Control of Struvite Deposition in
Wastewater Treatment Plants
Paul L. Bishop
Associate Vice President for Research
University of Cincinnati
11th Annual Central States Water Environment Association
Education Conference
April 4, 2006
Typical Municipal WWTP Flow Diagram
Incoming Wastew ater
Bar Screen
Dissolved A ir
Flotation Tank
Screenings
Gravity
Thickener
Grit Chamber
Further Dew atering
A eration Tanks
Secondary
Clarif ier
Chlorine Contact Tank
Plant Ef f luent
Return Activated Sludge
Primary
Clarif ier
A naerobic Sludge
Digester
Sludge Dew atering
Facility
Sludge to Incinerator,
Farmland or Landf ill
Centrate/Filtrate
Grits
Problems
 Anaerobic sludge digestion releases
ammonium, magnesium and phosphate,
which can form struvite in digesters and
downstream dewatering facilities
 Can result in scaling in pipelines and on walls
of process equipment
 Centrate or filtrate from sludge dewatering
is usually returned to the plant headworks
where it can add to the wastewater
burden
Struvite
 Magnesium ammonium phosphate
MgNH4PO4 · 6H2O
 Named after Russian diplomat, H.G. von Struve (17721851)
 White, yellowish white, or brownish white in color
 FW = 245.41
 Specific density = 1.7
 Very insoluble in water, pKso = 12.6 – 13.15 at 25oC
Struvite Chemistry
NH4+  NH3 (aq) + H+
pKa=9.3
H3PO4  H2PO4- + H+
pKa1= 2.1
H2PO4-  HPO42- + H+
pKa2= 7.2
HPO42-  PO43- + H+
pKa3= 12.3
MgOH+  Mg2+ + OH-
pK=2.56
MgNH4PO4.6H2O  Mg2+ + NH4+ +PO43- + 6H2O
pK=12.6
Struvite formation occurs when the conditions are
such that the concentration product exceeds the
struvite conditional solubility product
Conditional Solubility of Struvite vs pH
Mg2+
1
6
-1
4
2
-3
NH4 +
0
-2 1
-4
3
5
7
9
11 13
-5
-7
-9
-6
-8
MgNH4PO4.6H2O
-10
-12
-11
Log (ionization fraction)
Log (Ps)
8
-13
PO4 3-
-15
pH
Ps = conditional solubility product
Kso = solubility product
Ps  CT , Mg CT , NH3 CT , PO4
Kso

 Mg2  NH   PO3 g Mg2 g NH  g PO3
4
4
4
4
CT,Mg = total concentration of all soluble magnesium species
CT,NH3 = total concentration of all soluble ammonia species
CT,PO4 = total concentration of all soluble phosphate species
"i = ionization fraction for component i
gi = activity coefficient for component i
Struvite Formation in Sludge
Dewatering Process
Anaerobically digested sludge, anaerobic supernatant (centrate/filtrate)
Mixing & perturbations
Carbon dioxide stripping
pH elevation
Phosphate equilibrium shifts towards PO43[Mg2+] [NH4+][PO43-] exceeds struvite solubility product (super-saturation)
Nucleation and crystal growth
Struvite precipitates
MgNH4PO4 . 6H2O
Filtrate return line
Struvite encrusted roller
Ball check
Productivity lost!!
(Courtesy Schaner’s Waste Water Products, Inc.)
Problems with Current Struvite
Control Techniques
 Addition of iron chloride to form
vivianite (Fe3(PO4)2 . 8H2O)
Chloride concentration increases
Ferric ion acts as an acid, lowering pH
Large volume inorganic sludge generation
Phosphate recovery from ferric phosphate
salt(s) is nearly impossible
 Similar problems with ferric sulfate or alum
Objective
 Investigate the use of magnesium
hydroxide to remove nutrients in a
controlled fashion from digested
sludge
 Can use waste flue gas desulfurization
sludge as a source of Mg(OH)2
Characterization of Mg(OH)2: Basic
Properties that are Important to
Wastewater Treatment Applications
Magnesium Hydroxide Dissolution Kinetics
10
9.5
9
pH
8.5
8
7.5
7
6.5
6
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00
Time (min)
Titration Curves of Several
Neutralization Chemicals
14
12
pH
10
8
6
C
D
4
A
B
2
0
0
0.002
0.004
0.006
Titrant Added (eq.)
A = calcium hydroxide; B = pure magnesium
hydroxide; C = sodium carbonate; D = as-received
magnesium hydroxide slurry
0.0035
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Buffering Capacity(eq/pH)
Relative Neutralization Capacity
Relative Neutralization Capacity and
Buffering Capacity of Several
Neutralization Reagents (at pH = 8.5)
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
1
2
3
4
1
2
3
1 = pure magnesium hydroxide; 2 = sodium carbonate; 3 =
calcium hydroxide; 4 = as-received magnesium hydroxide
slurry.
4
Summary
Mg(OH)2 has unique features compared with other
commonly used chemicals:
 slow dissolution process
 high neutralization capacity
 high buffering intensity
Sludge Digestion Enhancement
Using Mg(OH)2
NH3-N, PO43--P, Mg2+, Ca2+ and SO42- Changes
During Anaerobic Sludge Digestion
Biogas Production Profiles During
Anaerobic Sludge Digestion
Biogas Volume (L)
300
250
200
150
Mg(OH)2 reactor
Control reactor
100
50
0
0.00
100.00
200.00
300.00
400.00
Digestion Time (hours)
500.00
600.00
Summary
Applying magnesium hydroxide into an anaerobic
sludge digester can:




Result in greater destruction of COD and SS
Enhance the production rate of biogas
Increase overall treatment efficiency
Reduce level of nutrients in the supernatant that must be
returned to the plant’s headworks
 Increase the nutrient content in the generated biosolids
for agricultural use
 Improved sludge dewaterability, which will ease the
operation of the down stream sludge dewatering facilities
Nutrient Removal from Anaerobically
Digested Sludge and Sludge Supernatant
Using Mg(OH)2
3-
9
160
140
120
100
80
60
40
20
0
8.5
8
7.5
0
100
200
300
Time (min)
Phosphate---No mix control
Phosphate---Mg(OH)2=100mg/L
pH---No mix control
pH---Mg(OH)2=100 mg/L
400
7
500
Phosphate---Mixed control
Phosphate---Mg(OH)2=250mg/L
pH---Mixed control
pH---Mg(OH)2=250 mg/L
pH
PO4 -P (mg/L)
Nutrient Removal from Digested Sludge
Pilot Scale Experimental Results on
Phosphate Removal from Centrate
70
air stripping only
50
40
30
settling period
aeration period
PO43- -P (mg/L)
60
air stripping + 200 mg/L MgCl2
air stripping + 400 mg/L MgCl2
air stripping + 100 mg/L Mg(OH)2
air stripping + 200 mg/L Mg(OH)2
air stripping + 400 mg/L Mg(OH)2
20
10
0
0
50
100
Time (min)
150
200
Total phosphorus mass balance without metal phosphate precipitation from centrate/filtrate
Influent
310
100
effluent
Primary + secondary
treatment systems
10
300
Sludge digester
300
Filtrate/centrate
sludge cake
Sludge dewatering
210
90
Total phosphorus mass balance with metal phosphate precipitation from centrate/filtrate
Influent
100
107
effluent
Primary + secondary
treatment systems
10
97
Treated filtrate/
centrate
7
Sludge digester
97
Metal phosphate
precipitation reactor
sludge cake +
chemical sludge
Filtrate/centrate
Sludge dewatering
68
P-containing chemical sludge
61
29
90
Summary
 Use of Mg(OH)2 to remove nutrients from
anaerobically digested sludge is effective
only if the sludge is well digested.
 Removing phosphate from the side waste
stream will:
 reduce the nutrient load to the headworks of the
treatment plant (this is a current practice that
adversely affects the overall treatment efficiency)
 lower the potential for struvite formation, which is
a frequently occurring O&M problem in many
municipal wastewater treatment plants
 generate a slow release fertilizer
Improving the Settleability and
Dewaterability of Activated Sludge:
Applications of Mg(OH)2
350
700
300
600
250
500
200
400
150
300
100
200
50
100
0
0
0
20
40
60
80
Mg(OH)2 Dosage (mg/L)
100
120
SV(ml/L)
SVI
Effect of Mg(OH)2 on Activated
Sludge Settleability
Surface Charge Density Changes vs
Mg(OH)2 Dosage
COO-
---Mg2+ ---
-OOC
Relative Surface Charge
NH3
NH3
2
1
0
-1 0
20
40
60
80
100
-2
-3
-4
-5
-6
-7
Magnesium Hydroxide Addition (mg/L)
Mixed Liquor Sedimentation Curves under
Different Mg(OH)2 Dosage Conditions
Height of water/sludge interface (cm)
180
160
Mg(OH)2: 0 mg/L
Mg(OH)2: 100 mg/L
140
Mg(OH)2: 300 mg/L
Mg(OH)2: 500 mg/L
120
100
80
60
40
0:00:00
0:14:24
0:28:48
0:43:12
0:57:36
Time (hour:minute:second)
1:12:00
1:26:24
Sludge Dewaterability Changes with the
Addition of Mg(OH)2
CST (seconds)
290
285
280
275
270
265
260
255
0
50
100
150
Mg(OH)2 Dosage (mg/L)
200
Summary
 By charge neutralization, sweep
flocculation and Mg2+ bridging between the
EPS matrices of the microorganisms,
Mg(OH)2 is effective in improving the
settleability of activated sludge
 Besides enhancing the overall sludge
digestion process efficiency, Mg(OH)2
application to anaerobic sludge digester
can also generate a digested sludge that
is easier to dewater
Conclusions
 Mg(OH)2 improved the biological phosphate uptake
and release behavior of activated sludge
 Mg2+ was found to stimulate the phosphate uptake
during aeration periods
 The pH increase caused by Mg(OH)2 addition
enhanced phosphate release during the anaerobic
sedimentation period
 Research results provide supporting evidence for the
potential application of Mg(OH)2 in EBPR processes
Conclusions
 Magnesium hydroxide can effectively improve the
settleability of mixed liquor during sedimentation in
secondary clarifier and the dewaterability of
anaerobically digested sludge in sludge dewatering
 Magnesium hydroxide can enhance the overall
process efficiency of anaerobic sludge digestion due
to improved pH/alkalinity and the supplementation
of Mg2
Conclusions
 Magnesium hydroxide is effective in removing nutrients
from anaerobic supernatant, thus reducing the nutrient
load returned to the headworks of the plant
 It minimizes the risk of struvite formation and generates a
good plant fertilizer
 Magnesium hydroxide is superior to other commonly
used chemicals in this regard FeCl3, alum and lime.
 Aeration (for mixing) plus magnesium chloride (Mg2+
source) plus struvite seeding proves to be a good process
for controlled struvite crystallization.
Potential Mg(OH)2 Application Locations
in Municipal WWTP
Incoming Wastewater
Bar Screen
Screenings
Gravity
Thickener
Grit Chamber
Grits
Further Dewatering
H
M
1
ag
yd n e
ro siu
xi m
de
Aeration Tanks
Secondary
Return Activated Sludge
Primary
Clarifier