a relationship between calcium phosphate and silica fouling in wastewater ro systems

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.


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


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.


Background – Membrane Autopsy - SEM


Background – Membrane Autopsy – SEM


Background – Membrane Autopsy - EDS


Background

A study was initiated to investigate whether there was a cause and effect relationship between silica polymerization and calcium phosphate precipitation.


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


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



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



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.


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


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%


No Iron, No Antiscalant - SEM

Filtered deposit of solution without antiscalant


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


No Iron, No Antiscalant – Elemental Mapping


No Iron, Product H - SEM

Filtered deposit from solution using Product H



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


No Iron, Product H - SEM

Filtered deposit from solution using Product H – another particle



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



No Iron, Product H – Elemental Mapping

Elemental Mapping of colloidal particle


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%


No Calcium, No Antiscalant - SEM




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)


No Calcium, No Antiscalant – Elemental Mapping

Elemental Mapping of Filter Deposit


No Calcium, Product B (Non-Linear Polymer) - SEM

Filtered deposit of solution using Product B



We

igh

t%

0

10

20

30

40

50

Mg Al Si P Fe

No Calcium, Product B (Non-Linear Polymer) - EDS


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


No Calcium, Product B (Non-Linear Polymer)



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%


Complete Feedwater, No Antiscalant - SEM



Complete Feedwater, No Antiscalant - EDS


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 ratios indicate co-precipitation of calcium and iron phosphates with silica

Amorphous Ferric Calcium Hydroxyphosphate? Fe1.66CaPO4(OH)4


Complete Feedwater, No Antiscalant – Elemental Mapping



Complete Feedwater, Product D - SEM

Filtered deposit of solution using Product D



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




Complete Feedwater, Product D – Elemental Mapping


Complete Feedwater, Product G (Silica Antiscalant)

SEM of Filtered deposit of solution using Product G



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



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.


Complete Feedwater, Product I - SEM

Filtered deposit of solution using Product I



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



Complete Feedwater, Product I – Elemental Mapping



Complete Feedwater, Product I - SEM

Filtered deposit of solution using Product I – another particle


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



Complete Feedwater, Product I – Elemental Mapping



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%


Membrane Autopsy – RO – Florida - SEM


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


Membrane Autopsy – RO – Florida – Elemental Mapping

Si Ka1


Membrane Autopsy – Texas - SEM


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


Membrane Autopsy – Texas – Elemental Mapping


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)


Phosphate Salts Likely to Form on RO membranes

Examples of amorphous phosphate salts:


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


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


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



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


Membrane Autopsy – Pilot RO – Florida – SEM



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


Membrane Autopsy – Pilot RO – Florida – Elemental Map


Membrane Autopsy – Pilot RO – Florida - SEM



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


Membrane Autopsy – Pilot RO – Florida – Elemental Map


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.


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.


Thank you

67

Mo Malki, American Water Chemicals

E-mail: [email protected]

www.membranechemicals.com


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

a relationship between calcium phosphate and silica fouling in wastewater ro systems

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