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)