PowerPoint Presentation - Tertiary (advanced) treatment
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Wastewater treatment steps
Primary: solids removal (physical)
Secondary: BOD treatment (biological)
Tertiary: Effluent polishing, Nutrient
and Toxins Removal (chemical, also
possibly physical and biological)
Tertiary (advanced) treatment
Secondary treatment:
removes 85% - 95% of BOD and TSS
removes 20% - 40% P
removes 0% - 50% N
Tertiary treatment:
removes over 99% of pollutants
very high cost
Goals of tertiary treatment
Effluent
polishing (BOD, TSS)
Nutrient removal (N, P)
Toxin removal
(pesticides, VOCs, metals)
Effluent polishing
Removal of additional BOD and TSS
Granular media filter beds
gravity
or pressurized
require frequent backwashing
air-washing
Microstraining/ microscreens
20-micrometer openings
Nutrient management
Nutrient = plants require them for growth
Potential problems from nutrients:
water
quality
aquatic ecosystem
human & animal health
Approaches:
dilution
treatment
(biological or physicochemical)
plant uptake
Nitrogen
Biochemically interconvertable forms :
organic
N (proteins, urea)
ammonia gas (NH3)
ammonium ion (NH4+)
nitrate (NO3)
nitrite (NO2)
elemental N2 gas (78% of air)
Mobile (esp. nitrate)
Limiting nutrient in salt waters
Nitrification-denitrification
Two-step biological method
Step 1: Aerobic Nitrification
Ammonia
to nitrate conversion
now nontoxic to fish
m.o.s: Nitrosomonas and Nitrobacter
Step 2: Anoxic-anaerobic Denitrification
nitrate
to N2 conversion
requires carbon source
m.o.: Pseudomonas
On-site biological methods
Upflow anaerobic sand filter
uses
septic tank effluent as carbon source
must monitor and manage recycle ratio:
– too low: incomplete denitrification
– too high: excess O2 shuts down denitrification
75% removal possible
On-site biological methods...
Aerobic chamber plus deep sand filter
uses
methanol as carbon source
must manage methanol dosing rate
85% - 95% removal possible
On-site biological methods...
Bardenpho system
uses
wastewater as carbon source
alternating anoxic and aerobic STRs
must monitor and manage sludge recycle
ratio
Oxidation ditch
endless
loop of anoxic and aerobic zones
less removal efficiency than Bardenpho
Physical-chemical N removal
Approach:
convert
all N to ammonia
then treat the ammonia
Three methods:
Breakpoint
chlorination
Ion exchange
Ammonia stripping
Often impractical for on-site systems
Ammonia stripping
Two-step physical-chemical method
Step 1: Raise pH to 10.5-11.5
convert
ammonium ions to ammonia gas
Step 2: Air-strip
cascade
wastewater countercurrent to air
flow
ammonia gas escapes to atmosphere
Pro: less costly, no sludge or Cl by-products
Cons: acids/bases, scale, freezing problems
Phosphorus
Forms:
organic
phosphorus
orthophosphate (PO4)
polyphosphates
phosphorus-containing rocks
Binds to soils and sediments
Limiting nutrient in fresh waters
Biological P removal
Luxury uptake
anaerobically- stressed m.o.s ingest more P
than needed
Methods:
Bardenpho
Sequencing
Batch Reactor
– 1 tank, 5 steps
– fill, aerate, settle, decant, idle
Physical-chemical P removal
Chemical precipitation (3 options)
add
alum (Al2SO4) to form aluminum
phosphate
add ferric chloride (FeCl3)
add lime (CaO)
Coagulation / flocculation
Clarifier/settler
More on P precipitation
Pros:
can
also serve as effluent polishing step if
added after 2ndary treatment
lime can aid ammonia stripping too
Cons:
expensive:
more tanks, clarifiers, and filters
must closely manage pH, chemical dosing,
and precipitate removal
produces a LOT of sludge
Toxin treatment and removal
Types of toxins
Organics (pesticides, solvent, petroleum,...)
Metals (lead, cadmium, mercury,...)
Sources of toxins
Impact on wastewater treatment
systems when toxins hit
Toxicity testing
Test for specific chemicals
Bioassays (response of fathead minnow,
water flea, others, over time)
Human toxicity (carcinogenicity, acute
or chronic disease)
Toxin strategy
Prevention
Protection
equalization
basins
holding tanks
contingency plans
Treatment
no
universal treatment method
each toxin different
Toxin Tertiary Treatment
Organics:
Biological treatment (incl. co-metabolism)
Oil-water separator
Air stripping
Thermal treatment (incineration, desorption,
distillation, evaporation)
Chemical oxidation
Sorption (activated carbon, kitty litter)
Land farming
Toxin Tertiary Treatment
Metals:
Chemical precipitation and filtration
Biological transformation
Sorption
Solidification (cement, asphalt, plastic
polymers)
Encapsulation
Plant uptake /phytoremediation (note
sludge application implications)