Primary and Secondary Treatment
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Sewage treatment is a multi-stage process designed to remove contaminants from wastewater, primarily domestic sewage, to produce an effluent that is suitable for discharge to the natural environment or for reuse. This complex process is broadly categorized into primary and secondary treatments, each employing distinct mechanisms to achieve purification. Primary treatment focuses on the physical re…
- Sewage Treatment — Removes pollutants from wastewater.
- Primary Treatment — Physical process. Removes large solids & grit.
- Steps: Screening Grit Removal Primary Sedimentation. - Output: Primary effluent (liquid), Primary sludge (solids).
- Secondary Treatment (Biological) — Uses microbes to reduce organic matter.
- Steps: Aeration Tank (aerobic microbes, floc formation) Secondary Sedimentation (activated sludge settles). - Aeration Tank: Oxygen supplied, aerobic microbes form flocs, consume organic matter. - Flocs: Masses of bacteria and fungi. - BOD (Biochemical Oxygen Demand): Measure of organic pollution. High BOD = high pollution. Secondary treatment significantly reduces BOD. - Activated Sludge: Settled microbial flocs, partly recycled to aeration tank.
- Anaerobic Sludge Digester — Treats primary & waste activated sludge.
- Anaerobic bacteria break down organic matter. - Produces Biogas: (major), , .
- Treated Effluent — Water after secondary treatment, low BOD, safe for discharge.
To remember the sequence and key aspects of sewage treatment:
Physical Separation Always Starts Aerobic Sludge Digestion
- Physical: Primary Treatment
- Separation: Screening & Grit Removal
- Always: Primary Aeration (Settling)
- Starts: Secondary Treatment
- Aerobic: Aeration Tank (Aerobic microbes, Flocs, BOD reduction)
- Sludge: Secondary Settling (Activated Sludge)
- Digestion: Anaerobic Sludge Digestion (Biogas)