Class 7 Science Wastewater Story Worksheets

To score full marks on a question about the WWTP process, you should describe the following stages in order:

• Primary Treatment: This is a physical process. First, wastewater passes through bar screens to remove large objects like rags, sticks, and plastics. Then, it goes to a grit and sand removal tank where the speed of water is decreased to allow sand, grit, and pebbles to settle down. Finally, the water is sent to a large settling tank where solid faeces (sludge) settles at the bottom and floatable materials like oil and grease are removed with a skimmer.
• Secondary Treatment: This is a biological process. The clarified water from primary treatment is pumped into an aeration tank where air is bubbled through it. This helps aerobic bacteria grow and consume the remaining organic waste. The bacteria then settle down as activated sludge.
• Tertiary/Final Treatment: The treated water is disinfected using chemicals like chlorine or non-chemical methods like UV radiation to kill any remaining disease-causing microbes before it is released into a water body.

Based on the CBSE pattern for the 2025-26 session, some expected long-answer questions from this chapter include:

• Describing the complete process of wastewater treatment in a WWTP with a simple diagram.
• Explaining the role of an active citizen in managing sanitation and minimising wastewater.
• Listing the harmful effects of discharging untreated sewage into rivers and explaining how it leads to waterborne diseases.
• Differentiating between a sewerage system and alternative low-cost disposal systems like septic tanks.

Preparing these topics is important for scoring well.

Sludge is the solid waste that settles at the bottom of sedimentation tanks during primary wastewater treatment. In the secondary stage, the settled bacteria are called activated sludge. The collected sludge is transferred to a separate tank called a digester. Here, anaerobic bacteria break it down, producing biogas, which is an important fuel. The remaining dried sludge can be used as manure, returning organic matter and nutrients to the soil.

An alert citizen can play a crucial role by adopting the following practices:

• Do not dispose of cooking oils, fats, and grease down the kitchen sink as they can harden and block the pipes.
• Avoid throwing solid wastes like tea leaves, food remains, sanitary towels, or plastics into drains.
• Use chemical cleaners, paints, and solvents sparingly as they can kill the helpful microbes in the WWTP.
• Practice water conservation by fixing leaks and turning off taps when not in use to reduce the load on the WWTP.

Aeration is critical because it supplies oxygen to the clarified water. This oxygen is essential for the rapid growth and activity of aerobic bacteria. These helpful bacteria consume the dissolved organic pollutants that were not removed during primary treatment. If the aeration step were skipped, the aerobic bacteria would not thrive, and the decomposition of organic matter would be extremely slow and incomplete. This would result in partially treated, polluted water being discharged, which could harm aquatic life and fail to meet safety standards.

Throwing cooking oil and fats down the drain is a very bad practice because they can cause serious problems in the plumbing and sewer systems. When flushed, the oil and fat cool down and solidify, sticking to the inner walls of the pipes. Over time, this buildup narrows the pipes and can lead to a complete blockage. This can cause wastewater to back up into the house or overflow in the streets. Furthermore, these fats are difficult to break down and can interfere with the efficiency of the wastewater treatment process.

A sewerage system is a large network of interconnected pipes (sewers) that collects wastewater from homes, industries, and other buildings and transports it to a centralized Wastewater Treatment Plant (WWTP). In contrast, a septic tank is a self-contained, underground treatment system for a single household or a small group of homes. In a septic tank, anaerobic bacteria decompose the waste, and the relatively clearer water flows out into a drain field. A septic tank is a better and more practical alternative in areas where:

• There is no centralized sewerage infrastructure, such as in rural villages or remote locations.
• The population density is low, making the construction of a full-scale WWTP and sewer network economically unviable.

Poor sanitation, especially the release of untreated wastewater, creates a direct pathway for diseases to spread. Untreated sewage is rich in harmful pathogens, including bacteria, viruses, and parasites. When this water contaminates drinking water sources like rivers, lakes, or groundwater, it leads to waterborne diseases. People who consume this contaminated water can suffer from illnesses like cholera, typhoid, dysentery, and jaundice. Flies and other insects that breed in open sewage can also physically carry these germs to food, further spreading disease.