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)