Class 7 Science Chapter 13 Important Questions

Clean water is a basic necessity of our life. We use clean water every-day for drinking, bathing, washing clothes, cooking food, washing utensils, and flushing toilets etc. When we use water for various purposes which make the water dirty. The water which is rich in lather, mixed with oil, black– brown water that goes down the drains from sinks, showers, toilets, laundries used water called wastewater.

Clean water is a basic need of human being. The used water should not be wasted. We must clean it up by removing pollutants. Clean water that is fit for use is unfortunately not available to all. It has been reported that more than one billion of our fellow human beings have no access to safe drinking water. This accounts for a large number of water-related diseases and even deaths. Women and girls walk for several kilometers to collect clean water.
The increasing scarcity of fresh- water due to population growth, pollution, industrial development, mismanagement and other factors. Realizing the urgency of the situation on the World Water Day, on 22 March 2005, the General Assembly of the United Nations proclaimed the period 2005–2015 as the International Decade for action on “Water for life”. All efforts made during this decade aim to reduce by half the number of people who do not have access to safe drinking water.

Cleaning of water is a process of removing pollutants before it enters a water body or is reused. This process of wastewater treatment is commonly known as “sewage treatment”. It takes place in several stages.

We know that sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease-causing bacteria and other microbes.
i) Organic impurities as – Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.
ii) Inorganic impurities as – Nitrates, Phosphates, metals.
iii) Nutrients as – Phosphorus and Nitrogen.
iv) Bacteria – Such as which cause cholera and typhoid.
v) Other microbes – Such as which cause dysentery.

The waste water passes into a sedimentation tank, in which organic material settle down and are drawn for disposal. This organic matter is then converted to methane, carbon dioxide and a humus like material by a process called digestion. This digested sludge is placed on sand beds for air drying. Dried sludge can be used as a fertilizer.

Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater.
1. Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed.
2. Water then goes to a grit and sand removal tank. The speed of the incoming wastewater is decreased to allow sand, grit and pebbles to settle down.
3. The water is then allowed to settle in a large tank which is sloped towards the middle. Solids like faeces settle at the bottom and are removed with a scraper. This is the sludge. A skimmer removes the floatable solids like oil and grease. Water so cleared is called clarified water.
The sludge is transferred to a separate tank where it is decomposed by the anaerobic bacteria. The biogas produced in the process can be used as fuel or can be used to produce electricity.
4. Air is pumped into the clarified water to help aerobic bacteria to grow. Bacteria consume human waste, food waste, soaps and other unwanted matter still remaining in clarified water.
After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water is then removed from the top.
The activated sludge is about 97% water. The water is removed by sand drying beds or machines. Dried sludge is used as manure, returning organic matter and nutrients to the soil. The treated water has a very low level of organic material and suspended matter. It is discharged into a sea, a river or into the ground. Nature cleans it up further. Sometimes it may be necessary to disinfect water with chemicals like chlorine and ozone before releasing it into the distribution system.

Yes, the aeroplanes have a special type of toilet called “vacuum toilets”. So, the human excreta or sewage in an aeroplane is disposed of through vacuum toilets. When we flush the toilet in an aeroplane, then a valve opens in a pipeline which has vacuum inside it. The vacuum in the pipeline sucks the human excreta from the toilet seat with a great force making a loud noise and carries it into a tank on board the aeroplane. When the aeroplane lands at an airport, then the tank containing all the human excreta is emptied into a sewer line into the ground through connecting pipes.

A chemical toilet uses a chemically treated reservoir located directly below the toilet seat. The chemicals reduce the odor or foul smell coming from the human waste and carry out partial disinfection of the human waste. Chemical toilets have a limited storage capacity for human waste, so their reservoirs need to be emptied into a sewer line after a certain time. All the portable toilets are chemical toilets. They can be shifted from one place to another. Chemical toilets are used at construction site and at a large outdoor gathering such as music, festivals and marriage etc.

To improve sanitation, low-cost onsite sewage disposal systems are being encouraged. Examples are septic tanks, chemical toilets, composting pits. Septic tanks are suitable for places where there is no sewerage system, for hospitals, isolated buildings or a cluster of 4 to 5 houses. Some organizations offer hygienic on-site human waste disposal technology. These toilets do not require scavenging. Excreta from the toilet seats flow through covered drains into a biogas plant. The biogas produced is used as a source of energy.

Poor sanitation and contaminated drinking water is the cause of a large number of diseases. A vast number of our people are still without sewerage facilities. A very large fraction of our people defecates in the open, on dry riverbeds, on railway tracks, near fields and many a time directly in water. Untreated human excreta is a health hazard. It may cause water pollution and soil pollution. Both the surface water and groundwater get polluted. Groundwater is a source of water for wells, tube-wells, springs and many rivers. Thus, it becomes the most common route for water borne diseases. They include cholera, typhoid, polio, meningitis, hepatitis and dysentery.

Oil and fats can clog the drinks reducing its effectiveness in filtering water. In open drains, they block the pores in the soil and thus hamper the filtration by soil. Hence, oil and fats should not be released in the drain.

It has been suggested that we should plant eucalyptus trees all along sewage ponds. These trees absorb all surplus wastewater rapidly and release pure water vapor into the atmosphere.

Untreated human excreta contain many pathogens and disease-causing microbes like cholera, diarrhea, typhoid, polio, meningitis, hepatitis etc. The germs of these diseases can easily reach human population through air, water or insects and through contaminated food and water. Thus, untreated human excreta are a health hazard.

We all have a role to play in keeping our environment clean and healthy. We must realise our responsibility in maintaining the water sources in a healthy state. Adopting good sanitation practices should be our way of life. As an agent of change our individual initiative will make a great difference. Influence others with our energy, ideas and optimism. A lot can be done if people work together. There is great power in collective action.

The two water disinfectants used during treatment of waste water are chlorine and ozone.