Wastewater treatment steps

1
Wastewater treatment steps

• Primary solids removal (physical)
• Secondary BOD treatment (biological)
• Tertiary Effluent polishing, Nutrient and Toxins
  Removal (chemical, also possibly physical and
  biological)

2
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

3
Goals of tertiary treatment

• Effluent polishing (BOD, TSS)
• Nutrient removal (N, P)
• Toxin removal
• (pesticides, VOCs, metals)

4
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

5
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

6
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

7
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

8
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

9
On-site biological methods...

• Aerobic chamber plus deep sand filter
• uses methanol as carbon source
• must manage methanol dosing rate
• 85 - 95 removal possible

10
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

11
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

12
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

13
Phosphorus

• Forms
• organic phosphorus
• orthophosphate (PO4)
• polyphosphates
• phosphorus-containing rocks
• Binds to soils and sediments
• Limiting nutrient in fresh waters

14
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

15
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

16
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

17
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

18
Toxicity testing

• Test for specific chemicals
• Bioassays (response of fathead minnow, water
  flea, others, over time)
• Human toxicity (carcinogenicity, acute or chronic
  disease)

19
Toxin strategy

• Prevention
• Protection
• equalization basins
• holding tanks
• contingency plans
• Treatment
• no universal treatment method
• each toxin different

20
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

21
Toxin Tertiary Treatment

• Metals
• Chemical precipitation and filtration
• Biological transformation
• Sorption
• Solidification (cement, asphalt, plastic
  polymers)
• Encapsulation
• Plant uptake /phytoremediation (note sludge
  application implications)