Difference Between Primary and Secondary Sewage Treatment
Biology · Microbes in Human Welfare · NEET
Primary treatment is a physical step: it removes solid particles from sewage by filtration and sedimentation, forming primary sludge and a clearer liquid called effluent. Secondary (biological) treatment then uses aerobic microbes that grow into flocs and eat the organic matter, which lowers the BOD of the water. Memory hook: "Primary = Physical (Particles), Secondary = Sludge-eating microbes (BOD down)."
Sewage flows from primary treatment (physical removal of particles, giving primary sludge and effluent) to secondary treatment (aerobic microbes form flocs and lower BOD). Remember that BOD falls only in the secondary stage.
Your doubts, answered
Is primary treatment physical or biological?
Primary treatment is purely physical. It removes large and small particles from the sewage by filtration (floating debris) and sedimentation (grit like soil and pebbles). No microbes are used to break down waste in this step. The biological work is done later, in secondary treatment.
Which sewage treatment removes suspended solids?
Primary treatment removes the suspended solids. Floating debris is taken out by sequential filtration and grit settles by sedimentation. The settled solids form the primary sludge, and the leftover liquid (supernatant) is the effluent that moves on to secondary treatment. This exact point was asked in NEET 2017.
Does primary treatment reduce BOD?
No. BOD is significantly reduced only in secondary (biological) treatment, where aerobic microbes consume the organic matter. Primary treatment only removes physical particles, so it does not meaningfully lower BOD. This is a very common NEET trap.
What are flocs?
Flocs are masses of bacteria associated with fungal filaments that form mesh-like structures. They appear in the aeration tanks of secondary treatment. As these flocs grow, they consume the organic matter in the effluent and reduce its BOD. Flocs belong to secondary treatment, never primary.
What is the difference between primary sludge and activated sludge?
Primary sludge is the settled solid from primary treatment (physical particles). Activated sludge is the sediment of bacterial flocs that forms after secondary treatment, when the flocs are allowed to settle. A small part of activated sludge is recycled as inoculum; the rest goes to the anaerobic sludge digester.
⚠️ The NEET trap ✗ Both primary and secondary treatment reduce BOD by using microbes. ✓ Only secondary (biological) treatment reduces BOD, because that is where aerobic microbes form flocs and consume organic matter. Primary treatment is only physical removal of particles. 🧠 Primary = no microbes, no BOD drop. If BOD falls, you are in secondary treatment.
Real NEET questions
NEET 2017
Which of the following in sewage treatment removes suspended solids?
A · Tertiary treatment
B · Secondary treatment
C · Primary treatment ✓
D · Sludge treatment
Solution: Primary treatment is a physical process that removes suspended solids from sewage through filtration (floating debris) and sedimentation (grit). Secondary treatment uses aerobic microbes to lower BOD, so it does not do the initial physical removal. Hence primary treatment is correct.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Which treatment comes first, primary or secondary?
Primary treatment comes first. Its effluent (the clearer liquid after solids settle) is then passed on to secondary treatment for biological breakdown.
What is the key difference between primary and secondary sewage treatment?
Primary treatment is physical removal of particles by filtration and sedimentation. Secondary treatment is biological: aerobic microbes form flocs and consume organic matter, which reduces BOD.
Are microbes used in primary treatment?
No. Microbes are used only in secondary treatment. Primary treatment relies on physical processes alone.
What happens to the sludge after secondary treatment?
The bacterial flocs settle as activated sludge. A small part is recycled as inoculum into the aeration tank, and the remaining major part is sent to the anaerobic sludge digester, where it is digested to form biogas.
