Wastewater Treatment: Primary and Secondary Treatment

Report
Wastewater Treatment:
Primary and Secondary
Treatment
Primary Settling
Basins
Primary Settling
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%
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
Diverse Microbial Community
• Create
a
very
rich
environment for growth of a
diverse
microbial
community
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)
Dispersed growth vs Fixed Growth
• Dispersed Growth – suspended organisms
– Activated sludge
– Oxidation ditches/ponds
– Aerated lagoons, stabilization
ponds
• Fixed Growth – attached organisms
– Trickling filters
– Rotating Biological Contactors
(RBCs)
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
Activated Sludge
w/w
Return
Activated
Sludge
(RAS)
Waste
Activated
Sludge
(WAS)
Mixed
Liquor
Air
Treated
w/w
Secondary
clarifier
Discharge to
River or Land
Application
Activated sludge
East Lansing WTP
Secondary Clarifier
East Lansing WWTP
Activated Sludge Design
• Major design parameter: food to microorganism
ratio:
F QS0

M VX
w here Q  flow rate
S 0  initial soluble BOD 5
V  volume
X  mixed liquor volatile
suspended solids (MLVSS)
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
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
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 microorganisms degrade waste materials as they
flow past
• Organisms slough off in clumps when film gets
too thick
Trickling Filters
Filter Material
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
Trickling Filter Plant Layout
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
Rotating Biological
Contactors
Rotating Biological
Contactors
Aeration
Film
mixes with
wastewater
Shearing of excess
microorganisms
Attached microorganisms
pick up organics
Rotating Biological
Contactors
Primary
Settling
Sludge
Treatment
Secondary
Settling
Sludge Treatment
Low-tech solutions
• Aerobic ponds
• Facultative ponds
• Anaerobic ponds
Aerobic ponds
•
•
•
•
Shallow ponds (<1 m deep)
Light penetrates to bottom
Active algal photosynthesis
Organic matter converted to CO2, NO3-,
HSO4-, HPO42-, etc.
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
Aerobic
Facultative
Anaerobic
Oxidation Ditches
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
Land and Wetland
Application
• Spray irrigation and infiltration
• Overland flow
• Wetlands
Source: Environmental
Science, 4th ed., B.J. Nebel
and R.T. Wright, PrenticeHall, N.J., c. 1981
Spray irrigation
Secondary
Treatment
Flooding,
channeling
spray irrigation
• Usually follows oxidation ponds, aerated
lagoons
• Application leads to filtering, biological
degradation, ion exchange, sorption, photodegradation
• Need about 1 acre/100 people
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
Overland flow
Secondary
Treatment
Application to land
slopped at 2-8%
• Water irrigated onto long narrow fields
• Use grasses that take up large amounts of
nitrogen
• Underlying soil should be fairly impervious
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
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
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
Wetlands
• Use of natural or artificial wetlands
• Floating plants act as filters and support for bacteria
(From: Environmental Science, 4th ed., B.J. Nebeland R.T. Wright, Prentice-Hall, N.J., © 1981)
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
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

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