What is the key difference between primary and secondary sewage treatment?
The key difference is that Primary treatment involves the physical removal of particles, whereas Secondary treatment is a Biological treatment.
In Primary treatment, large and small particles are removed from sewage through sequential filtration and sedimentation. The solids that settle form the primary sludge, and the supernatant forms the effluent.
Secondary treatment passes the primary effluent into aeration tanks where constant agitation and air allow useful aerobic microbes to grow into flocs. While growing, these microbes consume the major part of the organic matter, which significantly reduces the BOD (biochemical oxygen demand) of the effluent.
Explanation
The answer distinguishes the physical nature of primary treatment (filtration/sedimentation) from the biological nature of secondary treatment (microbial digestion/aeration). It highlights that primary treatment produces sludge while secondary treatment targets BOD reduction through flocs, strictly adhering to the provided text.
Solution Steps
Identify the mechanism of Primary Treatment as physical removal via filtration and sedimentation.
Identify the mechanism of Secondary Treatment as biological using aerobic microbes and aeration.
Contrast the outcomes: removal of solids/sludge vs. reduction of BOD/organic matter.