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Wastewater treatment steps

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:

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Wastewater treatment steps

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  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)

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