteria.

Discharging untreated sewage into rivers or seas is extremely harmful because:

• Water Pollution: It directly pollutes natural water bodies, making them unfit for consumption and other uses.
• Harm to Aquatic Life: The organic matter in sewage decomposes, consuming dissolved oxygen in the water, which is essential for the survival of fish and other aquatic plants and animals.
• Spread of Diseases: Untreated sewage often contains pathogens that can cause serious waterborne diseases such as cholera, typhoid, polio, meningitis, and malaria in humans who consume or come into contact with the contaminated water.

Oils and fats should not be released into drains because they can cause significant problems:

• Clogging Drains: When oils and fats enter drains, they can solidify and accumulate on the inner walls of pipes. This buildup gradually narrows the passage, eventually clogging the drains and reducing their effectiveness in carrying wastewater away.
• Hindering Soil Filtration: If drains are open and lead to the soil, oils and fats can block the pores in the soil. This blockage prevents water from percolating through the soil, which is a natural filtration process. This hampers the soil's ability to purify the water.

Therefore, it is crucial to dispose of oils and fats properly, usually by collecting them separately rather than pouring them down the drain.

The treatment of wastewater to obtain clarified water involves several physical, chemical, and biological processes. Here are the key steps:

1. Screening: Wastewater is first passed through bar screens. These screens have parallel bars that remove large floating objects like rags, sticks, plastic packets, and napkins, preventing them from entering the treatment system.
2. Grit and Sand Removal: After screening, the water flows into a grit and sand removal tank. In this tank, the flow of water is slowed down, allowing heavier materials like sand, grit, and pebbles to settle at the bottom.
3. Settling (Primary Treatment): The water then moves to a large settling tank. Here, the flow is further slowed, allowing solid organic matter (like faeces) to settle down as sludge at the bottom. A mechanical scraper removes this sludge. Simultaneously, lighter materials like oil and grease float to the surface and are removed by a skimmer. The water that remains at the top, now clearer, is called clarified water.
4. Aeration (Secondary Treatment): The clarified water is then transferred to an aeration tank. Air is pumped into this tank, which encourages the growth of aerobic bacteria. These bacteria play a crucial role in decomposing the remaining organic waste and human waste present in the water.
5. Activated Sludge Settling: After aeration, the water is allowed to settle again in another tank. The microbes (bacteria) that have decomposed the waste settle at the bottom, forming what is called activated sludge. This sludge is then removed from the top. A portion of this activated sludge is often returned to the aeration tank to help decompose new incoming waste.
6. Disinfection and Discharge: The treated water, now with a low level of organic material and suspended matter, is typically disinfected. This can be done by adding chemicals like chlorine or ozone to kill any remaining harmful microorganisms. Finally, this treated water is discharged into a sea, a river, or allowed to seep into the ground. In some cases, it may be further purified and distributed to towns for use.

The sludge collected can be dried and used as manure, or it can be used to produce biogas.

Sludge is the solid material that settles at the bottom of settling tanks during the wastewater treatment process. It primarily consists of human waste (like faeces) and other organic solids removed from the wastewater.

The treatment of sludge involves the following:

• Collection: Sludge is collected from the bottom of the primary settling tanks using a scraper.
• Anaerobic Digestion: The collected sludge is transferred to a separate tank where it is decomposed by anaerobic bacteria (bacteria that do not require oxygen). This process is called anaerobic digestion.
• Biogas Production: During anaerobic digestion, biogas is produced. This biogas is a mixture of gases, primarily methane, and can be used as a fuel source for heating or generating electricity.
• Drying and Use as Manure: After anaerobic digestion, the sludge is further processed. It is often dried in sand drying beds or using machines to remove most of its water content. The resulting dried sludge is nutrient-rich and can be used as manure to improve soil fertility in agriculture, returning organic matter and nutrients to the soil.

In the secondary treatment stage, the microbes that settle down after aeration also form activated sludge, which is also treated similarly for drying and use as manure.