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Select the INCORRECT statement regarding purification of water.
Screening is done before plain sedimentation.
Type-1 settling involves StockтАЩs law.
Sedimentation aided with coagulation is better than plain sedimentation.
Plain sedimentation is also known as type-2 settling.
Plain sedimentation is also known as type-2 settling.
Plain sedimentation involves the removal of discrete particles and is classified as Type-1 settling. Type-2 settling refers to flocculent settling, where particles interact and aggregate during sedimentation, often following the addition of coagulants.
Plain sedimentation involves the removal of discrete particles and is classified as Type-1 settling. Type-2 settling refers to flocculent settling, where particles interact and aggregate during sedimentation, often following the addition of coagulants.
vsтАЛ=18╬╜g(GтИТ1)d2тАЛ тАФ Terminal settling velocity of a spherical particle under laminar flow (Stoke's Law)
Re=╬╜vsтАЛdтАЛ тАФ Reynolds number used to determine the flow regime
In Type-1 settling (plain sedimentation), particles settle independently at a constant velocity without changing their size, shape, or mass. This is governed by Stoke's Law, which relates the settling velocity of a spherical particle to its diameter and the properties of the fluid. Type-2 settling involves flocculation, where particles collide and grow, leading to an increasing settling velocity over time.
Type-1 settling is independent (discrete) settling.
Type-2 settling is flocculent settling involving particle interaction.
Plain sedimentation uses gravitational force to remove suspended solids without chemical intervention.
Coagulation-flocculation (Type-2) significantly increases the removal efficiency of fine colloids.
Reduces load on downstream treatment units.
Cost-effective method for preliminary solid removal.
Ineffective for removing colloidal particles.
Requires large land area for settling tanks.
Preliminary treatment in water supply schemes.
Removal of grit and silt from raw water.
| Feature | Plain Sedimentation | Sedimentation with Coagulation |
|---|---|---|
Settling Type | Type-1 (Discrete) | Type-2 (Flocculent) |
Stoke's Law is valid when the Reynolds number Re<1.
Option A is correct because screening protects pumps and removes large debris before sedimentation.
Option B is correct as Type-1 settling assumes discrete particles following Stoke's Law.
Option C is correct because chemical coagulants neutralize surface charges to promote aggregation.
D is correct тАФ Plain sedimentation is classified as Type-1 settling (discrete particle settling), whereas Type-2 settling specifically refers to flocculent settling.
Remember the sequence: Screening тЖТ Plain Sedimentation тЖТ Coagulation/Flocculation тЖТ Secondary Sedimentation тЖТ Filtration.
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