Optimizing Chemical
Phosphorus Removal
JB Neethling
HDR Engineering
2013 Technical Conference and Exposition
Ohio Water Environment Association (OWEA)
Mason, OH
June 18-20, 2013
Copyright 2013 HDR Engineering, Inc. All rights reserved.
Agenda
• Chemical Phosphorus Removal Basics
• Phosphorus Species
• Performance
• Opportunities for Optimization
Phosphorus Species
Total P
Dissolved P Particulate P
Ortho P (Reactive P)
Inorganic Condensed P
Soluble Organic P
Particulate Acid Hyd P
Particulate Organic P
Particulate Reactive P
Fundamental Principle of Phosphorus Removal
There is no airborne (gaseous) form of phosphorus
Fundamental Principle of Phosphorus Removal
The exception
There is no airborne (gaseous) form of phosphorus
Convert to Particulate
Particu-
late P
Soluble P
Particu-
late P
SolP Liquid
Solid
Typical Chemical Treatment Opportunities
Primary Secondary Tertiary Polish
Solids Processing
Chemicals used for Phosphorus
Precipitation
Chemical
Formula
Removal
mechanism
Effect on pH
Aluminum
Sulfate (Alum)
Al2(SO4)3.14.3(H2O)
M.W. = 599.4
Metal hydroxides removes
alkalinity
Ferric Chloride FeCl3
M.W. = 162.3
Metal hydroxides removes
alkalinity
Poly Aluminum
Chloride
AlnCl(3n-m)(OH)m
Al12Cl12(OH)24
Metal hydroxides none
Ferrous sulfate
(pickle liquor)
Fe2SO4 Metal hydroxides Removes
alkalinity
Lime CaO, Ca(OH)2 Insoluble precipitate Raises pH to
above 10
Ferric Reaction with Phosphorus
The following illustrates a “stoichiometric reaction” of Fe+++ with P
1 mole of Fe reacts with 1 mole P 5.2 mg ferric per mg P 0.92 mg alkalinity per mg of ferric
FeCl3 + H3PO4 = FePO4 + 3HCl3
Alum Reaction with Phosphorus
Al2(SO4)3.14H2O+ 2H3PO4
2AlPO4 + 3H2SO4 + 18H2O
The following illustrates a “stoichiometric reaction” of Al+++ with P
2 mole of Al reacts with 2 mole P (or 1 mole Al per mole P) 9.6 mg alum per mg P 0.5 mg alkalinity per mg of Alum
Phosphorus Removal
Chemical Dose
Phosp
horu
s Conce
ntr
ation
Initial removal - Stoichiometric
1:1 Equilibrium control – need
higher dose
Break ~ 1 mg/L
Molar Dose Ratio From Tests
Slav Hermanowicz, Chemical Fundamentals of Phosphorus Precipitation, WERF Boundary Condition Workshop, Washington DC, 2006
0
2
4
6
8
10
12
14
16
18
20
0.01 0.1 1 10
ortho P res (mg/L)
Al/P
(m
ol/m
ol)
Full Scale 6.6 - 6.75
Lab data pH 6
Lab data pH 7.2
0.1
1
10
100
0.01 0.1 1 10
ortho P res (mg/L)
Fe
/P (m
ol/m
ol)
Lab data pH 6.5
Lab data pH 6.8
Lab data pH 7.2
Lab data pH 8
Full Scale data
Exact Molar Ratios Versus Effluent Soluble P
will Vary with Applications
• 1.5 to 2.0 Molar ratios for 80-98 percent removal
• 5.0 to 7.0 Molar ratios for higher efficiency and to reach low minimal soluble P concentrations
• Ratios are higher with PAC
• Factors that influence ratios
– pH
– Mixing method
– Wastewater characteristics
• Colloids and solids effect P-metal hydroxide complexations
• Organic subtrates
• Iron and aluminum can react with humic substances
Kinetics and Mixing of Phosphorus / Alum
Reaction
Szabó et al. (2006) The Importance Of Slow Kinetic Reactions In Simultaneous Chemical P Removal, WEFTEC 2006
Photomicrographs of Phosphate Precipitants
pH 7-->
← pH 3 Scott Smith, Wilfrid Laurier University
Fresh HFO
Scott Smith, Wilfrid Laurier University
Young HFO
Scott Smith, Wilfrid Laurier University
FePO4 precipitant After 4 days.
Aged HFO
HFO precipitant After 2 years. Hard !!
Scott Smith, Wilfrid Laurier University
Metal Hydroxide Removal of P
Found for Ferric Addition
• Metal hydroxide formed
• Co precipitation of P into hydrous ferric oxides structure
– Fe(OH)3, Fe(OH)4-
• Surface complexation between P and metal hydroxide compounds
• Phosphorus and Iron share oxygen molecule:
• FeOOH + HOPO3 = FeOOPO3 + H2O
• Hydroxide formation can be simply represented:
– FeCl3 + 3H2O => Fe(OH)3 + 3 HCl
– Al2(SO4)3.14H2O + 3 H2O => 2Al(OH)3(s) + 3H2SO4
Closer Look at the Chemical Species
33
L. Liu, D. S. Smith, D. Houweling
J.B. Neethling, H.D. Stensel
S. Murthy, Amit Pramanik and A. Z. Gu
Phosphorus Speciation and Removal in
Advanced Wastewater Treatment
Analytical Definitions of Phosphorus Species
• Filterable/nonfilterable (soluble?)
– Passing through filter paper
– Could be colloidal (very small particles)
• Reactivity to analytical procedure
– Measure for orthophosphate
• Pretreatment (acid hydrylosis, digestion, etc)
– Convert larger molecules to be reactive
(orthophosphate)
Phosphorus Species Categories
Total P
Dissolved P Particulate P
Ortho P (Reactive P)
Inorganic Condensed P
Soluble Organic P
Particulate Acid Hyd P
Particulate Organic P
Particulate Reactive P
Particulate Chemical Precipitant Measures as
Reactive P
Combined Secondary Effluent-
Jar Testing Results
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 10 20 30 40 50
Alum Dose-Al2(SO4)3.14H2O (mg/L)
Eff
luen
t o
rth
o-P
(m
g/L
)
P in=0.31mg/L
Filtered
unfiltered
Analytical Definition Based Phosphorus
Fractions/Species
Total P
Dissolved P Particulate P
Sol Reactive P SRP
PRP Sol Acid
Hydrolyzable Sol Dig.
Part Acid H.
Particle Digestible Meth
od
Colloidal P
Ortho P
Inorganic Condensed P
Sol Org
Chemical particulate P
Particulate Organic P
Nonreactive Phosphorus
Total P
Dissolved P Particulate P
Sol Reactive P SRP
PRP Sol NonReactive P
SNRP Particulate NonReactive P
Total Reactive P Total NonReactive P
Analytical Definition Based Phosphorus
Fractions/Species
Total P
Dissolved P Particulate P
Sol Reactive P SRP
Sol Non Reactive P sNRP
Colloidal P
Ortho P Acid Hydrolyzable P Sol
Org
Part Chemical P Part Organic P
Secondary Effluent TSS adds to Particulate NRP
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5 6
Pa
rtic
ula
te N
on
rea
cti
ve
Ph
os
ph
oru
s, u
g/L
TSS, mg/L
1.0% 1.5% 2.0% 2.5% 3.0%
160
5
3%
1.5%
Filtered Effluent TSS adds to Particulate NRP
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6 0.8 1 1.2
Pa
rtic
ula
te N
on
rea
cti
ve
Ph
os
ph
oru
s, u
g/L
TSS, mg/L
1.0% 1.5% 2.0% 2.5% 3.0%
35 ug/L
1 mg/L
3%
1.5%
Pilot Study Results Illustrated Challenges at
Limits of Technology
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
DSBP THS-1 ZW-500 DSD2
Ph
op
ho
rus [
mg
/L]
sRP sNRP pP
• No Treatment Technology Available for SNRP
• Portion May Not Be Bioavailable / Biodegradable
Particulate P
Soluble
NonReactive P
Soluble Reactive P Neethling et al, 2007
Effluent P Fractions From Advanced Tertiary
Treatment Processes
0.000
0.010
0.020
0.030
0.040
0.050
P C
on
cen
trat
ion
, mg
/L
pAHP
pOP
pRP
sAHP
DOP
sRP
Sedimentation
Filtration
Membrane Filtration
Lui, Gu, et al. (ongoing) Phosphorus Speciation and Removal in Advanced Wastewater Treatment, WERF Report
P fractions at Plant N
0.0
0.5
1.0
1.5
2.0
2.5
3.0
BNR INF BNR Chemical Premoval
Mono/Dualmedia
Filtration
P c
on
cen
trat
ion
(m
g/L
)
pOP
pAHP
pRP
DOP
sAHP
sRP
0.00
0.05
0.10
0.15
0.20
• sAHP seems to remain after BNR, associated with biomolecules
• Chemical addition converts sRP into pRP
• Chemical sRP (PO4) removal relies pRP removal
Comparison of P fractions at Plant P
0
1
2
3
4
5
6
7
8
9
BNR INF BNR Trident HS
P c
on
cen
trat
ion
(m
g/L
)
pOP
pAHP
pRP
DOP
sAHP
sRP0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
• Plant P and N are similar as (BNR+sedimentation +Filtration): composition very
different
• DOP dominant soluble fraction; pAHP major particulate form
• Multi stage barrier remove TP to lower level
BNR Eff
Opportunities for Optimization
Opportunities for Improvement
• Improve understanding of chemical kinetics
– Dose relationships
– Reuse formed metal hydroxides
• Enhance solids separation
Molar Dose Ratio From Tests
Slav Hermanowicz, Chemical Fundamentals of Phosphorus Precipitation, WERF Boundary Condition Workshop, Washington DC, 2006
0
2
4
6
8
10
12
14
16
18
20
0.01 0.1 1 10
ortho P res (mg/L)
Al/P
(m
ol/m
ol)
Full Scale 6.6 - 6.75
Lab data pH 6
Lab data pH 7.2
0.1
1
10
100
0.01 0.1 1 10
ortho P res (mg/L)
Fe
/P (m
ol/m
ol)
Lab data pH 6.5
Lab data pH 6.8
Lab data pH 7.2
Lab data pH 8
Full Scale data
Single Step Chemical Addition
Requires High Dose
One Step
P entering mg/L 5
P residual mg/L 0.1
Alum/P dose mol/mol 5
Alum/P dose mg/mg 48
Alum dose mg/L 235
Two Step Chemical Addition Reduce Dose
One Step Step 1 Step 2
Two Steps
P entering mg/L 5 5 1 5
P residual mg/L 0.1 1 0.1 0.1
Alum/P dose mol/mol 5 1.5 5 2.1
Alum/P dose mg/mg 48 14 48 21
Alum dose mg/L 235 58 43 101
Two Step Chemical Addition Reduce Dose
One Step Step 1 Step 2
Two Steps
P entering mg/L 5 5 1 5
P residual mg/L 0.1 1 0.1 0.1
Alum/P dose mol/mol 5 1.5 5 2.1
Alum/P dose mg/mg 48 14 48 21
Alum dose mg/L 235 58 43 101
Reuse Chemical Sludge to Reduce Dose and
Increase Reliability
• Return chemical sludge to upstream process
• Build solids inventory – operate in solids contact
mode
Conventional Tertiary Chemical P Removal
AS SCL SCL PCL Al/Fe
Filter Al/Fe
“ReUse” Chemical Sludge Upstream
AS SCL SCL PCL Al/Fe
Filter Al/Fe
Contact Clarification in Tertiary
AS SCL SCL PCL Al/Fe
Filter Al/Fe
ANX
Coeur d’Alene: Microfiltration and Solids Recycle
AS SCL PCL Al/Fe Al/Fe
ANX TMF
Implications for Design and Operation –
Coeur d’Alene Pilot
No Solids Inventory
Loss of Alum feed
Implications for Design and Operation –
Coeur d’Alene Pilot
With Solids Inventory
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
10-Jun 10-Jun 11-Jun 11-Jun 12-Jun 12-Jun 13-Jun 13-Jun 14-Jun 14-Jun 15-Jun
SE
PO
4-P
[m
g/L
]
PO
4-P
[m
g/L
]
TMF Effluent PO4-P Secondary Effluent PO4-P
Alum Off
Alum On
Summary and Conclusion - I
• Chemical reactions for phosphorus removal with
ferric or alum is a primarily a surface complexation
reaction
• Good mixing and contact time is needed to
maximize chemical efficiency
• Preformed Metal Hydroxides retain the ability to
react and remove phosphate
Summary and Conclusion - II
• Target phosphorus species for effective removal:
– Precipitate Reactive Phosphorus – Phosphate
• Increased dose can improve removal
– Filter particulate fractions
• High efficiency filters
– Soluble Non-Reactive P remains difficult to remove
• Reuse metal hydroxides to reduce chemical use
Optimizing Chemical
Phosphorus Removal
JB Neethling
HDR Engineering
2013 Technical Conference and Exposition
Ohio Water Environment Association (OWEA)
Mason, OH
June 18-20, 2013
Copyright 2013 HDR Engineering, Inc. All rights reserved.
