CBSE Notes Class 7 Science Chapter 13 Wastewater Story

Rich in lather and mixed with oil, the black-brown water that flows down drains from sinks, showers, toilets, and laundries are known as wastewater. This contaminated water contains various impurities and requires thorough treatment before it can be safely released into the environment.

1.0Introduction 

Access to clean water is a significant concern, with over a billion people without reliable sources of safe drinking water. This scarcity results in numerous water-related diseases and deaths, and many, including children, must travel long distances to find clean water. This challenge is a serious affront to human dignity.

Factors like population growth, pollution, industrial development, and poor management cause an increasing shortage of fresh water. To address this crisis, the UN designated 2005–2015 as the International Decade for Action on "Water for Life" to halve the number of people without access to safe drinking water. Although there has been some progress, substantial efforts are still needed to resolve this issue.

In Class 7 Science NCERT, Chapter 18, "Wastewater Story," one will learn that wastewater treatment is vital for protecting our environment and the health of both humans and animals. If wastewater is not treated properly, it can pollute water sources, damage natural habitats, and cause serious health problems.

2.0What Is Sewage?

Sewage is wastewater from homes, industries, hospitals, offices, and other sources. It also includes rainwater that collects on streets and rooftops during storms or heavy rainfall, carrying harmful substances. Sewage primarily consists of water mixed with dissolved and suspended impurities.

The impurities found in sewage are known as contaminants because they pollute the water. Sewage is a mixture of these contaminants.

Sewers: Large and small pipes installed in buildings to carry sewage water.

Sewerage refers to the network or sewer system that transports sewage water from its source to its disposal point, such as a treatment plant.

The sewerage system includes manholes located every 50 to 60 meters. Manholes are positioned where sewers intersect or change direction.

3.0Wastewater Treatment Plant (WWTP)

The treatment of wastewater involves physical, chemical, and biological processes to remove contaminants from the water. These practices ensure that sewage is managed correctly, reducing its environmental impact and making it safe for reuse or discharge.

Treatment Stages:

1. Physical Treatment

Removes large solids and debris.

• Screening: Wastewater first passes through bar screens where large objects such as rags, sticks, cans, plastic packets, and napkins are removed.
• Grit and Sand Removal: The water then flows into a grit and sand removal tank, where the speed of the incoming wastewater is reduced, allowing sand, grit, and pebbles to settle.
• Primary Settling: The water is transferred to a large tank with a sloped bottom. Here, heavier solids like faeces settle at the bottom and are removed.

2. Biological Treatment

Breaks down organic matter using microorganisms.

• Secondary Settling: After several hours, suspended microbes settle at the bottom of the tank as activated sludge. The clarified water is then removed from the top.
• Sludge Treatment: The activated sludge, about 97% water, is further treated using sand drying beds or machines to remove excess water. The dried sludge can be used as manure, returning organic matter and nutrients to the soil.

3. Chemical Treatment

 Addresses remaining pollutants and adjusts water quality.

• The treated water, containing minimal organic material and suspended matter, is released into a sea, river, or ground, undergoing further natural purification.
• Disinfection with chemicals like chlorine or ozone is sometimes necessary before the water is released into the distribution system.

By-Products of Wastewater Treatment

• Sludge: The solid waste removed during treatment. Dried biosolids can be used as manure, returning organic matter and nutrients to the soil.
• Biogas: Produced during the treatment process. It can be utilised as fuel or converted into electricity.

4.0Better Housekeeping Practices

To minimise sewage water and maintain effective drains, adopt these housekeeping practices:

• Dispose of Oil and Fats: Throw oil and fats in the dustbin instead of pouring them down the drain, as they can harden and block pipes.
• Avoid Chemicals: Do not dispose of chemicals like insecticides, pesticides, or medicines in the drain, as they can harm the microbes that help purify the water.
• Prevent Blockages: To prevent clogs, avoid throwing items like tea leaves, food scraps, cotton, and soft toys down the drain. Instead, place them in the dustbin.

5.0Sanitation and Disease

Contaminated drinking water and poor sanitation are significant causes of diseases. Untreated human excreta can pollute water and soil, leading to water-borne diseases like dysentery, hepatitis, meningitis, polio, typhoid, and cholera. Open drains can become breeding grounds for mosquitoes, flies, and disease-causing organisms.

The "Swachh Bharat" mission, initiated by the Government of India, aims to improve sanitation through campaigns focused on proper sewage disposal and providing toilets for all.

6.0Alternative Sewage Disposal Methods

Low-cost onsite sewage disposal systems are encouraged to enhance sanitation. Examples include:

• Septic Tanks are ideal for areas without a sewerage system, hospitals, isolated buildings, or small clusters of homes.
• Chemical Toilets: Used in various settings for hygienic waste disposal.
• Composting Pits: Converts waste into compost.

Some organisations provide advanced onsite human waste disposal technologies, where excreta flows through covered drains into a biogas plant. The resulting biogas is utilised as an energy source, eliminating the need for manual scavenging.