Lecture 8b

Question 1

what is the theory of coagulation?

Answer 1

the process of adding chemicals to surface waters to collect small particulate matters (ie colloidal matters) into clusters that can be removed from solution by subsequent sedimentation and filtration through granular media

Question 2

what are some characteristics of colloidal matters

Answer 2

• for larger particles, the ratio of surface area to mass is low and mass effects such as sedimentation by gravity forces predominate
• however, for colloids, the ratio of surface area to mass is high, and surface phenomena such as electrostatic repulsion and hydration become important

Question 3

what are the principal phenomena that control the behavior of colloids?

Answer 3

• electrical properties
• van der waals forces
• brownian motion

Question 4

what are electrical properties in relation to colloids?

Answer 4

surface charge of colloids and suspended particles coause them to remain in suspension without aggregation for a long time - STABLE colloids

Question 5

what are van der waal’s forces in relation to colloids?

Answer 5

• the tendency of particles in nature to attract each other weakly if they have no charge
• once the particles in water are not repelling each other, van der waal’s forces make the particles drift toward each other and join together into a group

Question 6

what is particle stability?

Answer 6

colloids remain stable when there is a balance between the repulsive electrostatic force of the charged particle and attractive force of van der waals forces

Question 7

what is brownian motion in relation to colloids?

Answer 7

colloids have a sufficiently small mass that collision with molecular size particles in water will cause constant movement of the colloids

Question 8

what are some of the mechanisms of coagulation and flocculation

Answer 8

• double layer compression
• charge neutralization
• enmeshment in precipitate (sweep coagulation)
• particle bridging

Question 9

what do coagulants do?

Answer 9

reduce net electron repulsive force at particle surface + addition of counter ions (+ charged ions)

Question 10

what does flocculation do?

Answer 10

results in agglomeration of destabilized particles

Question 11

what is double layer compression

Answer 11

positive charged ions reduce/suppress double layer so that net repulsive charge is reduced

Question 12

what is charge neutralization?

Answer 12

absorption of hydrolyzed metal salts (+ ve ions) onto colloidal surface and neutralizing the negatively charged colloids

Question 13

what is enmeshment in precipitate (sweep coagulation)

Answer 13

add alum or iron ions (coagulants) at the right pH to form flocs with colloids as its nulcei

Question 14

what is particle bridging?

Answer 14

adding polymers to enhance coagulation

Question 15

true or false, it is common that in practice 2-3 mechanisms of coagulation are used simultaneously in the coagulation process

Answer 15

FALSE!! its actually several mechanisms are used

Question 16

what does the removal of colloids by coagulation depend on?

Answer 16

• their nature and concentration
• the use of both coagulants and coagulant aids
• pH
• temperature
• ionic strength

Question 17

what are the most widely used coagulants for water and wastewater treatment?

Answer 17

aluminum salts and iron salts

Question 18

when is lime added to water and wastewater treatment?

Answer 18

to provide the necessary alkalinity and control of pH of the coagulation process

Question 19

what is aluminum sulphate?

Answer 19

the standard coagulant used in water treatment
- if sufficient alum is added and pH is adjusted to range of 6 TO 8, aluminum hydroxide precipitate is formed for the sweep floc coagulation mechanism

Question 20

what are the advantages of ferric salts

Answer 20

• coagulation is possible over a wide range of pH (4-9)
• floc settles better than alum floc
• better removal of NOM and odor/taste compounds

Question 21

what is a disadvantage of ferric salts?

Answer 21

it costs more than alum salts

Question 22

what are the advantages of using polymers for coagulants

Answer 22

• for some waters, cationic polymers are effective as a primary coagulant, but more commonly are used as coagulant aids
• do no affect pH, can be used in low alkalinity water
• dosage of cationic polymer is much less than metal coagulants
• anionic and non-ionic polymers are effective coagulant aids. promote larger and tougher floc by bridging mechanism
• more effective in water containing higher concentration of divalent cations (calcium and magnesium)
• reduce the amount of salt to be added

Question 23

how are acids and alkalies used to adjust water and wastewater

Answer 23

• used to adjust the pH of water for optimum coagulation
• typical acids usd to lower the pH are sulphuric and phosphoric acid
• alkalies used to raise the pH are lime, sodium hydroxide, and soda ash

Question 24

in regards to mixing, what is rapid mixing (flash coagulation)

Answer 24

• the process where the chemcials are quickly dispersed and mixed into the water
• vigorous, turbulent mixing causes lots of particle collision - leads to coagulation

Question 25

in regards to mixing, what is gentle mixing (flocculation)

Answer 25

• process by which these precipitates are brought in contact with one another so they can agglomerate and form larger particles called flocs
• gentle (strong enough to form collisions but NOT strong enough to break apart flocs)

Question 26

what are the different types of mixers for rapid mixing (flash mixing)

Answer 26

• in-line mechanical mixing (in-line blending)
• in-line static mixing
• mechanical mixing in stirred tanks

Question 27

what are the types of mixers for gentle mixing?

Answer 27

• paddle flocculators
• baffled chambers
• axial flow impellers

Question 28

what are the benefits of coagulation

Answer 28

• remove infectious pathogens
• remove toxic compound adsorbed to the surface of particles
• remove precursors to formation of disinfection by product (NOM)
• make water palatable (colloid removal)