Question 1 What is the function of bar screen in a wastewater treatment plant?
Question 2 What is sludge? Explain how it is treated?
Question 3 Describe the various steps involved in getting clear water from wastewater?
Question 4 What is meant by treatment of sewage?
Question 5 What is the purpose of bubbling air into the aeration tank during the treatment of sewage?
Question 6 Why oils and fats should not be released in the drain?
Question 7 Name the various wastes which should be thrown down the drain?
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.
The various (processes or stages) in the treatment of wastewater or sewage are:
(1) 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 waste water. 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.
(2) 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. This grit and sand is removed from the tank from time to time
(3) 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. Thus, the solid part of sewage is called sludge.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. 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). 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 particles 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.
(4) 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). 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 .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.
(5) 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 used in the aeration tank settle down at the bottom of the tank forming a fine sludge called activated sludge.
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.
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 to eliminate completely the harmful micro-organisms in the water to be discharged back into the environment.
The treated wastewater can also be passed through lavers of sand (called sand filters). Sand filtration removes most of the remaining suspended matter from the treated wastewater.
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). Eucalyptus trees help in purifying wastewater quite rapidly
Activity : Treatment of polluted water
(1) Fill a glass jar three-fourths with tap water.
(2) 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.
(3) Cap the glass jar, shake it well and keep it in the sun for at least two days.
(4) During these two days, the organic matter present in the glass jar will decay partially and give us polluted water.
(5) 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.
(6) Shake the contents of the glass jar well and pour the contents over a strainer 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.
(7) 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.
(8) Transfer the polluted water from the top of this beaker into another beaker by decantation. Pass air into this polluted water for several hours by using an aerator from a domestic aquarium.
(9) 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.
(10) 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.
(11) 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 hot give any foul smell.
Good Housekeeping Practices
Some of the good housekeeping practices which help us to minimise the undesirable going from our house down the drain (into sewers) are as follows:
(1) 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 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.
(2) 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.In an open drain, the fats clog (or block) the soil pores reducing its effectiveness in filtering water.
(3) 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.