Download - Nutrient Removal
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
Pretreatment Sed Tank
Aerobic Biological
Process
SedTank
Final Effluent
InfluentBOD 300SS 300TKN 50PO4 15
BOD <20SS <30TKN >20PO4 >10
PrimarySludge
SecondarySludge
Nutrient levels in a Conventional Aerobic Treatment Plant
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
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+ + OH- NH3 + H2O
• Phosphorus– Precipitation by metal ions
Ca(OH)2 + HPO42- Ca5(OH)(PO4)3
Al2(SO4)3 + PO43- AlPO4 + SO4
2-
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
• Nitrification1. NH4
+ + 1.5 O2 NO2- + 2H+ + H2O
Nitrosomonas2. NO2
- + 0.5 O2 NO3- (nitrified
effluent)Nitrobacter
• DenitrificationNO3
- + CH2 + H+ N2 + CO2 + H2O
denitrifying bacteria (many)
Anoxic(denitrification)
Aerobic
Influent
Effluent
N2
Basic Nitrogen Removal System(Ludzak-Ettinger Process)
Sedimentation Tank
RAS
QR
Modified L-E Processhas recycle (QR)
Aerobic +Nitrification
Anoxic(denitrification)
Aerobic
Methanol
Influent
Effluent
N2
Re-aeration for Excess MethanolRemoval
Alternative Nitrogen Removal System
Sedimentation Tank
RASRAS
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
AerobicLow BOD
Carbon Oxidation(PHA oxidised to CO2,
releases energy)
Phosphate uptake (Luxury)(PO4 polyP)*
AnaerobicHigh BOD
Carbon uptake(fatty acids stored as
poly hydroxy alkanoates PHA)*
Phosphate released from cells(polyP PO4, energy released)
* These processes need energy to drive them
Aeration Tank
Sed.Tank
Waste ChemicalSludge (P)
Lime
RAS
Phosphorus Stripped Sludge
Supernatant Return
Waste Sludge
Influent Effluent
Primary Effluent(BOD, Elutriation)
PhoStrip Process (Sidestream)
AnaerobicStripper
P
AerationBOD Rem
Nitrific-ation
PhoStrip
Denitrification
Final Effluent
Waste Chemical Sludge (P)
RAS
Phosphorus FreeSludge
Methanol
Aerobic
Anoxic
AnaerobicSedimentation
Combined N & P Removal
N2
P
NH3 to NO3
HRT= 3-6 hHRT=
0.5-1.0hHRT=
0.5-1.0h
Anaerobic
AnoxicAerobic Settling Tank
Nitrified Re-cycle (100-200%Q)
RAS (50-100%Q)
(= 6% P)
AnoxicRe-cycle(100%Q)
Q
Combined N & P Removal (Mainstream) (UNIVERSITY OF CAPE TOWN PROCESS , UCT)
WAS (P)
N2
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)