CBSE Class 7 Science Chapter 18: Wastewater Story NCERT Solutions

Here are the completed sentences:

(a) Cleaning of water is a process of removing pollutants.

(b) Wastewater released by houses is called sewage.

(c) Dried sludge is used as manure.

(d) Drains get blocked by oils and fats.

Sewage is a liquid waste that primarily consists of water, along with a complex mixture of impurities. These impurities include waste from houses, offices, factories, and hospitals. It contains suspended solids, organic and inorganic matter, nutrients, and harmful microorganisms like saprotrophic and disease-causing bacteria.

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

• Water Pollution: It contaminates natural water bodies, making them unfit for drinking, bathing, and other uses.
• Harm to Aquatic Life: The organic matter in sewage decomposes, consuming dissolved oxygen in the water, which can suffocate aquatic plants and animals.
• Spread of Diseases: Untreated sewage often contains pathogens that can cause serious water-borne diseases such as cholera, typhoid, polio, meningitis, malaria, and dengue in humans who consume contaminated water or come into contact with it.

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

• Clogging Drains: When oils and fats are poured down the drain, they can solidify and accumulate, forming blockages. This reduces the effectiveness of the drainage system in carrying wastewater away.
• Hindering Soil Filtration: If wastewater containing oils and fats flows into open drains or soil, these substances can clog the pores in the soil. This blockage prevents water from being naturally filtered by the soil, reducing the soil's ability to purify the water.

Therefore, it is essential to dispose of oils and fats properly, often by collecting them separately or allowing them to solidify before disposal, rather than pouring them down the drain.

The treatment of wastewater to obtain clarified water involves a series of 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. Clarification: The water then moves to a large settling tank. Here, the flow is further slowed, allowing solid organic matter, such as faeces, to settle down. This settled solid material is called sludge and is removed using a scraper. Lighter materials like oil and grease float to the surface and are removed by a skimmer. The water that remains after this stage is called clarified water.
4. Aeration: 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 matter, including human waste, present in the water.
5. Activated Sludge Settling: After aeration, the water is allowed to settle again. The microbes that decomposed the organic matter settle at the bottom, forming activated sludge. This sludge is then removed from the top.
6. Disinfection and Discharge: The treated water, now with a low level of organic material and suspended matter, is removed from the top of the settling tank. It can be disinfected using chemicals like chlorine or ozone to kill any remaining harmful microorganisms. Finally, this treated water is discharged into a sea, river, or allowed to seep into the ground. In some cases, it is distributed to towns for use after disinfection.

The sludge collected can be used to produce biogas, and the dried activated sludge is used as manure.

Sludge is the solid material that settles at the bottom of the settling tank 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:

1. Collection: Sludge is collected from the bottom of the primary settling tank using a scraper.
2. Anaerobic Digestion: The collected sludge is transferred to a separate tank where it is decomposed by anaerobic bacteria. These bacteria break down the organic matter in the absence of oxygen.
3. Biogas Production: This anaerobic decomposition process produces biogas, a mixture of gases (primarily methane) that can be captured and used as a fuel source for heating or electricity generation.
4. Activated Sludge Treatment: In a later stage of wastewater treatment, after aeration, microbes and remaining human waste settle down to form activated sludge. This activated sludge contains about 97% water.
5. Water Removal: The water is removed from the activated sludge using sand drying beds or mechanical dewatering machines.
6. Manure Use: The dried sludge, now significantly reduced in volume and pathogen content, is used as manure. It enriches the soil with organic matter and essential nutrients, completing a cycle of waste management.