Microbes in Sewage Treatment - Practice Questions & MCQ

• We know that large quantities of waste water are generated everyday in cities and towns. 
• A major component of this waste water is human excreta. This municipal waste-water is also called sewage. 
• It contains large amounts of organic matter and microbes. 
• Before disposal, hence, sewage is treated in sewage treatment plants (STPs) to make it less polluting.
• Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage.
• This treatment is carried out in following stages:

Primary treatment:

• These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. 
• These are removed in stages:
  • Initially, floating debris is removed by sequential filtration. 
  • Then the grit (soil and small pebbles) are removed by sedimentation. 

• All solids that settle form the primary sludge, and the supernatant forms the effluent.
• The effluent from the primary settling tank is taken for secondary treatment.

Secondary treatment or Biological Treatment:

• The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. 
• This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). 
• While growing, these microbes consume the major part of the organic matter in the effluent. 
• This significantly reduces the BOD (biochemical oxygen demand) of the effluent.
• Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. 
• This sediment is called activated sludge. 
• A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. 
• The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. 
• Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. 
• During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. 
• These gases form biogas and can be used as a source of energy as it is inflammable.

Note - Biological Oxygen Demand

• BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. 
• The sewage water is treated till the BOD is reduced. 
• The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water.
• The greater the BOD of waste water, more is its polluting potential.