7. Primary Settling Tank Design
• Size
– rectangular: 3-24 m wide x 15-100 m
long
– circular: 3-90 m diameter
• Detention time: 1.5-2.5 hours
• Overflow rate: 25-60 m3/m2·day
• Typical removal efficiencies
– solids: 50-60%
– BOD5: 30-35%
8. Secondary Treatment
• Provide BOD removal beyond what is achieved
in primary treatment
– removal of soluble BOD
– additional removal of suspended solids
• Basic approach is to use aerobic biological
degradation:
organic carbon + O2 → CO2
• Objective is to allow the BOD to be exerted in
the treatment plant rather than in the stream
10. Basic Ingredients
• High density of microorganisms (keep
organisms in system)
• Good contact between organisms and wastes
(provide mixing)
• Provide high levels of oxygen (aeration)
• Favorable temperature, pH, nutrients (design
and operation)
• No toxic chemicals present (control
industrial inputs)
12. Activated Sludge
• Process in which a mixture of wastewater and
microorganisms (biological sludge) is agitated and
aerated
• Leads to oxidation of dissolved organics
• After oxidation, separate sludge from wastewater
• Induce microbial growth
– Need food, oxygen
– Want Mixed Liquor Suspended Solids
(MLSS) of 3,000 to 6,000 mg/L
16. Activated Sludge Design
• Major design parameter: food to microorganism
ratio:
(MLVSS)
solids
suspended
volatile
liquor
mixed
volume
BOD
soluble
initial
rate
flow
where
5
X
V
S
Q
VX
QS
M
F
0
0
17. Activated Sludge Design
• td = approximately 6 - 8 hr
• Long rectangular aeration basins
• Air is injected near bottom of aeration tanks
through system of diffusers
• Aeration system used to provide mixing
• MLVSS and F/M controlled by wasting a
portion of microorganisms
18. F/M Parameter
• Low F/M (low rate of wasting)
– Starved (hungry) organisms
– more complete degradation
– larger, more costly aeration tanks
– more O2 required
– higher power costs (to supply O2)
– less sludge to handle
• High F/M (high rate of wasting)
– organisms are saturated with food
– low treatment efficiency
19. Trickling Filters
• Rotating distribution arm sprays primary
effluent over circular bed of rock or other
coarse media
• Air circulates in pores between rocks
• “Biofilm” develops on rocks and micro-
organisms degrade waste materials as they
flow past
• Organisms slough off in clumps when film gets
too thick
21. Trickling Filters
• Not a true filtering or sieving process
• Material only provides surface on which
bacteria to grow
• Can use plastic media
– lighter - can get deeper beds (up to 12 m)
– reduced space requirement
– larger surface area for growth
– greater void ratios (better air flow)
– less prone to plugging by accumulating
slime
24. Rotating Biological
Contactors
• Called RBCs
• Consists of series of closely spaced discs
mounted on a horizontal shaft and rotated
while ~40% of each disc is submerged in
wastewater
• Discs: light-weight plastic
• Slime is 1-3 mm in thickness on disc
29. Aerobic ponds
• Shallow ponds (<1 m deep)
• Light penetrates to bottom
• Active algal photosynthesis
• Organic matter converted to CO2, NO3
-,
HSO4
-, HPO4
2-, etc.
30. Facultative ponds
• Ponds 1 - 2.5 m deep
• td = 30 - 180 d
• not easily subject to
upsets due to
fluctuations in Q,
loading
• low capital, O&M costs
Facultative
Aerobic
Anaerobic
32. Anaerobic Ponds
• Primarily used as a pretreatment process for
high strength, high temperature wastes
• Can handle much high loadings
• 2 stage:
– Acid fermentation: Organics Org.
acids
– Methane fermentation Org. Acids
CH4 and CO2
33. Land and Wetland
Application
• Spray irrigation and infiltration
• Overland flow
• Wetlands
Source: Environmental
Science, 4th ed., B.J. Nebel
and R.T. Wright, Prentice-
Hall, N.J., c. 1981
34. Spray irrigation
• Usually follows oxidation ponds, aerated
lagoons
• Application leads to filtering, biological
degradation, ion exchange, sorption, photo-
degradation
• Need about 1 acre/100 people
Secondary
Treatment
Flooding,
channeling
spray irrigation
35. Spray irrigation
• Problems
– climate
– pathogens
– need buffer zone
Source: Environmental Science, 4th ed., B.J. Nebel
and R.T. Wright, Prentice-Hall, N.J., c. 1981
36. Overland flow
• Water irrigated onto long narrow fields
• Use grasses that take up large amounts of
nitrogen
• Underlying soil should be fairly impervious
Secondary
Treatment
Application to land
slopped at 2-8%
37. Overland Flow:
• Treats 1 MGD on 200
acres
• Settling pond then
irrigated
• Fields planted with
reed canary grass
• Below ~1 ft topsoil is
compacted clay
Source: Environmental Science, 4th ed., B.J. Nebel
and R.T. Wright, Prentice-Hall, N.J., c. 1981
38. Overland Flow:
• W/W applied to one side of
field, percolates through
topsoil to a collecting gutter
• Water in gutter (clear and
nutrient-free)
• Collected in another
reservoir and spray-irrigated
onto forage crops
Source: Environmental Science, 4th ed.,
B.J. Nebel and R.T. Wright, Prentice-Hall,
N.J., c. 1981
39. Overland Flow:
• Advantages
– free water
– free nutrients
– plants can be fed
to animals
– low-cost
– low-maintenance
– water meets
discharge
regulations
• Disadvantages
– will not work in
cold climates
– pathogen
dispersion in air
– need buffer zones
– need large amount
of land
41. Facility Options
• Considerations for wastewater treatment
facility options
– costs
•capital
•operation and maintenance (including energy)
– availability of space
– degree of treatment required
– municipal or municipal plus industrial
– Flow rate
42. Facility Options
• Considerations for wastewater treatment
facility options
– distance from residential properties
•problems with: odors, flies, other nuisances
– agricultural usage or land application
options
– presence of pathogens
– experience of design engineers
