Lecture 8c

Question 1

what is sedimentation?

Answer 1

suspended particles are removed from liquid phase by gravity settling

Question 2

what are the 3 types of settling for sedimentation removal in water treatment?

Answer 2

• pretreatment of surface waters
• settling of coagulated/flocculated waters prior to filtration
• settling of coagulated flocculated water chemical water softening

Question 3

what are the 2 types of settling for sedimentation removal in wastewater treatment?

Answer 3

• removal of grit and other coarse solids, suspended solids before biological treatment
• removal of biological solids produced during biological treatment

Question 4

what are the 4 different types of settling classes for settling properties of particles?

Answer 4

1. discrete settling (settling of non-flocculant particles) - TYPE 1
2. flocculant settling - TYPE 2
3. hindered settling - TYPE 3
4. compression settling - TYPE 4

Question 5

what is type 1 discrete settling?

Answer 5

particles settle discretely at a constant settling velocity. They settle individually and do not flocculate

Question 6

what are some applications of type 1 settling?

Answer 6

• presettling of grit or sand prior to water treatment
• settling of sand particles during cleaning of rapid sand filters
• in grit chambers in wastewater treatment

Question 7

what are some of the assumptions that we make for the ideal settling tank

Answer 7

• type 1 discrete settling
• four zones in basin: inlet, outlet, sludge, and settling
• even distribution of flow entering and leaving settling zone
• uniform particle distribution throughout
• particles that enter outlet zone not removed from water

Question 8

true or false most actual settling systems do no behave like TYPE 1 (discrete settling) system

Answer 8

true

Question 9

what are the particle interactions for TYPE 2 (flocculent settling)

Answer 9

flocculation in which particles collide and adhere to other particles (particle growth)

Question 10

what are the particle interactions for TYPE 3 (hindered or zone settling)

Answer 10

fluidic in which particles are so close together that flow is restricted and particles move as a block

Question 11

what are the particle interactions for TYPE 4 (compression settling)

Answer 11

mechanical - particles physically interact in a compressive mode

Question 12

what are the two types of sedimentation tanks?

Answer 12

rectangular and circular

Question 13

some facts about rectangular sedimentation tanks

Answer 13

• usually have chain driven scrapers for sludge removal
• typically 3m deep for water treatment

Question 14

some facts about circular sedimentation tanks

Answer 14

• inflow at center, outflow along perimeter weir or radial collection trough
• circular rake arms for sludge collection
• typically more than 3m deep

Question 15

what are the 4 zones for sediment tank design?

Answer 15

inlet zone
settling zone
outlet zone
sludge zone

Question 16

what are some considerations for sedimentation basin design?

Answer 16

• overflow rate (surface loading rate) - for initial dimensions/sizing
• detention time: need sufficient time for settling, if flocculation expected - need time for flocs to grow
• scour velocity: particles settled at bottom may be re-suspended (scoured) if horizontal velocity too high
• critical horizontal velocity (min. horizontal velocity that will produce scour)
• horizontal flow velocity
• weir loading rate (wastewater)
• short-circuiting and hydraulic instability (L/W ratio, min. depth requirements

Question 17

what happens in a sedimentation tank during thermal stratification?

Answer 17

effective volumetric capacity of basin is reduced therefore required td < actual td

Question 18

what happens in a sedimentation tank during wind circulation?

Answer 18

stuff gets moved around (random patrick wind noises)