Control of Struvite Deposition in Control of Struvite Deposition in Wastewater Treatment PlantsWastewater Treatment Plants
Paul L. BishopAssociate Vice President for Research
University of Cincinnati
11th Annual Central States Water Environment Association Education Conference
April 4, 2006
Incoming Wastewater
Bar Screen
Grit Chamber
Aeration Tanks
PrimaryClarifier
SecondaryClarifier
Chlorine Contact Tank
Plant Effluent
Dissolved AirFlotation Tank
Anaerobic SludgeDigester
Sludge DewateringFacility
Screenings
Grits
Further Dewatering
GravityThickener
Sludge to Incinerator,Farmland or Landfill
Re
turn
Act
iva
ted
Slu
dg
e
Ce
ntr
ate
/Filt
rate
Typical Municipal WWTP Flow DiagramTypical Municipal WWTP Flow Diagram
ProblemsProblems 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
StruviteStruvite
Magnesium ammonium phosphate
MgNH4PO4 · 6H2O
Named after Russian diplomat, H.G. von Struve (1772-1851)
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
NH4+ NH3 (aq) + H+ pKa=9.3
H3PO4 H2PO4- + H+ pKa1= 2.1
H2PO4- HPO4
2- + H+ pKa2= 7.2
HPO42- PO4
3- + H+ pKa3= 12.3
MgOH+ Mg2+ + OH- pK=2.56
MgNH4PO4.6H2O Mg2+ + NH4+ +PO4
3- + 6H2O pK=12.6
Struvite ChemistryStruvite Chemistry
Struvite formation occurs when the conditions are such that the concentration product exceeds the struvite conditional solubility product
-12
-10
-8
-6
-4
-2
0
2
4
6
8
1 3 5 7 9 11 13
pH
Log (
Ps)
-15
-13
-11
-9
-7
-5
-3
-1
1
Log (
ioniz
ati
on f
racti
on)
Mg2+
NH4+
PO43-
MgNH4PO4.6H2O
Ps = conditional solubility product
Kso = solubility product
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
i = activity coefficient for component i
P s C C CK so
T M g T N H T P O
M g N H P O M g N H P O
, , ,3 42
4 43 2
4 43
Conditional Solubility of Struvite vs pHConditional Solubility of Struvite vs pH
Anaerobically digested sludge, anaerobic supernatant (centrate/filtrate)
Mixing & perturbations
Carbon dioxide stripping
pH elevation
Phosphate equilibrium shifts towards PO43-
[Mg2+] [NH4+][PO4
3-] exceeds struvite solubility product (super-saturation)
Nucleation and crystal growth
Struvite precipitates
Struvite Formation in Sludge Struvite Formation in Sludge Dewatering ProcessDewatering Process
Filtrate return line Ball check
Struvite encrusted roller Productivity lost!!Productivity lost!!
(Courtesy Schaner’s Waste Water Products, Inc.)
MgNHMgNH44POPO4 4 . . 6H6H22OO
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
Problems with Current Struvite Problems with Current Struvite Control TechniquesControl Techniques
Addition of iron chloride to form
vivianite (Fe3(PO4)2 . 8H2O)
Similar problems with ferric sulfate or alum
ObjectiveObjective
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)Characterization of Mg(OH)22: Basic : Basic
Properties that are Important to Properties that are Important to
Wastewater Treatment ApplicationsWastewater Treatment Applications
6
6.5
7
7.5
8
8.5
9
9.5
10
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00
Time (min)
pH
Magnesium HydroxideMagnesium Hydroxide Dissolution KineticsDissolution Kinetics
0
2
4
6
8
10
12
14
0 0.002 0.004 0.006
Titrant Added (eq.)
pH
A BCD
A = calcium hydroxide; B = pure magnesium hydroxide; C = sodium carbonate; D = as-received magnesium hydroxide slurry
Titration Curves of Several Titration Curves of Several Neutralization ChemicalsNeutralization Chemicals
00.10.20.30.40.50.60.70.80.9
1
1 2 3 4
Rel
ativ
e N
eutr
aliz
atio
n C
apac
ity
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
1 2 3 4
Buf
feri
ng C
apac
ity(
eq/p
H)
1 = pure magnesium hydroxide; 2 = sodium carbonate; 3 = calcium hydroxide; 4 = as-received magnesium hydroxide slurry.
Relative Neutralization Capacity and Relative Neutralization Capacity and Buffering Capacity of Several Buffering Capacity of Several
Neutralization Reagents (at pH = 8.5)Neutralization Reagents (at pH = 8.5)
Mg(OH)2 has unique features compared with other
commonly used chemicals:
slow dissolution process
high neutralization capacity
high buffering intensity
SummarySummary
Sludge Digestion EnhancementSludge Digestion Enhancement
Using Mg(OH)Using Mg(OH)22
NHNH33-N, PO-N, PO443-3--P, Mg-P, Mg2+2+, Ca, Ca2+2+ and SO and SO44
2-2- Changes Changes
During Anaerobic Sludge DigestionDuring Anaerobic Sludge Digestion
0
50
100
150
200
250
300
0.00 100.00 200.00 300.00 400.00 500.00 600.00Digestion Time (hours)
Bio
gas
Vol
um
e (L
)
Mg(OH)2 reactor
Control reactor
Biogas Production Profiles During Biogas Production Profiles During Anaerobic Sludge DigestionAnaerobic Sludge Digestion
SummarySummary
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 Nutrient Removal from Anaerobically
Digested Sludge and Sludge Supernatant Digested Sludge and Sludge Supernatant
Using Mg(OH)Using Mg(OH)22
020406080
100120140160
0 100 200 300 400 500
Time (min)
PO
43- -
P (
mg/
L)
7
7.5
8
8.5
9
pH
Phosphate---No mix control Phosphate---Mixed controlPhosphate---Mg(OH)2=100mg/L Phosphate---Mg(OH)2=250mg/LpH---No mix control pH---Mixed controlpH---Mg(OH)2=100 mg/L pH---Mg(OH)2=250 mg/L
Nutrient Removal from Digested SludgeNutrient Removal from Digested Sludge
0
10
20
30
40
50
60
70
0 50 100 150 200
Time (min)
PO
43- -
P (
mg/
L)
air stripping only
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
aera
tion per
iod
sett
ling p
erio
d
Pilot Scale Experimental Results on Pilot Scale Experimental Results on Phosphate Removal from CentratePhosphate Removal from Centrate
Primary + secondarytreatment systems
Sludge digester
Sludge dewatering
effluent
sludge cake
Influent
Primary + secondarytreatment systems
Sludge digester
Sludge dewatering
effluent
sludge cake +chemical sludge
Influent
Filtrate/centrate
Filtrate/centrateMetal phosphateprecipitation reactor
100
100
310
210
300
300
10
90
107 10
29 90
61
7Treated filtrate/centrate
P-containing chemical sludge68
97
97
Total phosphorus mass balance without metal phosphate precipitation from centrate/filtrate
Total phosphorus mass balance with metal phosphate precipitation from centrate/filtrate
SummarySummary 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 Improving the Settleability and Dewaterability of Activated Sludge: Dewaterability of Activated Sludge: Applications of Mg(OH)Applications of Mg(OH)22
0
50
100
150
200
250
300
350
0 20 40 60 80 100 120
Mg(OH)2 Dosage (mg/L)
SV
I
0
100
200
300
400
500
600
700
SV
(ml/L
)
Effect of Mg(OH)2 on Activated Effect of Mg(OH)2 on Activated Sludge SettleabilitySludge Settleability
-7
-6
-5
-4
-3
-2
-1
0
1
2
0 20 40 60 80 100
Magnesium Hydroxide Addition (mg/L)
Re
lativ
e S
urf
ace
Ch
arg
e
COO- -OOC---Mg2+ ---
NH3 NH3
Surface Charge Density Changes vs Surface Charge Density Changes vs Mg(OH)2 DosageMg(OH)2 Dosage
40
60
80
100
120
140
160
180
0:00:00 0:14:24 0:28:48 0:43:12 0:57:36 1:12:00 1:26:24 Time (hour:minute:second)
He
igh
t o
f w
ate
r/s
lud
ge
inte
rfa
ce
(c
m)
Mg(OH)2: 0 mg/L
Mg(OH)2: 100 mg/L
Mg(OH)2: 300 mg/L
Mg(OH)2: 500 mg/L
Mixed Liquor Sedimentation Curves under Mixed Liquor Sedimentation Curves under Different Mg(OH)Different Mg(OH)22 Dosage Conditions Dosage Conditions
255
260
265
270
275
280
285
290
0 50 100 150 200
Mg(OH)2 Dosage (mg/L)
CS
T (
seco
nd
s)
Sludge Dewaterability Changes with the Sludge Dewaterability Changes with the Addition of Mg(OH)Addition of Mg(OH)22
SummarySummary 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
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 thepotential application of Mg(OH)2 in EBPR processes
ConclusionsConclusions
ConclusionsConclusions
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
ConclusionsConclusions 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)Potential Mg(OH)22 Application Locations Application Locations
in Municipal WWTPin Municipal WWTP