Which of the following statements is/are incorrect ? 1. BOD (biochemical oxygen demand) of the effluent refers to the amount of the carbon dioxide that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. 2. The greater the BOD of wastewater, the more is its polluting potential.
Treatment of wastewater is done by the heterotrophic microbes naturally present in the sewage . This treatment is carried out in two stages: 1. Primary treatment : These treatment steps basically involve the 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) is 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. 2. Secondary treatment or Biological treatment: The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and the 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. BOD refers to the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidized by bacteria. Hence, Statement 1 is correct. 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 wastewater, the more is its polluting potential. Hence, Statement 2 is correct. Once the BOD of sewage or wastewater 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 sulfide, and carbon dioxide. These gases form biogas and can be used as a source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams .