Advances in technology have expanded the scope of Biology Topics we can investigate and understand.
Explain why it is harmful to discharge untreated sewage into rivers or seas.
The sewage (or wastewater) released by houses and other buildings contains a large number of harmful substances and disease-causing micro-organisms. If this untreated sewage is discharged into rivers, lakes or ground, it will contaminate our freshwater sources (like rivers, lakes and groundwater). Drinking of water contaminated with sewage will spread diseases in people which can even lead to deaths. The discharge of untreated sewage into rivers (and sea) can also kill fish and other aquatic animals. Since it is very harmful, untreated sewage should not be allowed to run into rivers, lakes, sea or underground. Sewage water should be treated suitably (or processed) before releasing it back to the environment.
The processing of sewage to remove pollutants and make it harmless so that it may be disposed of safely in the rivers, etc., is called sewage treatment (or wastewater treatment). The main purpose of sewage treatment is to reduce the pollutants (or harmful substances) present in it to such a low level that it may not remain a health hazard for any one. Sewage is treated at the sewage treatment plants which have been established in most of the cities and towns. Sewage is carried from houses and other places by underground pipes called ‘sewers’ to sewage treatment plant. A sewage treatment plant removes solid and liquid wastes, as well as most of the disease-causing micro organisms from the sewage so that the water leaving the sewage treatment plant is fit to use. We will now describe how sewage is purified at the sewage treatment plant.
Waste-Water Treatment Plant (or Sewage Treatment Plant)
A place where wastewater (or sewage) from houses and other buildings is brought for processing is called wastewater treatment plant. Waste-Water Treatment Plant is written in short form as WWTP. The wastewater treatment plant is also called sewage treatment plant. A modern wastewater treatment plant (or sewage treatment plant) works by treating the wastewater or sewage by a series of physical, chemical and biological processes till it becomes fit to be discharged into the environment. The various processes (or stages) in the treatment of wastewater or sewage are described below :
(i) SCREENING.The wastewater (or sewage) entering the sewage treatment plant is first passed through bar screens (This process is called screening). The bar screen removes the large rubbish objects like rags, sticks, cans, plastic bags (polythene bags), napkins, sanitary towels, etc., from the wastewater
(because they cannot pass through the holes of bar screen) [see Figure (a)]. Thus, screening removes large pieces of rubbish from the wastewater (or sewage). These big pieces of rubbish caught by the bar screen are removed from time to time.
(ii) GRIT AND SAND REMOVAL. The wastewater (or sewage) passing through the bar screen is made to flow slowly through a tank called ‘grit and sand removal tank’. As wastewater flows slowly, the grit and sand present in it settle down at the bottom of the tank [see Figure 2(b)]. This grit and sand is removed from the tank from time to time.
(iii) FIRST SEDIMENTATION TANK. The wastewater (or sewage) is then passed into a sedimentation tank and allowed to stand there for a while. Most of the solid organic matter (faeces, etc.) settles down on the sloping bottom of the sedimentation tank in the form of a sludge [see Figure (c)]. Thus, the solid part of sewage is called sludge. In this way, the first sedimentation tank separates the solid organic sludge from the rest of wastewater.
(a) The sludge is taken out from the bottom of first sedimentation tank and put into a large, closed tank called digester tank [see Figure (d)]. In the digester tank, many types of anaerobic bacteria decompose the organic matter present in sludge to produce biogas (also called sewage gas). (The anaerobic bacteria do not need oxygen of air to carry out the process of decomposition. That is why the digester tank is a closed tank). The biogas produced here can be used as a fuel directly or it can be used to generate electricity. The digested sludge left after the removal of biogas is taken out, dried and used as a manure (or fertiliser). Thus, the wastewater treatment (or sewage treatment) gives us two useful products :
- biogas, and
- sludge. The biogas is used as a fuel whereas sludge (or rather ‘digested sludge’) is used as a manure (or fertiliser). The use of dried sludge as a manure returns the nutrients to the soil.
(b) The wastewater left in the first sedimentation tank still has some organic waste in the form of tiny suspended particles (which do not settle at the bottom of the tank), as well as soluble organic matter. There may also be some oil and grease floating on the surface of wastewater in the sedimentation tank. These floating materials are removed by a skimmer. Thus, mainly watery waste is left in the first sedimentation tank. It is also called clarified water.
(iv) AERATION TANK. The mainly watery waste (or clarified water) from the first sedimentation tank is passed into aeration tank (The tank in which air is put into water is called aeration tank). The watery waste already contains aerobic bacteria in it. Compressed air is bubbled through the watery waste in the aeration tank to provide oxygen to activate aerobic bacteria and make them grow rapidly in this water [see Figure (e)]. The large number of aerobic bacteria produced in this way digest (or consume) any human waste, food waste, soaps and other unwanted and harmful matter still remaining in the wastewater, leaving behind fairly pure water. In this way, aerobic bacteria clean the clarified watery waste and make it almost harmless.
(v) SECOND SEDIMENTATION TANK. From the aeration tank, the treated watery waste goes into the second sedimentation tank and allowed to stand there. In this tank, the micro-organisms (aerobic bacteria, etc.) used in the aeration tank settle down at the bottom of the tank forming a fine sludge called ‘activated sludge’ [see Figure (f)]. It is called activated sludge because active micro-organisms (or living micro-organisms) such as aerobic bacteria, and oxygen are present in it. Some of the activated sludge is added back to the aeration tank to increase the population of aerobic bacteria there and speed up the cleaning of watery waste.
The remaining activated sludge is sent to the digester tank (for the production of biogas). The water left in the second sedimentation tank has a very low level of organic material and suspended matter. This water does not contain much harmful things, so it is quite safe. This water is then discharged into a river (or sea), or made to percolate into the ground [see Figure (f)). The natural processes clean this water further. Please note that the water in a river is cleaned naturally by’ processes that are similar to those adopted in a wastewater treatment plant (like the cleaning of water done by aerobic bacteria).
Sometimes it may be necessary to disinfect the treated water with chemicals like chlorine or ozone before releasing it into the distribution system. The purpose of disinfection in the treatment of wastewater is eliminate completely the harmful micro-organisms in the water to be discharged back into the environment. The treated wastewater can also be passed through layers of sand (called sand filters). Sand filtration removes most of the remaining suspended matter from the treated wastewater.
It has been suggested that we should plant eucalyptus trees all along the sewage ponds. This is because eucalyptus trees absorb all the surplus wastewater from the sewage ponds rapidly and release pure water vapour into the atmosphere (through transpiration). In this way, eucalyptus trees help in purifying wastewater quite rapidly.
Treatment of Polluted Water
Polluted water is a kind of wastewater. We will now perform an activity to treat a sample of polluted water and make it clean. This will help us in understanding the processes that take place at the wastewater treatment plant (or sewage treatment plant). In order to perform this activity, we will have to make polluted water ourselves. This can be done as follows.
Fill a glass jar three-fourths with tap water. Add a little of soil, some dirty organic matter such as grass pieces and orange peels, a small amount of detergent, and a few drops of ink (or any other colour) to it. Cap the glass jar, shake it well and keep it in the sun for at least two days. During these two days, the organic matter present in the glass jar will decay partially and give us polluted water (which is a kind of wastewater). If we open the cap of the glass jar, we will get a foul smell coming out from the polluted water inside it. We will now treat this self-made polluted water to convert it into clean water. This can be done as follows.
(i) Shake the contents of the glass jar well and pour the contents over a strainer (chhalni) kept over a beaker. The larger pieces of grass and orange peels will not pass through the holes of strainer and hence get removed. The remaining part of polluted water will collect in the beaker kept below the strainer.
(ii) Allow the polluted water to stand in the beaker for some time. The solid’soil’present in the polluted water will settle down at the bottom of the beaker.
(iii) Transfer the polluted water from the toptif this beaker into another beaker by decantation. Pass air into this polluted water for several hours an aerator is not available, use a mechanical stirrer (or mixer) to stir the polluted water several times (this will also provide air to the polluted water).
(iv) When the aeration is complete, we will find that the foul smell from polluted water has almost disappeared. This aerated water has, however, some suspended impurities in it. Filter the aerated water through the layers of sand, fine gravel and medium gravel arranged on a filter paper kept in a funnel (see Figure). The sand filter removes the tiny suspended impurities from the treated water. Collect the clean water as a filtrate in a beaker kept below the funnel (see Figure).
(v) Add a small piece of chlorine tablet to the clean water collected in the beaker. Mix well until the water is clear and colourless. This is the clean and clear safe water obtained by the treatment of polluted water. The treated water does not give any foul smell. It, however, smells of chlorine. But the smell of chlorine is not foul like that of polluted water. The chlorine disinfects the water (makes it germ-free), and also removes the colour by bleaching.
Some Good Housekeeping Practices
We can minimise the waste which goes from our house into the drains by taking some simple precautions. Some of the good housekeeping practices which help us to minimise the undesirable wastes going from our house down the drain (into sewers) are as follows:
(i) The wastes such as solid food remains, used tea leaves, sanitary towels, polythene bags, used cotton, hair, and soft toys, etc., should not be thrown down the drain. This is because if these wastes are thrown in piped drains’ carrying wastewater, they block the drains (or choke the drains), and do not allow the free flow of wastewater through them. By blocking the drains, these wastes also do not allow free flow of oxygen (of air) to the useful microbes in the drain which help to decompose the wastes. This hampers the natural process of purification of wastewater. Solid food remains, used tea leaves, sanitary towels, polythene bags, used cotton, hair and soft toys, etc., should always be thrown in a dustbin.
(ii) Waste cooking oils and fats should not be thrown in the kitchen sink to go down the drain. This is because cooking oils and fats can harden and block the drainage pipes. On the other hand, in an open drain, the fats clog (or block) the soil pores reducing its effectiveness in filtering water. The waste oils and fats should always be thrown in the dustbin.
(iii) The chemicals like paints, solvents, insecticides, medicines and motor oil, etc., should not be thrown down the drain because they may kill the microbes which help purify wastewater naturally. Please note that by minimising the throwing of undesirable wastes into the drainage system of our house, we can reduce the load of wastewater treatment plants.
Sanitation And Disease
Sanitation means to create hygienic environment (or clean environment) around us which is essential for preventing diseases and keeping good health. Poor sanitation and contaminated drinking water is the cause of a large number of diseases in our country. This happens as follows.
A vast number of people in our country still do not have sewerage facilities for the safe disposal of human excreta. Due to this, a very large number of our people defecate in the open in fields, near railway tracks, on dry river beds and many a time directly in water (in ponds, lakes and rivers, etc.). The untreated human excreta is a health hazard (because it can cause a large number of water-borne diseases).
The untreated human excreta causes soil pollution and water pollution. Since the human excreta is carried along by rainwater, therefore, both surface water (in lakes and rivers) as well as groundwater get polluted (or contaminated) with it. The river water and groundwater are the sources of drinking water for many people (The groundwater is drawn out through wells, tube-wells and handpumps). The drinking of water contaminated with untreated human excreta becomes the most common route for the spreading of water-borne diseases. Some of the water borne diseases spread in this way are : Cholera, Typhoid, Polio, Meningitis, Hepatitis and Dysentery.
In order to prevent water-borne diseases, people should not defecate in the open spaces (or near the water bodies). People should realise their responsibility in maintaining the ‘water sources’ in a healthy state by preventing their pollution (or contamination).
Clean water is a basic need of human beings. Unfortunately, clean water that is fit for drinking is not available to all the people. It has been estimated that more than one billion people in the world do not get safe drinking water. The drinking of contaminated water by the people is responsible for a large number of water- related diseases and even deaths, throughout the world. By realising the grave situation created by the extreme shortage of clean and safe drinking water in many parts of the world, the General Assembly of United Nations on the World Water Day on 22nd March 2005 has declared the period 2005-2015 as the International Decade for action on ‘Water for Life’. All efforts will be made during this decade to reduce by half the number of people who do not have access to safe drinking water.