Q.Treatment of wastewater is done in a sewage treatment plant to make it less polluting. Explain the following with reference to this treatment process :
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Start your 14-day free trial to unlock the full solution →Wastewater treatment involves primary physical removal of solids to form primary sludge, followed by secondary biological treatment where aerobic microbes form activated sludge, which is then digested anaerobically to produce biogas.
Wastewater, or sewage, contains a large amount of organic matter and pathogenic microbes, making it highly polluting. Before discharge into natural water bodies, it must undergo treatment in a Sewage Treatment Plant (STP) to reduce its polluting potential. This treatment typically involves two main stages: primary treatment and secondary treatment. The terms primary sludge, activated sludge, and anaerobic sludge digesters refer to key components and processes within these stages.
(a) Primary Sludge
Primary treatment is the initial physical process of wastewater treatment. It involves sequential filtration and sedimentation.
- Filtration: Floating debris is removed by sequential filtration through screens.
- Grit Removal: Grit (soil and small pebbles) is removed by sedimentation in settling tanks.
Once these physical processes are complete, the wastewater enters large primary settling tanks. In these tanks, the larger, heavier solid particles, both organic and inorganic, settle down due to gravity. This settled material is called primary sludge. The supernatant, which is the effluent from the primary settling tank, is then taken for secondary treatment. Primary sludge primarily consists of the solid waste that has settled out, including human faeces, food waste, and other suspended solids.
Primary treatment does not significantly reduce the Biochemical Oxygen Demand (BOD) of the wastewater, as it mainly removes physical solids, not dissolved organic matter.
(b) Activated Sludge
Activated sludge is a crucial component of the secondary, or biological, treatment stage. The effluent from the primary settling tank is transferred into large aeration tanks. In these tanks, the wastewater is continuously agitated mechanically, and air is pumped into it. This creates an aerobic environment, encouraging the vigorous growth of useful aerobic microbes.
These aerobic microbes consume the major part of the organic matter present in the effluent. During their growth, they form masses of bacteria associated with fungal filaments, creating a mesh-like structure. These structures are called flocs. The microbes in the flocs consume the organic matter, significantly reducing the Biochemical Oxygen Demand (BOD) of the wastewater. BOD is a measure of the amount of oxygen required by microbes to decompose organic matter in a given volume of water. A significant reduction in BOD indicates that the polluting potential of the wastewater has been substantially lowered.
Once the BOD of the wastewater is reduced, the effluent is passed into a secondary settling tank. Here, the bacterial flocs are allowed to settle. This settled material is called activated sludge. A small part of this activated sludge is pumped back into the aeration tank to serve as an inoculum, meaning it provides the necessary microbial population to start the next cycle of biological treatment. The remaining major portion of the activated sludge is then pumped into large tanks called anaerobic sludge digesters. …
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