CBSE – 7th Standard Science

Chapter 18 – Waste water Story

Chapter 18 

Waste water Story

  • Clean water is a basic need of human being.  Clean water that is fit for use is unfortunately not available to all.
  • Increasing scarcity of freshwater due to population growth, pollution, industrial development, mismanagement and other factors.
  • 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.
  • Sewage is wastewater released by homes, industries, hospitals, offices and other users.
  • It also includes rainwater that has run down the street during a storm or heavy rain.
  • The water that washes off roads and rooftops carries harmful substances with it.
  • Sewage is a liquid waste.
  • Most of it is water, which has dissolved and suspended impurities.
  • These impurities are called contaminants.
  • Sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and microbes.
  • Organic impurities –Human faeces, animal waste, oil, urea (urine), pesticides, herbicides, fruit and vegetable waste, etc.
  • Inorganic impurities – Nitrates, Phosphates, metals. Nutrients – Phosphorus and Nitrogen.
  • Bacteria – Such as which cause cholera and typhoid, other microbes cause dysentery.
  • Network of big and small pipes, called sewers, forming the sewerage. It is like a transport system that carries sewage from the point of being produced to the point of disposal.



  • Wastewater is passed through bar screens.
  •  Large objects like rags, sticks, cans, plastic packets, napkins are removed.
  • 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.
  • 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 scraper. This is the sludge.


  • 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.
  • A skimmer removes the floatable solids like oil and grease. Water so cleared is called clarified water.
  • 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.
  • Waste generation is a natural part of human activity. But we can limit the type of waste and quantity of waste produced.
  •  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

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  • Cooking oil and fats should not be thrown down the drain.
  • They can harden and block the pipes.
  • In an open drain the fats clog the soil pores reducing its effectiveness in filtering water. Throw oil and fats in the dustbin.
  • Chemicals like paints, solvents, insecticides, motor oil, and medicines may kill microbes that help purify water.
  • So do not throw them down the drain.
  • Used tealeaves, solid food remains in sink, these wastes choke the drains.


  • They do not allow free flow of oxygen.
  • This hampers the degradation process
  • 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, many a time directly in water.
  • Untreated  human excreta is a health hazard it causes water pollution and soil pollution.
  • Groundwater is a source of water for wells, tubewells, springs and many rivers it becomes the most common route for water borne diseases.
  • They include cholera, typhoid, polio, meningitis, hepatitis and dysentery.
  • To improve sanitation, low cost onsite sewage disposal systems are being encouraged, septic tanks, chemical toilets, composting pits.
  • Septic tanks are suitable for places where there is no sewerage system.
  • In hygienic on-site human waste disposal technology, excreta flow through covered drains into a biogas plant.
  • The biogas produced is used as a source of energy.
  • A design of a toilet in which humans excreta is treated by earthworms has been tested in India.


  •  It has been found to be a novel, low water-use toilet for safe processing of human waste. The operation of the toilet is very simple and hygienic.
  • The human excreta is completely converted to vermi cakes  a resource much needed for soil.


  • Used water is wastewater. Wastewater could be reused.
  • Wastewater is generated in homes, industries, agricultural fields and in other human activities.
  • Sewage is a liquid waste which causes water and soil pollution.