Primary treatment of sewage is the FIRST stage of sewage treatment. It is a PHYSICAL process: floating debris is removed by filtration, then grit (soil and small pebbles) settles down by sedimentation. The solids that settle form the primary sludge, and the clear liquid on top is the effluent (which goes to secondary treatment). Memory hook: "Primary = Physical" (both start with P). No microbes do the work here — it is just filtering and settling.
Primary treatment is physical: filtration removes floating debris, sedimentation settles grit, and sewage separates into effluent (liquid, sent to secondary treatment) and primary sludge (settled solids).
Your doubts, answered
Is primary treatment physical or biological? This confuses many students.
Primary treatment is a PHYSICAL process only. It uses filtration and sedimentation to remove particles. No microbes reduce the waste here. Microbes act in the NEXT stage, called secondary (biological) treatment. NEET loves this exact difference, so remember: Primary = Physical, Secondary = Biological.
What is actually removed in primary treatment of sewage?
Only the suspended solids (physical particles) are removed. First, large floating debris (like plastic, cloth, paper) is removed by sequential filtration. Then the grit (soil and small pebbles) is removed by sedimentation (settling down). The dissolved organic matter is NOT removed here — that is handled in secondary treatment.
What is the difference between primary sludge and effluent?
After settling, the sewage separates into two parts. The solids that SETTLE at the bottom form the primary sludge. The liquid supernatant (clear watery part) on top is called the effluent. The effluent from the primary settling tank is taken forward for secondary treatment. So: sludge = settled solids, effluent = liquid on top.
What is the correct order of steps in primary treatment?
The order is: (1) sequential filtration removes floating debris first, then (2) sedimentation removes grit (soil and small pebbles). NEET can ask the sequence, so remember debris (filtration) comes BEFORE grit (sedimentation).
What is grit in sewage treatment?
Grit means the heavy inorganic matter — soil particles and small pebbles present in the sewage. It is removed by sedimentation, meaning it is allowed to settle down under gravity. Grit is removed after the floating debris is filtered out.
⚠️ The NEET trap ✗ Primary treatment removes organic matter and lowers BOD using microbes. ✓ Primary treatment is purely PHYSICAL (filtration + sedimentation) and removes suspended solids. BOD is lowered by microbes in SECONDARY treatment, not primary. 🧠 If a question links BOD reduction or microbes to primary treatment, it is a trap. Primary = physical removal only.
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 sequential filtration and sedimentation (floating debris first, then grit). Secondary (biological) treatment uses aerobic microbes to reduce BOD, while tertiary and sludge treatment occur later, so they do not perform the initial removal of suspended solids.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the physical removal of solid particles from sewage by filtration and sedimentation, producing primary sludge and effluent.
Does primary treatment reduce BOD?
No. Primary treatment removes only solids and does not significantly reduce BOD. BOD is reduced by aerobic microbes during secondary treatment.
What are the two physical steps in primary treatment?
First, sequential filtration removes floating debris. Second, sedimentation removes grit (soil and small pebbles).
Where does the effluent go after primary treatment?
The effluent (supernatant liquid) from the primary settling tank is passed on to secondary (biological) treatment.
Why is primary treatment important for NEET?
NEET frequently tests that primary treatment is physical (removes suspended solids), while secondary is biological. The Primary vs Secondary difference is a common one-mark question.
