Wastewater Treatment

Question 0

On-site waste water treatment

Answer 0

• ~25% of population uses this
• called conventional septic system
  - - septic tank
  - - leach field

Question 1

Wastewater collection and treatment system

Answer 1

To remove contaminants (e.g. BOD, nutrients) before effluent release to environment

*Regulation: Clean Water Act

Question 2

Septic tank

Answer 2

• solids settle, undergo anaerobic digestion (without O2)

* excess liquid drains into leach fields

Question 3

Leach fields

Answer 3

• organics & inorganics in liquid are degraded in soil

* liquid is lost through various processes e.g. Evaporation, plant uptake

Question 4

Municipal waste water treatment

Answer 4

1) pretreatment
2) primary treatment process
3) secondary treatment process

Question 5

Pretreatment

Answer 5

Removes material that may cause operational problems

1) bar screens
2) grit chambers
3) equalization

Question 6

Bar screens

Answer 6

To remove larger objects, material is sent to a landfill

Question 7

Grit chambers

Answer 7

To remove inert material e.g. Glass, sand

Question 8

Equalization

Answer 8

To dampen variation in waste water flow into a WWTP

• collects waste water in large basins
• it’s then pumped to WWTP at a constant rate

Question 9

Primary treatment

Answer 9

• removes wastes in a settling tank or primary clarifier
• waste water is held in a tank for several hours
• particles settle to the bottom and can be removed by mechanical scrapers
• greases and oils rise to surface and are skimmed from the top

Question 10

Design parameters for Primary Treatment

Answer 10

• overflow rate (flow rate m^3/ area m^2)

* detention time (volume m^3/ flow rate m^3/hr)

Question 11

Secondary treatment

Answer 11

• microorganisms degrade the organic material
• BOD & SS removal beyond what’s achieved in primary treatment
• requires healthy microorganisms

Question 12

Healthy microorganisms w/ in secondary treatment

Answer 12

1) high O2 levels
2) favorable pH range
3) no toxic chemicals

Question 13

Dispersed or suspended growth

Answer 13

• microorganisms suspended in liquid waste water

* e.g. Activated sludge, oxidation ponds

Question 14

Activated sludge

Answer 14

• secondary treatment - suspended
• in aeration tank microorganisms consume & oxidize waste, “activated” by O2, waste water flows to settling tank, settled material is returned to aeration tank or removed from system

Question 15

Fixed or attached growth

Answer 15

• microorganisms grow on a surface

* e.g. Trickling filters, Rotating Biological Contactors RBCs

Question 16

Trickling Filters

Answer 16

• secondary treatment - attached

* microorganisms grow on the surface of “filter media” (rocks or plastic)

Question 17

Trickling Filter process

Answer 17

• rotating arm sprays primary effluent over filter media
• biofilm (layer of microorganisms) develops of filter media
• microbes degrade waste as it passes over them

Question 18

Rotating Biological Contactors (RBCs)

Answer 18

*secondary treatment - attached

• series of discs (~3m diameter)
  - half of each disc is submerged
  - discs rotate
  - biofilm forms on discs

Question 19

Phosphorus removal

Answer 19

• excess phosphorus can cause eutrophication
• removal accomplished with chemical precipitation
• aluminum hydrate or lime

Question 20

Nitrogen removal

Answer 20

N can be present in various forms in waste water (NH3, NO3-, NO2-) and can cause problems in receiving water

Question 21

Biological method in Nitrogen Removal

Answer 21

• nitrification: ammonium (NH4+) to nitrate (NO3-)

* denitrification: nitrate (NO3-) to nitrogen gas (N2)

Question 22

Chemical method in Nitrogen Removal- Ammonia Stripping

Answer 22

• increase in pH
• this converts ammonium ions (NH4+) to ammonia gas (NH3)
• the water is purged to remove NH3 from water

Question 23

Final treatment

Answer 23

Disinfection- commonly by the addition of chlorine gas

Question 24

Sludge treatment

Answer 24

• Final disposal of biosolids/ sludge

* treated sludge or can be disposed in landfills or can be land applied