Chapter 8: Water Treatment

Question 1

Physical characteristics of water (6)

Answer 1

Color, turbidity, temperature, particulars, taste, odor

Question 2

Characteristics of groundwater (5)

Answer 2

1. Constant composition
2. High mineral content (Fe, Mn)
3. Low turbidity
4. Low or no DO
5. High hardness

Question 3

Characteristics of surface water (5)

Answer 3

1. Variable composition
2. Low mineral content
3. High turbidity
4. DO present
5. Low hardness

Question 4

Colloids

Answer 4

Discrete particles that are separated by a dispersal medium (fluid). (EX: milk, fog, butter, blood, paint)

Question 5

Properties of colloids

Answer 5

1. Negative electrostatic surface charges
2. 1 nm to 1,000 nm (1µm)
3. High surface to volume ratios
4. Called “stable ” when colloidal particles remain in suspension without coagulation and flocculation.

Question 6

Rapid sand filtration/conventional treatment steps (3)

Answer 6

1. Pre-sedimentation (removes easily settleable material)
2. Pre-chlorination (Cl added at rapid mix stage)
3. Direct filtration (coagulation, flocculation, filtration, disinfection)

Question 7

Coagulation (rapid mixing)

Answer 7

Destabilization and initial coalescing of colloidal pollutant particles. Particles stick to each other.

Question 8

Flocculation (slow mixing)

Answer 8

Formation of loosely attached larger aggregates of pollutant particles (flocs) via the addition of coagulants.

Question 9

How does coagulation and flocculation occur? (3)

Answer 9

1. Brownian motion of particles (particles < 0.5 µm)
2. Stirring
3. Differential settlement

Question 10

Coagulants for water treatment (6)

Answer 10

Alum or ferric iron salts, ferric chloride, polymers, slaked lime, ferric sulfate, ferrous sulfate

Question 11

Sedimentation

Answer 11

Gravity separates particles from water.

Question 12

Reynold’s Number

Answer 12

Ratio of inertial forces to viscous forces

Question 13

Type 1 sedimentation

Answer 13

Particles that settle discretely at a constant velocity. Occurs in dilute suspensions

Question 14

Type II Sedimentation

Answer 14

Particles that flocculate during sedimentation. Changing size and settling velocity due to flocculation.

Question 15

Type III/Zone, Sedimentation

Answer 15

High concentration of particles (>1000 mg/L) settle as a mass, a distinct clear zone and sludge zone are present.

Question 16

Type IV/Compression Settling

Answer 16

Setting of particles that are of such concentration that the settling can occur only by compression of the mass.

Question 17

Charge neutralization

Answer 17

Coagulation by reducing the energy barrier height through adding positive charge counterions as coagulant to the negatively charged colloidal particles.

Question 18

Sweep coagulation

Answer 18

Add high concentration of Fe or Al salts so that they form a cloud of metal hydroxide and settle down sweeping the colloidal particles downward

Question 19

Sand size

Answer 19

500 µm

Question 20

Size of particles passing through sand

Answer 20

0.15d

Question 21

Size of soil

Answer 21

1-100 µm

Question 22

Size of bacteria

Answer 22

<300 µm

Question 23

Size of viruses

Answer 23

0.005-0.01 µm

Question 24

Size of flocs

Answer 24

100-2000 µm

Question 25

Slow sand filters

Answer 25

Removal by physical straining using fine sand and biological degradation. Clean by scraping top layers. Low loading rates (0.05-2 m/s)

Question 26

Rapid granular filtration

Answer 26

Coarser grain size (0.4 < d < 1) with higher loading rates (5-15 m/s). Like sedimentation/coagulation/flocculation

Question 27

Backwashing

Answer 27

Forcing water/air up from the clear well back through the filter to release trapped particles.

Question 28

Disinfectants (3)

Answer 28

Chlorine, ozone, UV light

Question 29

CT concept

Answer 29

Combination of disinfectant residual concentration, C (mg/L) and the effective disinfection contact time, T (in minutes) to determine the required dose of a disinfectant for effective pathogen reduction.

Question 30

Infiltration

Answer 30

Groundwater that enters sewer systems through cracks and/or leaks in the sewer pipes

Question 31

Inflow

Answer 31

Stormwater that enters into sanitary sewer systems at a point of direct connection to the system

Question 32

Municipal wastewater treatment stages (4)

Answer 32

1. Preliminary treatment: removes materials that can cause operational problems (sand, grit, debris, etc.) 2. Primary Treatment: removes mainly suspended solids 3. Secondary Treatment: remove remaining organic suspended solid and organic dissolved solids 4. Tertiary Treatment: Further removal of nutrients, suspended solids, dissolved organics and microorganisms

Question 33

Equalization

Answer 33

Reduced difference between night time flow and morning high flow.

Question 34

In-line equalization

Answer 34

Dampen variations in both concentration and flow rate

Question 35

Off-line equalization

Answer 35

Dampens flow rate but has a smaller effect on concentration

Question 36

Dispersed growth

Answer 36

When microorganisms are in wastewater (EX: Activated sludge, Biological nutrient removal (BNR), Oxidation ditches/ponds, Aerated lagoons, stabilization ponds)

Question 37

Fixed growth

Answer 37

When microorganisms are fixed on surfaces (EX: Trickling filters, Rotating Biological Contactors (RBCs))

Question 38

Activated sludge

Answer 38

Process in which a mixture of wastewater and microorganisms is agitated and aerated.

Question 39

Aeration tank

Answer 39

Contains mixed liquor (suspended solids), the combination of influent wastewater and return (recycled) activated sludge.

Question 40

Trickling filters

Answer 40

Not a true filtering or sieving process, as material (rock or plastic) only provides a surface on which bacteria to grow. Less energy is needed, but has a poorer effluent quality.