Coagulation\Flocculation

Question 1

In drinking water treatment, why is coagulation/flocculation used before sedimentation/filtration for particles <10 µm?

Answer 1

To aggregate fine, often negatively charged particles so they can be cost-effectively removed by sedimentation/filtration

Question 2

T/F — Most suspended particles in raw water carry a positive charge, so they naturally attract each other.

Answer 2

False - Many carry a negative charge and repel; coagulants reduce this stability.

Question 3

Coagulation is the process of ______ colloidal particles to permit particle growth; flocculation is the ______ of these destabilized particles.

Answer 3

destabilizing; agglomeration.

Question 4

Which step directly produces settleable “flocs”?
A) Coagulation
B) Flocculation
C) Disinfection
D) Aeration

Answer 4

B) Flocculation

Question 5

What two opposing forces govern whether colloids stick together?

Answer 5

Electrostatic repulsion vs van der Waals attraction.

Question 6

T/F — Coagulation/flocculation must overcome repulsion so attractive forces or chemical reactions can hold particles together.

Answer 6

True

Question 7

When does van Der Waals cause attraction?

Answer 7

At very close distances

Question 8

What is the “energy barrier” in colloid stability diagrams?

Answer 8

The net energy hump from electrostatic repulsion that prevents particles from approaching closely enough for van der Waals attraction to dominate.

Question 9

What enables particles to aggregate?

Answer 9

Reducing the energy barrier

Question 10

Name the four basic coagulation destabilization mechanisms.

Answer 10

Diffuse layer compression, charge neutralization, sweep flocculation, inter-particle bridging.

Question 11

Which mechanism relies on polymers chemically linking multiple particles?
A) Diffuse layer compression
B) Charge neutralization
C) Sweep flocculation
D) Inter-particle bridging

Answer 11

D) Inter-particle bridging

Question 12

What happens to the electrical double layer when ionic strength increases?

Answer 12

It compresses, lowering the energy barrier between particles.

Question 13

T/F — Higher ionic strength expands the double layer and stabilizes colloids.

Answer 13

False - it compresses the double layer and promotes aggregation.

Question 14

On an energy-vs-distance plot, high ionic strength shifts which curve?
A) Attraction up
B) Repulsion down (net barrier decreases)
C) Both up
D) No change

Answer 14

B) Repulsion down, reducing the net energy barrier.

Question 15

Practically, why does lowering the energy barrier matter?

Answer 15

It lets particles approach closely so van der Waals forces can hold them together, enabling floc growth

Question 16

T/F — In full-scale plants, increasing ionic strength is a common primary method to destabilize colloids.

Answer 16

False - it’s generally not practical to add enough ions.

Question 17

If diffuse layer compression isn’t practical, what mechanisms are typically used?

Answer 17

Charge neutralization, sweep flocculation, and polymer bridging.

Question 18

Define charge neutralization in coagulation.

Answer 18

Addition of oppositely charged ions (e.g., Al³⁺, Fe²⁺/Fe³⁺) that adsorb to negatively charged particles to reduce net surface charge to ~0

Question 19

How do cations behave?

Answer 19

Cations adsorb to surface of (-ve) particles

Question 20

T/F — Overdosing a cationic coagulant can restabilize particles.

Answer 20

True - particles can become net positively charged and repel again

Question 21

What dosing target avoids restabilization?

Answer 21

Dose near the zero-charge point—just enough to neutralize particle surface charge, not exceed it.

Question 22

What is “sweep flocculation”?

Answer 22

Precipitating hydroxides/carbonates at suitable pH so particles adsorb onto the precipitate en masse (“sweep”), forming large fluffy flocs.

Question 23

Which precipitates commonly form during sweep flocculation?
A) NaCl, KCl
B) Al(OH)₃, Fe(OH)₃, CaCO₃, Mg(OH)₂
C) SiO₂, TiO₂
D) NH₄Cl, NaNO₃

Answer 23

B) Al(OH)₃, Fe(OH)₃, CaCO₃, Mg(OH)₂

Question 24

T/F — Polymer bridging relies mainly on electrostatic attraction.

Answer 24

False - it’s chemical binding of polymer strands to multiple particles.

Question 25

When would you prefer polymer bridging over metal salts?

Answer 25

When using polymers as primary coagulants/coagulant aids to link particles, especially where charge neutralization alone is insufficient.

Question 26

The two most common coagulant metals in water treatment are: A) Na and K B) Ca and Mg C) Al and Fe D) Cu and Zn

Answer 26

C) Al and Fe.

Question 27

Give common commercial forms of these coagulants.

Answer 27

Alum (Al₂(SO₄)₃·14.3H₂O) and ferric chloride (FeCl₃·6H₂O).

Question 28

Around what pH is alum most effective for charge neutralization?

Answer 28

5.5, where precipitate formation is maximized and dominant species are positively charged.

Question 29

T/F — Sweep flocculation with alum requires low pH near 5.5 to be effective.

Answer 29

False - sweep floc can be effective at higher pH because it doesn’t rely solely on charge neutralization.

Question 30

Name two practical drawbacks of ferric chloride.

Answer 30

Higher cost; handling issues (staining, corrosion).

Question 31

Adding alum produces H⁺ and can lower pH; sufficient ______ buffers this effect.

Answer 31

alkalinity (e.g., HCO₃⁻).

Question 32

If source water lacks alkalinity, what is commonly added? A) Chlorine B) Lime or soda ash C) CO₂ D) Ozone

Answer 32

B) Lime or soda ash

Question 33

List two key purposes of a jar test.

Answer 33

Optimize coagulant/coagulant-aid dose and pH; determine mixing intensity and sequence/timing effects.

Question 34

T/F — Jar testing can evaluate the effect of lag time between rapid mix and flocculation.

Answer 34

True

Question 35

The visual difference in turbidity across jars mainly helps assess: A) Disinfection efficiency B) Coagulant dosage effectiveness C) Pipe roughness D) Chlorine demand

Answer 35

B) Coagulant dosage effectiveness

Question 36

After a jar test, which simple metric often indicates better coagulation?

Answer 36

Lower turbidity in settled samples.

Question 37

T/F — Flocculation uses intense, high-shear mixing.

Answer 37

False - it uses gentle/slow mixing.

Question 38

What two competing processes set floc size during flocculation?

Answer 38

Aggregation (collisions) vs shear-induced breakup

Question 39

Increasing collision frequency generally (before excessive shear) leads to: A) Smaller flocs B) Larger flocs C) No change D) Floc dissolution

Answer 39

B) Larger flocs (until breakup balances growth)

Question 40

T/F — Turbidity typically keeps decreasing indefinitely as you extend flocculation time

Answer 40

False - beyond a point, further time gives no additional floc formation

Question 41

What operational strategy follows from the “plateau” in floc formation vs time?

Answer 41

Use tapered flocculation and avoid overly long, high-G mixing once growth plateaus.

Question 42

What is G in flocculation design?

Answer 42

The mean velocity gradient (s⁻¹), reflecting mixing intensity/shear

Question 43

Why use tapered flocculation (decreasing G in series)? A) To increase shear on big flocs B) To reduce shear on growing flocs and continue net growth C) To neutralize charge D) To disinfect

Answer 43

B) To reduce shear on growing flocs and continue net growth

Question 44

Typical flocculation G range is __ to __ s⁻¹.

Answer 44

20 to 70 s⁻¹

Question 45

How to calculate G.

Answer 45

mean vel. gradient = vel./dist

Question 46

Give the relation between G, power P, viscosity μ, and volume V.

Answer 46

G = (P/(μV))^(1/2)

Question 47

Increasing which parameter decreases G, all else constant? A) P B) μ C) V D) A and B only

Answer 47

C) V (larger volume lowers G for given P, μ)

Question 48

What is the dimensionless design parameter combining intensity and time?

Answer 48

G·t (typical 10⁴ to 10⁵)

Question 49

By roughly what factor should G vary from influent to effluent in tapered flocculation?

Answer 49

By about a factor of 2 (decreasing).

Question 50

Why are variable-speed drives useful in flocculation basins? A) For aesthetics B) To optimize mixing under changing raw water conditions C) To boost chlorine residual D) To increase pH

Answer 50

B) To optimize mixing under changing raw water conditions