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A Relationship Between Calcium Phosphate And Silica Fouling In Wastewater Ro Systems

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.

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A Relationship Between Calcium Phosphate And Silica Fouling In Wastewater Ro Systems

  1. 1. Mo Malki American Water Chemicals, Inc. (AWC) A RELATIONSHIP BETWEEN CALCIUM PHOSPHATE AND SILICA FOULING IN WASTEWATER RO SYSTEMS
  2. 2. Background <ul><li>A wastewater recycling RO plant suddenly started to experience silica scaling after having operated without any scaling issues for about 1 year. </li></ul><ul><li>Feedwater silica was reasonably low had not changed since plant start-up: </li></ul><ul><li>18 – 22 ppm SiO 2 @ 85% Recovery </li></ul><ul><li> 120 – 146 ppm SiO 2 in concentrate stream </li></ul><ul><li>The same phenomenon was also seen at a nearby wastewater RO plant. </li></ul>
  3. 3. <ul><li>In both cases, the only change in feedwater chemistry was a sudden increase in orthophosphate. </li></ul><ul><li>Initially, cleaning with citric acid would substantially improve tail element permeate production, verifying that calcium phosphate scaling had formed. </li></ul><ul><li>Similar results were seen with online cleaning performed by reducing feed pH to 6 for extended periods of time. </li></ul><ul><li>Despite the improvement, performance would never return to baseline, with tail element specific flux declining consistently despite low pH cleaning after each scaling event. </li></ul>Background
  4. 4. Background <ul><li>Eventually citric acid cleaning would no longer result in a measurable improvement. </li></ul><ul><li>In some tail elements, there was a complete loss of permeate production, but no increase in differential pressure ( Δ P) was observed across the tail pressure vessel. </li></ul><ul><li>A membrane autopsy was performed on a tail end element after an unsuccessful citric acid cleaning. </li></ul><ul><li>The non-acid soluble scale consisted substantially of silica. </li></ul>
  5. 5. Background – Membrane Autopsy - SEM
  6. 6. Background – Membrane Autopsy – SEM
  7. 7. Background – Membrane Autopsy - EDS
  8. 8. Background <ul><li>A study was initiated to investigate whether there was a cause and effect relationship between silica polymerization and calcium phosphate precipitation. </li></ul>
  9. 9. <ul><li>Standard Test Procedure </li></ul><ul><li>Make cation solution for entire test in one batch </li></ul><ul><li>Make anion solution for entire test in one batch </li></ul><ul><li>Divide each of the solutions into 1L volumetric flasks </li></ul><ul><li>Pour cation and anion solution at a controlled rate into 2L cylindrical dish and place on a hotplate stirrer set to maintain target temperature </li></ul><ul><li>Continue mixing at a controlled speed during the experiment to maintain uniform temperature throughout the solution. </li></ul><ul><li>Immediately take first turbidity reading </li></ul><ul><li>Take turbidity reading every 30 minutes for each running test. </li></ul>Simulation of Scaling Conditions
  10. 10. HIGH SILICA, NO PHOSPHATE, NO ANTISCALANT 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 Feed Reject Calcium 10.91 28.12 Magnesium 1.50 3.87 Bicarbonate 9.60 30.00 Orthophosphate 0.00 0.00 Silica 93.37 291.78 Iron 0.05 0.16 Aluminum 0.10 0.31 pH 7.1 7.4
  11. 11. HIGH SILICA, NO PHOSPHATE, NO ANTISCALANT 30 minute hold time prior to filtering in order to ensure sufficient time for silica polymerization Element Wt % At % Si K 4.48 8.54 Fe K 95.52 91.46
  12. 12. HIGH SILICA, NO PHOSPHATE, NO ANTISCALANT 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. 0 min 30 min Filtrate Silica 291 291 291 pH 7.38 7.41 7.41
  13. 13. WASTEWATER PLANT FEEDWATER ANALYSIS (PPM) Raw Feed Reject@85% recovery Ca 80.70 80.70 538.00 Mg 25.20 25.20 168.00 Na 256.45 256.45 1709.67 K 18.80 18.80 125.33 Fe 0.12 0.12 0.80 Mn 0.05 0.05 0.33 Al 0.006 0.006 0.04 Cl 252.00 252.00 1680.00 SO4 233.00 316.81 2112.07 HCO3 408.70 302.15 2014.33 PO4 4.35 4.35 29.00 SiO2 22.20 22.20 148.00 pH 7.80 7.00 7.50
  14. 14. WASTEWATER RO FEED, NO IRON Feed PO 4 =4.35 ppm, Fe=0, Feed pH=7.0, Temp=31°C, Antiscalant Dosage=5 ppm, Recovery=85%
  15. 15. No Iron, No Antiscalant - SEM Filtered deposit of solution without antiscalant
  16. 16. No Iron, No Antiscalant - EDS Amorphous Calcium Phosphate Ca 9 (HPO 4 ) x (PO 4 ) 6-x (OH) x Elemental Analysis of Filter Deposit – No Antiscalant Element Weight% Atomic% Na K 6.78 9.18 Mg K 2.03 2.60 Al K 1.93 2.23 Si K 36.79 40.77 P K 15.33 15.40 S K 2.54 2.47 Cl K 4.92 4.32 Ca K 29.66 23.03
  17. 17. No Iron, No Antiscalant – Elemental Mapping
  18. 18. No Iron, Product H - SEM Filtered deposit from solution using Product H
  19. 19. No Iron, Product H - EDS Localized elemental analysis of colloidal particle Element Weight% Atomic% Mg K 0.48 0.76 Al K 1.93 2.73 Si K 7.88 10.72 P K 1.14 1.40 Ca K 88.57 84.39
  20. 20. No Iron, Product H - SEM Filtered deposit from solution using Product H – another particle
  21. 21. No Iron, Product H - EDS Localized elemental analysis of colloidal particle Element Weight% Atomic% Al K 1.87 1.96 Si K 94.81 95.24 P K 2.22 2.03 Ca K 1.10 0.77
  22. 22. No Iron, Product H – Elemental Mapping Elemental Mapping of colloidal particle
  23. 23. WASTEWATER RO FEED, NO CALCIUM Feed PO 4 =4.35 ppm, Feed Fe=0.12 ppm, Ca=0, Feed pH=7.0, Temp=31°C, Antiscalant Dosage=5 ppm, Recovery=85%
  24. 24. No Calcium, No Antiscalant - SEM Filtered deposit of solution without antiscalant
  25. 25. No Calcium, No Antiscalant - EDS Elemental Analysis of Filter Deposit 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: Fe r PO 4 (OH) 3r-3 Most Common: Fe 2 PO 4 (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) Element Weight% Atomic% Na K 1.93 3.08 Mg K 6.58 9.92 Al K 2.25 3.06 Si K 19.00 24.80 P K 24.71 29.25 Fe K 45.53 29.89
  26. 26. No Calcium, No Antiscalant – Elemental Mapping Elemental Mapping of Filter Deposit
  27. 27. No Calcium, Product B (Non-Linear Polymer) - SEM Filtered deposit of solution using Product B
  28. 28. No Calcium, Product B (Non-Linear Polymer) - EDS Elemental Analysis of Filter Deposit Element Weight% Atomic% Mg K 10.93 16.41 Al K 2.16 2.92 Si K 18.82 24.45 P K 22.33 26.31 Fe K 45.75 29.90
  29. 29. No Calcium, Product B (Non-Linear Polymer) Elemental Mapping of Filter Deposit
  30. 30. WASTEWATER RO FEED, COMPLETE Feed PO 4 =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%
  31. 31. Complete Feedwater, No Antiscalant - SEM Filtered deposit of solution without antiscalant
  32. 32. Complete Feedwater, No Antiscalant - EDS Elemental Analysis of Filter Deposit Elemental ratios indicate co-precipitation of calcium and iron phosphates with silica Amorphous Ferric Calcium Hydroxyphosphate? Fe 1.66 CaPO 4 (OH) 4 Element Weight% Atomic% Na K 7.55 11.39 Mg K 1.53 2.18 Al K 2.57 3.31 Si K 22.17 27.40 P K 17.30 19.39 S K 1.33 1.44 Cl K 6.47 6.33 Ca K 12.40 10.74 Fe K 28.67 17.81
  33. 33. Complete Feedwater, No Antiscalant – Elemental Mapping Elemental Mapping of Filter Deposit
  34. 34. Complete Feedwater, Product D - SEM Filtered deposit of solution using Product D
  35. 35. Complete Feedwater, Product D - EDS Elemental Analysis of Filter Deposit Element Weight% Atomic% Mg K 1.66 2.38 Al K 1.54 1.99 Si K 29.58 36.71 P K 25.48 28.68 Cl K 2.11 2.07 Ca K 13.97 12.15 Fe K 25.67 16.02
  36. 36. Complete Feedwater, Product D – Elemental Mapping Elemental Mapping of Filter Deposit
  37. 37. Complete Feedwater, Product G (Silica Antiscalant) SEM of Filtered deposit of solution using Product G
  38. 38. 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. Element Weight% Atomic% Na K 1.18 1.84 Mg K 1.83 2.69 Al K 1.83 2.43 Si K 22.65 28.84 P K 23.10 26.66 S K 1.51 1.68 Ca K 20.68 18.44 Fe K 27.22 17.43
  39. 39. Complete Feedwater, Product G (Silica Antiscalant) Elemental Mapping of Filter Deposit
  40. 40. Complete Feedwater, Product I - SEM Filtered deposit of solution using Product I
  41. 41. Complete Feedwater, Product I - EDS LOCALIZED ANALYSIS Localized elemental analysis of colloidal particle Element Weight% Atomic% Na K 3.46 4.19 Al K 3.12 3.22 Si K 93.42 92.59
  42. 42. Complete Feedwater, Product I – Elemental Mapping Elemental Mapping of Filter Deposit
  43. 43. Complete Feedwater, Product I - SEM Filtered deposit of solution using Product I – another particle
  44. 44. Complete Feedwater, Product I - EDS Localized Analysis Localized elemental analysis of colloidal 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
  45. 45. Complete Feedwater, Product I – Elemental Mapping Elemental Mapping of Filter Deposit
  46. 46. Feed Water Analysis – Municipal RO - Florida   Feed Concentrate Ca 120 490 Mg 3.8 16 Ba 0.0069 0.029 Sr 0.62 2.6 Fe 2+ 0.42 0.69 Total Fe 0.43 1.8 Al <0.05 <0.05 Mn 0.025 0.1 Na 18 69 Cl 35.75 141 HCO 3 384.117 1684.7712 SO 4 <10 <10 PO 4 1.51 6 SiO 2 29.13 122.53 pH 7.29 7.79 Recovery ~75%  
  47. 47. Membrane Autopsy – RO – Florida - SEM
  48. 48. 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
  49. 49. Membrane Autopsy – RO – Florida – Elemental Mapping Si Ka1
  50. 50. Membrane Autopsy – Texas - SEM
  51. 51. Membrane Autopsy – Texas - EDS Element Wt % At % NaK 1.08 2.02 MgK 0.91 1.61 AlK 0.23 0.36 SiK 13 19.98 P K 3.32 4.63 S K 0.76 1.03 CaK 26.18 28.21 FeK 54.53 42.16 Total 100 100
  52. 52. Membrane Autopsy – Texas – Elemental Mapping
  53. 53. [(OH) 4 SiOH] - + HOSi(OH) 3  (OH) 3 Si-O-Si(OH) 3 + H 2 O + OH - Reference: R.K.Iler, The Chemistry of Silica, Wiley (1979) OH OH OH OH Si OH - OH OH OH OH Si OH O  H H OH OH OH OH Si OH - OH O  H H OH OH OH OH Si OH - OH O  H H O  H H OH - OH
  54. 54. Phosphate Salts Likely to Form on RO membranes <ul><li>Examples of amorphous phosphate salts: </li></ul><ul><li>Amorphous Calcium Phosphate </li></ul><ul><li>Ca 9 (HPO 4 ) x (PO 4 ) 6-x (OH) x </li></ul><ul><li>Amorphous Ferric Hydroxyphosphate </li></ul><ul><li>Fe r PO 4 (OH) 3r-3 </li></ul><ul><li>Most Common: Fe 2 PO 4 (OH) 3 </li></ul><ul><li>Amorphous Aluminum Hydroxyphosphate Al 2 PO 4 (OH) 3 </li></ul><ul><li>Amorphous Ferric Calcium Hydroxyphosphate Fe 1.66 CaPO 4 (OH) 4 </li></ul>
  55. 55. OH OH OH OH Si OH - OH OH OH Fe 2 PO 4 OH O  H H OH OH OH OH Si OH - O OH OH OH OH Si OH - OH O  H H O OH - OH OH  H H OH OH OH OH Si O  H H  H H OH OH OH OH Si O  H H O  H H
  56. 56. Membrane Surface Calcium phosphate formation will result in a disproportionate amount of Silica deposition on the membrane surface Amorphous Calcium Phosphate Ca 9 (HPO 4 ) x (PO 4 ) 6-x (OH) x PO 4 3 ¯ Ca 2+ Ca 2+ -Si-O-Si- I I I I PO 4 3 ¯ -Si-O-Si- I I I I -Si-O-Si- I I I I PO 4 3 ¯ Ca 2+ PO 4 3 ¯ Ca 2+ PO 4 3 ¯ Ca 2+ PO 4 3 ¯ Ca 2+ -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I -Si-O-Si- I I I I OH-Si-OH I I OH OH OH-Si-OH I I OH OH -Si-O- S- I I I I O O -Si-O- Si- I I I I O OH - Si-O-Si- I I I I O -Si-O-Si- I I I I O O O -Si-O-Si- I I I I O -Si-O-Si- I I I I O O -Si-O-Si- I I I I OH OH OH OH - Si- I I I O O O -Si-O-Si- I I I I O O O -Si-O-Si- I I I I O O -Si-O-Si- I I I I O O -Si-O-Si- I I I I O O O -Si-O-Si- I I I I O -Si-O-Si- I I I I O O O -Si-O-Si- I I I I O -Si-O-Si- I I I I O O O -Si-O-Si- I I I I O O O I I O -Si-OH I I O OH -Si-O-Si- I I I I O O OH-Si- I I O O -Si-O-Si- I I I I OH O - Si-O-Si- I I I I O O O OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH -Si-OH OH OH OH
  57. 57. Membrane Autopsy – Pilot RO - Florida RO Pilot Feedwater Analysis   Feed Concentrate Ca 470.00 783.33 Mg 61.00 101.67 Ba 0.11 0.18 Sr 1.20 2.00 Fe ND ND Mn ND ND Al ND ND Na 310.00 516.67 Cl 865.00 1441.67 HCO3 1097.90 1829.84 SO4 135.24 225.40 PO4 0.37 0.61 SiO2 135.24 225.40 pH 7.58 7.80 Temperature (°c) 25.40 26.20
  58. 58. Membrane Autopsy – Pilot RO – Florida – SEM
  59. 59. Membrane Autopsy – Pilot RO – Florida - EDS Element Weight% Atomic% Na K 0.75 1.11 Mg K 2.50 3.48 Al K 0.30 0.37 Si K 10.07 12.13 P K 26.78 29.26 S K 20.18 21.30 Cl K 1.57 1.50 Ca K 35.59 30.05 Fe K 0.56 0.34 I L 1.69 0.45 Totals 100.00
  60. 60. Membrane Autopsy – Pilot RO – Florida – Elemental Map
  61. 61. Membrane Autopsy – Pilot RO – Florida - SEM
  62. 62. Membrane Autopsy – Pilot RO – Florida - EDS Element Weight% Atomic% Na K 1.27 1.82 Mg K 1.69 2.30 Al K 0.32 0.39 Si K 18.92 22.27 P K 21.15 22.58 S K 22.76 23.46 Cl K 2.25 2.10 Ca K 29.48 24.31 Fe K 0.61 0.36 I L 1.55 0.41 Totals 100.00
  63. 63. Membrane Autopsy – Pilot RO – Florida – Elemental Map
  64. 64. Membrane Autopsy – Pilot RO – Florida – Elemental Map
  65. 65. Conclusion <ul><li>Polymer based antiscalants catalyze iron phosphate precipitation, resulting in heavier silica fouling – typically dispersants are believed to inhibit silica polymerization. </li></ul><ul><li>Certain antiscalants that are effective at inhibiting calcium carbonate appear to catalyze calcium phosphate scaling and result in heavier silica scale formation. </li></ul>
  66. 66. Conclusion <ul><li>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. </li></ul><ul><li>Effective inhibition of calcium phosphate scales and other phosphate salts prevents silica deposition on RO membrane surfaces when SiO 2 < 300 ppm. </li></ul><ul><li>Silica antiscalants are ineffective at inhibiting silica polymerization that occurs as a result of phosphate salt precipitation. </li></ul>
  67. 67. Thank you Mo Malki, American Water Chemicals E-mail: momalki@amh2ochem.com www.membranechemicals.com
  68. 68. Product A King Lee PTP100 Product B PWT Spectraguard Product C Avista Vitec 3000 Product D Genesys LF Product E Nalco PC-191 Product F Flocon 260 Product G Avista Vitec 4000 Product H AWC A-109 Product I AWC A-110

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