Sedimentation

Question 1

In drinking water treatment, where does sedimentation sit relative to coagulation/flocculation—and why?

Answer 1

It typically follows coagulation/flocculation to remove formed flocs by gravity settling

Question 2

In wastewater treatment, sedimentation usually occurs:
A) Only before biological treatment
B) Only after biological treatment
C) Both before (primary) and after (secondary) biological treatment
D) After disinfection

Answer 2

C) Both before (primary) and after (secondary) biological treatment

Question 3

How do the impurities behave in water and wastewater?

Answer 3

– in flowing liquids remain in suspension
– in quiescent liquids will settle under the influence
of gravity

Question 4

T/F — In quiescent water, suspended impurities will tend to settle under gravity.

Answer 4

True

Question 5

Why don’t the same impurities necessarily settle in flowing water?

Answer 5

Turbulent mixing/flow keeps them in suspension despite gravity

Question 6

What type of sedimentation tanks can you get?

Answer 6

Rectangular and circular

Question 7

What two factors determine the classification of settling into four distinct types?

These factors describe the state of the particle mixture.

Answer 7

The concentration of particles present and their ability to interact.

These factors influence how particles settle and the resulting settling

Question 8

What characterizes Class I - Discrete Particle Settling?

Answer 8

Particles settle as individual entities without interacting with neighboring particles.

This occurs at low particle concentrations.

Question 9

What is the key feature of Class II - Flocculent Settling?

The particles are no longer independent.

Answer 9

Particles interact and coalesce (flocculate), forming larger aggregates with higher settling velocities.

The particle size and mass change during the settling process.

Question 10

What occurs in the final stage, Class IV - Compression Settling?

This is the final stage in a sedimentation tank.

Answer 10

The settled particles are in such a high concentration that they form a compressed structure, and further settling is due to the compression of the sludge layer under its own weight.

This process is very slow and is mainly about compaction, not free settl

Question 11

Which class of settling occurs at low suspended solids concentrations where particles settle as individual entities?

This is the simplest form of settling.

Answer 11

Class I: Discrete Settling.

There is no significant interaction between particles.

Question 12

Definition

What is Class I - Discrete Particle Settling?

Answer 12

The settling of low concentrations of discrete, non-flocculent particles that settle individually without interacting.

This is the simplest form of sedimentation.

Question 13

Multiple Choice

Which of the following is a key application of Discrete Settling?
A) Thickening activated sludge
B) Removing sand and grit in pre-treatment
C) Compressing sludge in a secondary clarifier

Think about where non-interacting, heavy particles are removed.

Answer 13

B) Removing sand and grit in pre-treatment

Grit chambers are designed based on the principles of discrete settling.

Question 14

Formula Recall

What is the formual for Drag Force?

Answer 14

FD = ½ × CD × A × ρ × v_s²

Question 15

Formual Recall

What is the formula for the Drag Coefficient (C_D) in the laminar flow regime?

Answer 15

C_D = 24/Re

This simplifies the settling velocity equation to Stokes’ Law.

Question 16

Besides laminar flow, what other key limitation applies to the standard Stokes’ Law equation?

The equation assumes a specific particle shape.

Answer 16

It is only valid for spherical particles.

For non-spherical particles, a shape factor must be introduced.

Question 17

What are the key assumptions for “Ideal Sedimentation” in a horizontal flow basin?

Answer 17

1. Uniform particle concentration at the inlet
2. Steady flow
3. No re-suspension of settled particles.
4. Flow-through period equals hydraulic detention time
5. Particles are discrete.

These assumptions allow for a straightforward calculation of removal eff

Question 18

True or False

In an ideal sedimentation basin, the flow-through period is shorter than the theoretical hydraulic detention time due to short-circuiting.

Answer 18

False

A key assumption is that the flow-through period is equal to the hydrau

Question 19

What is the purpose of the “sludge zone” in a horizontal flow sedimentation basin?

It’s where the removed material collects.

Answer 19

To store and collect the settled particles (sludge) for removal.

Proper sludge removal is critical to maintain the tank’s capacity and pe

Question 20

Definition

What is the “Surface Overflow Rate” (v0) in an ideal sedimentation basin?

Answer 20

The settling velocity of the smallest particle that is 100% removed.

It represents the upward velocity of the water in an imaginary sense an

Question 21

What determines the removal efficiency of a particle with a settling velocity v2 that is less than the surface overflow rate v0?

Answer 21

Its starting depth. Only particles that start at a depth from which they can reach the bottom during the detention time will be removed.

Question 22

Formual Recall

What is the formual fro fraction removed?

Answer 22

v2/v0

Question 23

When are particles partially removed?

Answer 23

When particles have a vecloity v2, less than v0

Question 24

Formual Recall

What is the removal efficiency formual?

Answer 24

v_sA/Q

Question 25

# Simple Q&A What does the surface overflow rate represent?

Answer 25

It physically represents the settling velocity of the smallest particle that is fully removed

Question 26

Why is a settling column test necessary for real-world sedimentation basin design?

Answer 26

Because real water contains a mixture of particles with different settling velocities. The test determines the overall removal for a given overflow rate. | It provides essential data that the ideal model cannot.

Question 27

What is the purpose of having a port at a specific height (h) in the settling column? | It allows for sampling at a known depth

Answer 27

To withdraw samples at specific time intervals to measure the concentration of particles remaining in suspension at that depth. | This data is used to construct the particle settling velocity distributi

Question 28

# Test Procedure What is the first step in conducting a discrete settling column test?

Answer 28

Thoroughly mix the suspension in the column to ensure a uniform distribution of particles at the start (t=0). | This initial sample gives the starting concentration C0 .

Question 29

# Formual Recall What is the formula of the proportion of particles remaining x1?

Answer 29

x1 = C1/C0

Question 30

# Formula Recall What is the removal fraction of the particles that are removed with velcoity vs>v0?

Answer 30

r0 = 1 - x0

Question 31

# Definition What is Flocculent (Class II) Settling?

Answer 31

The settling of flocculent particles in dilute suspension where particles coalesce to form larger aggregates (flocs) during descent. | This occurs in primary sedimentation for wastewater and in water treatme

Question 32

What are the two principal causes of flocculation during sedimentation?

Answer 32

1. Differences in settling velocities (faster particles overtake and coalesce with slower ones) 2. Velocity gradients in the fluid causing particles to collide

Question 33

Primary settling in a wastewater treatment plant is an example of which class of settling? | The organic particles in sewage are flocculent.

Answer 33

Class II - Flocculent Settling.

Question 34

# Definition What is Hindered (Zone) Settling (Class III)?

Answer 34

Settling where particles are so close that they interfere with each other, settling as a mass (or zone) with a distinct interface between the sludge and clarified water.

Question 35

Where does Hindered Settling typically occur in a wastewater treatment plant?

Answer 35

In the final clarifiers following the activated sludge process, and in sludge thickeners. | It occurs at suspended solids concentrations greater than about 500 mg/L

Question 35

How does the rate of Compression Settling compare to other settling classes?

Answer 35

It occurs very slowly over a long period.

Question 36

What is the main trade-off when choosing an upflow clarifier over a horizontal flow clarifier? | It's a compromise between efficiency and cost.

Answer 36

Upflow clarifiers are less efficient but are often smaller and cheaper to construct.

Question 37

Name 3 types of flow behaviours?

Answer 37

1. Horizontal 2. Radial 3. Upflow

Question 38

How does particle removal work in an upflow clarifier?

Answer 38

Water moves upward at velocity v0. Particles that sink faster than v0 are removed, while particles that sink slower than v0 are washed out with the effluent.

Question 39

# Simple Q&A Why is an upflow clarifier always less efficient than a horizontal-flow clarifier with the same surface overflow rate (v₀)?

Answer 39

Because an upflow clarifier removes NO particles with vs < v0, while a horizontal flow tank removes a fraction of them depending on their starting depth.

Question 40

What is the main advantage of choosing an upflow clarifier despite its lower efficiency?

Answer 40

They are often smaller and cheaper to construct.

Question 41

What is "short-circuiting" in a sedimentation tank?

Answer 41

When a portion of the inflow travels directly to the outlet in a much shorter time than the theoretical detention time.

Question 42

How can "density currents" negatively impact sedimentation?

Answer 42

Colder, denser water can flow along the bottom, carrying incoming solids directly to the outlet and bypassing the settling zone.