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Nutrient Removal

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  1. Nutrient Removal Objective: To understand the fundamental principles of nutrient removal using chemical and biological methods To know examples of the major wastewater treatment processes for nutrient removal. Reference: “Operation of municipal wastewater treatment plants. Manual of Practice 11, Vol2 (1996). Water Environment Federation “ http://www.staff.ncl.ac.uk/p.j.sallis/teach.html see section ‘CIV912’; user and password both cassie

  2. Nutrient Removal Introduction Chemical Methods Principle of Biological Nitrogen Removal Biological Nitrogen Removal Processes Principle of Biological Phosphorus Removal Biological Phosphorus Removal Processes Combined Biological N & P Removal Processes

  3. Nutrient levels in a Conventional Aerobic Treatment Plant Final Effluent BOD <20 SS <30 TKN >20 PO4 >10 Aerobic Biological Process Sed Tank Sed Tank Pretreatment Influent BOD 300 SS 300 TKN 50 PO4 15 Primary Sludge Secondary Sludge

  4. Nutrient Cycles • Eutrophication potential • Nutrient balance C:N:P (100:5:1) 10,000 pe x 200 l/d x 15mgN/l  500kg algae/d 10,000 pe x 200 l/d x 5mgP/l  1200kg algae/d

  5. Nutrient Removal - Standards - UWWT Directive (1991): Pop >10,000 N<15mg/l P<2mg/l Pop >100,000 N<10mg/l P<1mg/l or 80% removal of Total P 70 - 80% removal of Total N (The above applies to “sensitive waters”)

  6. Chemical Methods • Nitrogen • Ammonia stripping at high pH (Lime, CaO) NH4+ + OH- NH3  + H2O • Phosphorus • Precipitation by metal ions Ca(OH)2 + HPO42- Ca5(OH)(PO4)3 Al2(SO4)3 + PO43- AlPO4 + SO42-

  7. Biological Nutrient Removal • Assimilation • C, N, P, S etc uptake for synthesis of new cells • Dissimilation • C, N, S, oxidized/reduced to provide energy • Aerobic (oxic) • in the presence of molecular oxygen (O2) • Anoxic • very low concentration of molecular oxygen (O2) • significant levels of electron acceptors (NO3-, SO4-) • Anaerobic • no oxygen, lack of electron acceptors (only CO2)

  8. Biological Nitrogen Removal • Wastewaters contain: Org-N, ammonia, (nitrate) • Dissimilatory metabolism • Nitrification 1. NH4+ + 1.5 O2 NO2- + 2H+ + H2O Nitrosomonas 2. NO2- + 0.5 O2 NO3-(nitrified effluent) Nitrobacter • Denitrification NO3- + CH2 + H+ N2 + CO2 + H2O denitrifying bacteria (many)

  9. Basic Nitrogen Removal System (Ludzak-Ettinger Process) Effluent N2 Influent Anoxic (denitrification) Aerobic QR RAS Modified L-E Process has recycle (QR) Sedimentation Tank

  10. Alternative Nitrogen Removal System Effluent Influent N2 Methanol Aerobic + Nitrification Anoxic (denitrification) Aerobic RAS RAS Re-aeration for Excess Methanol Removal Sedimentation Tank

  11. Biological Phosphorus Removal • Selection of Bacteria in Sludge • Luxury uptake of Phosphorus • (Acinetobacter, Pseudomonas) • Cyclic Environmental Conditions • High BOD when anaerobic • Low BOD when aerobic • Sidestream • P is stripped from sludge in separate unit process • Mainstream • P is concentrated to high levels in the sludge (biomass)

  12. Selection of Bacteria Anaerobic High BOD Carbon uptake (fatty acids stored as poly hydroxy alkanoates PHA)* Phosphate released from cells (polyP  PO4, energy released) Aerobic Low BOD Carbon Oxidation (PHA oxidised to CO2, releases energy) Phosphate uptake (Luxury) (PO4  polyP)* * These processes need energy to drive them

  13. PhoStrip Process (Sidestream) Aeration Tank Sed. Tank Influent Effluent RAS Waste Sludge Anaerobic Stripper Phosphorus Stripped Sludge Primary Effluent (BOD, Elutriation) Supernatant Return P Waste Chemical Sludge (P) Lime

  14. N2 Combined N & P Removal Methanol Aeration BOD Rem Nitrific -ation Denitrification RAS PhoStrip Phosphorus Free Sludge P Final Effluent Waste Chemical Sludge (P) Aerobic Anoxic Sedimentation Anaerobic

  15. Anaerobic Settling Tank Anoxic Aerobic N2 Combined N & P Removal (Mainstream) (UNIVERSITY OF CAPE TOWN PROCESS , UCT) Anoxic Re-cycle Nitrified Re-cycle (100-200%Q) (100%Q) NH3 to NO3 HRT= 3-6 h HRT= 0.5-1.0h HRT= 0.5-1.0h Q WAS (P) RAS (50-100%Q) (= 6% P)

  16. Operational Considerations • Maintain discrete environments • excess recycle rate gives completely mixed system • Limitations • Combined System optimized for N (denitrification), biological P removal non-optimized (requires chemical supplementation) • Efficiency • denitrification re-uses Oxygen bound in the nitrate • Contingency • provide P removal by chemical means (when biological process fails)