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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 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 Nutrient levels in a Conventional Aerobic Treatment Plant Pretreatment Sed Tank Influent BOD 300 SS 300 TKN 50 PO4 15 Primary Sludge Final Effluent BOD SS TKN PO4 Aerobic Biological Process <20 <30 >20 >10 Sed Tank Secondary Sludge Nutrient Cycles • Eutrophication potential – Nutrient balance C:N:P (100:5:1) 10,000 pe x 200 l/d x 15mgN/l algae/d 10,000 pe x 200 l/d x 5mgP/l algae/d 500kg 1200kg 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”) Chemical Methods • Nitrogen – Ammonia stripping at high pH (Lime, CaO) NH4+ + OHNH3 + H2O • Phosphorus – Precipitation by metal ions Ca(OH)2 + HPO42- Ca5(OH)(PO4)3 Al2(SO4)3 + PO43- AlPO4 + SO42- 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) 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) Basic Nitrogen Removal System (Ludzak-Ettinger Process) Effluent N2 Influent Anoxic (denitrification) Aerobic QR RAS Sedimentation Tank Modified L-E Process has recycle (QR) Alternative Nitrogen Removal System Effluent Influent Aerobic + Nitrification RAS Sedimentation Tank N2 Methanol Anoxic (denitrification) RAS Aerobic Re-aeration for Excess Methanol Removal 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) 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 PhoStrip Process (Sidestream) Influent Aeration Tank Sed. Tank Effluent RAS Waste Sludge Phosphorus Stripped Sludge Anaerobic Stripper Primary Effluent (BOD, Elutriation) Supernatant Return P Lime Waste Chemical Sludge (P) Combined N & P Removal Methanol Aeration BOD Rem Nitrific -ation N2 Denitrification RAS Phosphorus Free Sludge Aerobic PhoStrip P Waste Chemical Sludge (P) Anoxic Sedimentation Anaerobic Final Effluent Combined N & P Removal (Mainstream) (UNIVERSITY OF CAPE TOWN PROCESS , UCT) Anoxic Re-cycle (100%Q) Q HRT= 0.5-1.0h Anaerobic Anoxic Aerobic Nitrified Re-cycle (100-200%Q) N2 HRT= 0.5-1.0h NH3 to NO3 HRT= 3-6 h RAS (50-100%Q) Settling Tank WAS (P) (= 6% P) 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)