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This paper was presented at AMTA and the IDA. It shows that allowing calcium phosphate scale to form will result in silica scaling. It also compares the performance of different antiscalants in control of calcium phosphate.
Citation preview
July 2007
AMTA/SEDA Joint Conference & ExpositionMiami Beach, FL – July 18-21, 2011
AMTA/SEDA 2011 Joint Conference & Exposition
1
Mo MalkiAmerican Water Chemicals, Inc. (AWC)
A RELATIONSHIP BETWEEN CALCIUM PHOSPHATE AND SILICA FOULING IN
WASTEWATER RO SYSTEMS
AMTA/SEDA Joint Conference & Exposition
Background
A wastewater recycling RO plant suddenly started to experience silica scaling after having operated without any scaling issues for about 1 year.
Feedwater silica was reasonably low had not changed since plant start-up:
18 – 22 ppm SiO2 @ 85% Recovery
120 – 146 ppm SiO2 in concentrate stream
The same phenomenon was also seen at a nearby wastewater RO plant.
AMTA/SEDA Joint Conference & Exposition
In both cases, the only change in feedwater chemistry was a sudden increase in orthophosphate.
Initially, cleaning with citric acid would substantially improve tail element permeate production, verifying that calcium phosphate scaling had formed.
Similar results were seen with online cleaning performed by reducing feed pH to 6 for extended periods of time.
Despite the improvement, performance would never return to baseline, with tail element specific flux declining consistently despite low pH cleaning after each scaling event.
Background
AMTA/SEDA Joint Conference & Exposition
Background
Eventually citric acid cleaning would no longer result in a measurable improvement.
In some tail elements, there was a complete loss of permeate production, but no increase in differential pressure (ΔP) was observed across the tail pressure vessel.
A membrane autopsy was performed on a tail end element after an unsuccessful citric acid cleaning.
The non-acid soluble scale consisted substantially of silica.
AMTA/SEDA Joint Conference & Exposition
Background – Membrane Autopsy - SEM
AMTA/SEDA Joint Conference & Exposition
Background – Membrane Autopsy – SEM
AMTA/SEDA Joint Conference & Exposition
Background – Membrane Autopsy - EDS
AMTA/SEDA Joint Conference & Exposition
Background
A study was initiated to investigate whether there was a cause and effect relationship between silica polymerization and calcium phosphate precipitation.
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Standard Test Procedure
Make cation solution for entire test in one batch
Make anion solution for entire test in one batch
Divide each of the solutions into 1L volumetric flasks
Pour cation and anion solution at a controlled rate into 2L cylindrical dish and place on a hotplate stirrer set to maintain target temperature
Continue mixing at a controlled speed during the experiment to maintain uniform temperature throughout the solution.
Immediately take first turbidity reading
Take turbidity reading every 30 minutes for each running test.
Simulation of Scaling Conditions
AMTA/SEDA Joint Conference & Exposition
HIGH SILICA, NO PHOSPHATE, NO ANTISCALANT
Feed RejectCalcium 10.91 28.12
Magnesium 1.50 3.87Bicarbonate 9.60 30.00
Orthophosphate 0.00 0.00Silica 93.37 291.78
Iron 0.05 0.16Aluminum 0.10 0.31
pH 7.1 7.4
Water chemistry of high silica case, recovery=68%, Temp=25°C
Measured turbidity same that of deionized water as upon mixing anion and cation solutions - this indicates no crystal nucleation
AMTA/SEDA Joint Conference & Exposition
HIGH SILICA, NO PHOSPHATE, NO ANTISCALANT
Element Wt % At %Si K 4.48 8.54Fe K 95.52 91.46
30 minute hold time prior to filtering in order to ensure sufficient time for silica polymerization
AMTA/SEDA Joint Conference & Exposition
HIGH SILICA, NO PHOSPHATE, NO ANTISCALANT
0 min 30 min FiltrateSilica 291 291 291
pH 7.38 7.41 7.41
Reactive Silica was Measured Before and After Filtration using UV/VIS Spectrometer
It was therefore established that even when silica was ~300 ppm, no silica polymerization occurred in the absence of scale formation.
AMTA/SEDA Joint Conference & Exposition
WASTEWATER PLANT FEEDWATER ANALYSIS (PPM)
Raw FeedReject@85%
recoveryCa 80.70 80.70 538.00Mg 25.20 25.20 168.00Na 256.45 256.45 1709.67K 18.80 18.80 125.33Fe 0.12 0.12 0.80Mn 0.05 0.05 0.33Al 0.006 0.006 0.04Cl 252.00 252.00 1680.00
SO4 233.00 316.81 2112.07HCO3 408.70 302.15 2014.33PO4 4.35 4.35 29.00SiO2 22.20 22.20 148.00pH 7.80 7.00 7.50
AMTA/SEDA Joint Conference & Exposition
WASTEWATER RO FEED, NO IRON
Feed PO4=4.35 ppm, Fe=0, Feed pH=7.0, Temp=31°C, Antiscalant Dosage=5 ppm, Recovery=85%
AMTA/SEDA Joint Conference & Exposition
No Iron, No Antiscalant - SEM
Filtered deposit of solution without antiscalant
AMTA/SEDA Joint Conference & Exposition
Quantitative results
Wei
ght%
0
10
20
30
40
Na Mg Al Si P S Cl Ca
No Iron, No Antiscalant - EDS
Element Weight%
Atomic%
Na K 6.78 9.18Mg K 2.03 2.60Al K 1.93 2.23Si K 36.79 40.77P K 15.33 15.40S K 2.54 2.47Cl K 4.92 4.32Ca K 29.66 23.03
Amorphous Calcium Phosphate Ca9(HPO4)x(PO4)6-x(OH)x
Elemental Analysis of Filter Deposit – No Antiscalant
AMTA/SEDA Joint Conference & Exposition
No Iron, No Antiscalant – Elemental Mapping
AMTA/SEDA Joint Conference & Exposition
No Iron, Product H - SEM
Filtered deposit from solution using Product H
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
20
40
60
80
100
Mg Al Si P Ca
No Iron, Product H - EDS
Element Weight% Atomic%Mg K 0.48 0.76Al K 1.93 2.73Si K 7.88 10.72P K 1.14 1.40
Ca K 88.57 84.39
Localized elemental analysis of colloidal particle
AMTA/SEDA Joint Conference & Exposition
No Iron, Product H - SEM
Filtered deposit from solution using Product H – another particle
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
20
40
60
80
100
Al Si P Ca
No Iron, Product H - EDS
Element Weight% Atomic%Al K 1.87 1.96Si K 94.81 95.24
P K 2.22 2.03Ca K 1.10 0.77
Localized elemental analysis of colloidal particle
AMTA/SEDA Joint Conference & Exposition
No Iron, Product H – Elemental Mapping
Elemental Mapping of colloidal particle
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WASTEWATER RO FEED, NO CALCIUM
Feed PO4=4.35 ppm, Feed Fe=0.12 ppm, Ca=0, Feed pH=7.0, Temp=31°C, Antiscalant Dosage=5 ppm,
Recovery=85%
AMTA/SEDA Joint Conference & Exposition
No Calcium, No Antiscalant - SEM
Filtered deposit of solution without antiscalant
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
10
20
30
40
50
Na Mg Al Si P Fe
No Calcium, No Antiscalant - EDS
Elemental Analysis of Filter Deposit
Element Weight% Atomic%Na K 1.93 3.08Mg K 6.58 9.92Al K 2.25 3.06Si K 19.00 24.80P K 24.71 29.25Fe K 45.53 29.89
Elemental ratios indicate co-deposition of ferric phosphate, magnesium phosphate and aluminum phosphate with silica
Ferric phosphate typically precipitates as Amorphous Ferric Hydroxyphosphate General Formula: FerPO4(OH)3r-3
Most Common: Fe2PO4(OH)3
Reference: D.W.De Haas et al, The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal Part 4: Experimental periods using ferric chloride, Water SA, 26, 4 (2000)
AMTA/SEDA Joint Conference & Exposition
No Calcium, No Antiscalant – Elemental Mapping
Elemental Mapping of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
No Calcium, Product B (Non-Linear Polymer) - SEM
Filtered deposit of solution using Product B
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
10
20
30
40
50
Mg Al Si P Fe
No Calcium, Product B (Non-Linear Polymer) - EDS
Elemental Analysis of Filter Deposit
Element Weight% Atomic%Mg K 10.93 16.41Al K 2.16 2.92Si K 18.82 24.45P K 22.33 26.31Fe K 45.75 29.90
AMTA/SEDA Joint Conference & Exposition
No Calcium, Product B (Non-Linear Polymer)
Elemental Mapping of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
WASTEWATER RO FEED, COMPLETE
Feed PO4=4.35 ppm, Feed Fe=0.12 ppm, Ca=80.7 ppm, Feed pH=7.0, Temp=27°C,
Antiscalant Dosage=5 ppm, Recovery=85%
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, No Antiscalant - SEM
Filtered deposit of solution without antiscalant
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, No Antiscalant - EDS
Quantitative results
We
igh
t%
0
5
10
15
20
25
30
Na Mg Al Si P S Cl Ca Fe
Element Weight% Atomic%Na K 7.55 11.39Mg K 1.53 2.18Al K 2.57 3.31Si K 22.17 27.40P K 17.30 19.39S K 1.33 1.44Cl K 6.47 6.33Ca K 12.40 10.74Fe K 28.67 17.81
Elemental Analysis of Filter Deposit
Elemental ratios indicate co-precipitation of calcium and iron phosphates with silica
Amorphous Ferric Calcium Hydroxyphosphate? Fe1.66CaPO4(OH)4
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, No Antiscalant – Elemental Mapping
Elemental Mapping of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product D - SEM
Filtered deposit of solution using Product D
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
5
10
15
20
25
30
Mg Al Si P Cl Ca Fe
Element Weight% Atomic%Mg K 1.66 2.38Al K 1.54 1.99Si K 29.58 36.71P K 25.48 28.68Cl K 2.11 2.07Ca K 13.97 12.15Fe K 25.67 16.02
Complete Feedwater, Product D - EDS
Elemental Analysis of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
Elemental Mapping of Filter Deposit
Complete Feedwater, Product D – Elemental Mapping
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product G (Silica Antiscalant)
SEM of Filtered deposit of solution using Product G
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
5
10
15
20
25
30
Na Mg Al Si P S Ca Fe
Element Weight% Atomic%Na K 1.18 1.84Mg K 1.83 2.69Al K 1.83 2.43Si K 22.65 28.84P K 23.10 26.66S K 1.51 1.68
Ca K 20.68 18.44Fe K 27.22 17.43
Complete Feedwater, Product G (Silica Antiscalant)
Elemental Analysis of Filter Deposit
Elemental ratios indicate co-precipitation of calcium and iron phosphates with silica
Silica polymerization was not inhibited by the Silica Antiscalant when phosphate salts formed.
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product G (Silica Antiscalant)
Elemental Mapping of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product I - SEM
Filtered deposit of solution using Product I
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
20
40
60
80
100
Na Al Si
Element Weight% Atomic%Na K 3.46 4.19Al K 3.12 3.22Si K 93.42 92.59
LOCALIZED ANALYSIS
Complete Feedwater, Product I - EDS
Localized elemental analysis of colloidal particle
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product I – Elemental Mapping
Elemental Mapping of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product I - SEM
Filtered deposit of solution using Product I – another particle
AMTA/SEDA Joint Conference & Exposition
Element Weight% Atomic%
Na K 8.43 15.81
Al K 3.15 5.03
Si K 14.26 21.89
P K 10.75 14.97
S K 12.51 16.82
K K 1.54 1.70
Ca K 8.49 9.13
Fe K 3.95 3.05
Ba L 36.92 11.59
Localized Analysis
Complete Feedwater, Product I - EDS
Localized elemental analysis of colloidal particle
AMTA/SEDA Joint Conference & Exposition
Complete Feedwater, Product I – Elemental Mapping
Elemental Mapping of Filter Deposit
AMTA/SEDA Joint Conference & Exposition
Feed Water Analysis – Municipal RO - Florida
Feed ConcentrateCa 120 490Mg 3.8 16Ba 0.0069 0.029Sr 0.62 2.6
Fe2+ 0.42 0.69Total Fe 0.43 1.8
Al <0.05 <0.05Mn 0.025 0.1Na 18 69Cl 35.75 141
HCO3 384.117 1684.7712SO4 <10 <10PO4 1.51 6SiO2 29.13 122.53pH 7.29 7.79
Recovery ~75%
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – RO – Florida - SEM
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – RO – Florida – EDS
Weight% Atomic%
Al 0.6 0.89
Si 1.59 2.25
P 12.8 16.4
S 26.6 33
Ca 20.9 20.7
Fe 37.5 26.7
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – RO – Florida – Elemental Mapping
Si Ka1
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Membrane Autopsy – Texas - SEM
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Texas - EDS
Element Wt % At %NaK 1.08 2.02MgK 0.91 1.61AlK 0.23 0.36SiK 13 19.98P K 3.32 4.63S K 0.76 1.03CaK 26.18 28.21FeK 54.53 42.16Total 100 100
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Texas – Elemental Mapping
AMTA/SEDA Joint Conference & Exposition
OH
OH
OHOH Si
OH-
OH
OH
OHOH Si
OH
O
H H
OH
OH
OHOH Si
OH-OH
OH H
OH
OH
OHOH Si
OH-OH
O
H H
O
H H
OH-OH
[(OH)4SiOH]- + HOSi(OH)3 (OH)3Si-O-Si(OH)3 + H2O + OH-
Reference: R.K.Iler, The Chemistry of Silica, Wiley (1979)
AMTA/SEDA Joint Conference & Exposition
Phosphate Salts Likely to Form on RO membranes
Examples of amorphous phosphate salts:
Amorphous Calcium Phosphate Ca9(HPO4)x(PO4)6-x(OH)x
Amorphous Ferric Hydroxyphosphate FerPO4(OH)3r-3
Most Common: Fe2PO4(OH)3
Amorphous Aluminum Hydroxyphosphate Al2PO4(OH)3
Amorphous Ferric Calcium Hydroxyphosphate Fe1.66CaPO4(OH)4
AMTA/SEDA Joint Conference & Exposition
OH
OH
OHOH Si
OH-
OH
OH
OHFe2PO4
OH
O
H H
OH
OH
OHOH Si
OH- O
OH
OH
OHOH Si
OH-OH
O
H H
O
OH-OH
OH H H
OH
OH
OHOH Si
O
H H
H H
OH
OHOH Si
O
H H
O
H H
AMTA/SEDA Joint Conference & Exposition
PO43 ¯
Ca2+
Ca2+
-Si-O-Si-
II
II
PO43 ¯
-Si-O-Si-I
II
I
-Si-O-Si-I
I I
I
PO43 ¯
Ca2+
PO43 ¯
Ca2+
PO43 ¯
Ca2+
PO43 ¯
Ca2+
-Si-O-Si-
I
II
I
-Si-O-Si-I
I I
I
-Si-O-Si-
I
II
I-Si-O
-Si-
II
II
-Si-O
-Si-
III I -S
i-O-S
i-II
I I-Si-O-Si-
I
I I
I -Si-O-Si-
I
II
I-Si-O-Si-
I
II
I -Si-O
-Si-
III I
-Si-O-Si-
II
II
OH-Si-OHII
OH
OH
OH-Si-O
H
II
OH
OH
-Si-O- S-I
I I
IO O
-Si-O- Si-I
I I
IO
OH
- Si-O-Si-I
I I
IO
-Si-O-Si-I
I I
IO
O O
-Si-O-Si-I
I I
IO
-Si-O-Si-I
I I
IO O
-Si-O-Si-I
I I
I
OH OH
OH
OH - Si-I
I
IO
O O
-Si-O-Si-I
I I
IO
O O
-Si-O-Si-I
I I
IO O
-Si-O-Si-I
I I
IO O
-Si-O-Si-I
I I
IO
O O
-Si-O-Si-I
I I
IO
-Si-O-Si-I
I I
IO
O O
-Si-O-Si-I
I I
IO
-Si-O-Si-I
I I
IO
O O
-Si-O-Si-I
I I
IO
O O
I
I
O
-Si-OHI
I
O
OH
-Si-O-Si-I
I I
IO O
OH-Si-I
IO
O
-Si-O-Si-I
I I
IOH
O
- Si-O-Si-I
I I
IO
O O
OHOHOH
OH
OH
OH
OH
OHOH
OH
OH
OH
OH
OH
OHOHOH OH
OHOHOH OH OHOH OHOH OHOH
OH
OH
-Si-OH
OHOHOH
Membrane SurfaceCalcium phosphate formation will result in a disproportionate amount of Silica deposition on the membrane surface
Amorphous Calcium Phosphate Ca9(HPO4)x(PO4)6-x(OH)x
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Pilot RO - Florida
Feed ConcentrateCa 470.00 783.33Mg 61.00 101.67Ba 0.11 0.18Sr 1.20 2.00Fe ND NDMn ND NDAl ND NDNa 310.00 516.67Cl 865.00 1441.67
HCO3 1097.90 1829.84SO4 135.24 225.40PO4 0.37 0.61SiO2 135.24 225.40pH 7.58 7.80
Temperature (°c) 25.40 26.20
RO Pilot Feedwater Analysis
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Pilot RO – Florida – SEM
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
10
20
30
40
Na Mg Al Si P S Cl Ca Fe I
Element Weight% Atomic%Na K 0.75 1.11Mg K 2.50 3.48Al K 0.30 0.37Si K 10.07 12.13P K 26.78 29.26S K 20.18 21.30Cl K 1.57 1.50Ca K 35.59 30.05Fe K 0.56 0.34I L 1.69 0.45
Totals 100.00
Membrane Autopsy – Pilot RO – Florida - EDS
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Pilot RO – Florida – Elemental Map
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Pilot RO – Florida - SEM
AMTA/SEDA Joint Conference & Exposition
Quantitative results
We
igh
t%
0
5
10
15
20
25
30
Na Mg Al Si P S Cl Ca Fe I
Element Weight% Atomic%Na K 1.27 1.82Mg K 1.69 2.30Al K 0.32 0.39Si K 18.92 22.27P K 21.15 22.58S K 22.76 23.46Cl K 2.25 2.10Ca K 29.48 24.31Fe K 0.61 0.36I L 1.55 0.41
Totals 100.00
Membrane Autopsy – Pilot RO – Florida - EDS
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Pilot RO – Florida – Elemental Map
AMTA/SEDA Joint Conference & Exposition
Membrane Autopsy – Pilot RO – Florida – Elemental Map
AMTA/SEDA Joint Conference & Exposition
Conclusion
Polymer based antiscalants catalyze iron phosphate precipitation, resulting in heavier silica fouling – typically dispersants are believed to inhibit silica polymerization.
Certain antiscalants that are effective at inhibiting calcium carbonate appear to catalyze calcium phosphate scaling and result in heavier silica scale formation.
AMTA/SEDA Joint Conference & Exposition
Conclusion
Antiscalants that can individually inhibit calcium phosphate scale and iron phosphate scale lose their efficacy when both calcium and iron are present together with phosphate.
Effective inhibition of calcium phosphate scales and other phosphate salts prevents silica deposition on RO membrane surfaces when SiO2 < 300 ppm.
Silica antiscalants are ineffective at inhibiting silica polymerization that occurs as a result of phosphate salt precipitation.
AMTA/SEDA Joint Conference & Exposition
Thank you
67
Mo Malki, American Water Chemicals
E-mail: [email protected]
www.membranechemicals.com
AMTA/SEDA Joint Conference & Exposition
Product A King Lee PTP100Product B PWT SpectraguardProduct C Avista Vitec 3000Product D Genesys LFProduct E Nalco PC-191Product F Flocon 260Product G Avista Vitec 4000Product H AWC A-109Product I AWC A-110