Framework for Developing ph Guidance for Drinking Water Treatment and Distribution

Transcription

1 Framework for Developing ph Guidance for Drinking Water Treatment and Distribution Michael Schock U.S. Environmental Protection Agency ORD, NRMRL, WSWRD, TTEB, Cincinnati, Ohio 45268

2 Problems with Existing ph Guidance Historically, ph range developed as secondary or aesthetic guidance but often implemented as regulation Current limit of ph can limit optimization of Pb and Cu release (corrosion control) ph 6.5 may need to be reconsidered in light of current metal solubility knowledge Current upper limit is not reflective of reported finished water phs No health effects seen for drinking water at high ph 2

3 ph Interactions with Treatment Processes Iron and Manganese Removal Radionuclide or As Treatment (IX, AA, aeration) Corrosion Control Treatment ph Control Disinfection Effectiveness Coagulation & Filtration Disinfection By-Products Formation Rates 3

4 ph Variations in Treatment Optimum ph range will vary across treatment processes Treated water ph often will need to be adjusted prior to distribution Corrosion control DBP control Accuracy of ph measurement can especially impact optimization for corrosion control Biases from atmospheric contact Calibration/measurement issues at low ionic strength 4

5 Alkalinity/DIC Relationship I=0.005, 25ºC 5

6 Effect of DIC on β for the CO 3 -H 2 O System 25ºC, I=0.0 Buffer Intensity, DIC (in mg C/ L) More ph stability at high ph than with large DIC addition ph 6

7 Difference between Alkalinity and Buffer Intensity for Different ph s Alkalinity ph DIC mg/l as CaCO 3 mg/l as C Buffer Intensity (β/10000) Computations for 10 C, I=

8 Non-Carbonate Ions and Buffer Intensity Carbonate, orthophosphate, plus silicate, I=0.01, 25ºC TIC=4.8 mg C/L Buffer Int ensity, mg/l SiO mg/l PO 4 H 2 O Tot al Supplemental stability from silicic acid at elevated ph ph 8

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10 Air Contact Affects ph Stability in Tanks & Reservoirs 50 Alkalinit y (mg/las CaCO 3 ) CO 2 Lost -- phincreases CO 2 Absorbed -- phdecreases ph 10

11 Examples of ph impact on metal speciation and release

12 Importance of Speciation in Mn Mobility Mn (0.05 mg/l) DIC = 18 mg C/L.8 Eh (volts) MnO 2 Area of main concern Mn Mn ++ 3 O 4 MnCO C ph 12

13 Divalent Pb Solubility: The Classic Model mg Pb/L DIC 1 mg C/L 5 mg C/L 10 mg C/L 20 mg C/L 35 mg C/L 50 mg C/L 75 mg C/L 100 mg C/L 0.01 Target ph ph 13

14 Eh-pH Diagram for Pb in Water 1.2 Eh (volts) DIC = 18 mg C/L Pb = mg/l Pb ++ PbO 2 (plattnerite) PbCO 3 Pb 3 (CO 3 ) 2 (OH) 2 (s) Pb(CO 3 ) 2 2 Pb(OH) 4 2 Observed fast PbO 2 formation Pb metal ph 14

15 Orthophosphate Treatment for Pb(II) 0.40 mg Pb/L ph = 7.0 ph = 7.5 ph = 8.0 ph = mg C/L 4.8 mg C/L More effective with low TIC Lower dosages at low TIC Point of diminishing returns higher with high TIC ph plays more of a role in optimization as DIC increases mg PO 4 /L 15

16 ph Is Dominant Variable for Cu(I) 16

17 Copper(II) Solubility & ph Adjustment If ph > 7.5, no problems if DIC < 35 If DIC < 5, no problems if ph > 7 If DIC > 35-40, scaling & buffering prevents sufficient ph adjustment to solve problems To minimize Cu for discharge optimization, ph > 9 needed mg Cu/L ACTION LEVEL 1.0 ph = 6.5 ph = 7.0 ph = 7.5 ph = 8.0 ph = 8.5 ph = 9.0 ph = 9.5 ph = mg C/L DIC 17

18 Problems with ph measurement

19 Open-System ph Air Contact Issue Alkalinity 20 mg/l, Room Temperature Closed- System Sample A Sample B Sample C No precautions taken. Open-system ph on samples from polyethylene bottles with variable amounts of air space. 19

20 ph Drift in Low Alkalinity Sample Open to air, 3.27 mg/l Initial (closed system) 8.16 ± minute minutes minutes minutes minutes

21 Improved ph Measurement Methodology Closed system conditions Samples sealed until analysis Analysis in stoppered flask, minimal air contact Fast, combination electrode Certified and traceable buffers Gentle stirring Multiple re equilibrations to random variation (Optional) PC monitoring of stability Do not trust ph meter auto stabilization 21

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23 Rarely Understood: Stirring Issues Stirring needed to displace surface layer from prior sample Stirring too fast Causes Streaming potential bias Drives CO 2 out, changing ph Heats sample 23

24 ph 7.0, DIC = 10, SiO 2 = ph Flasks ph Beaker ph ph continues to rise as CO 2 exsolves from the water in the beaker Time (minutes) 24

25 ph 8, DIC = ph Flasks ph Beaker ph Ironically, the longer you wait for ph to stabilize, the more inaccurate it gets!! Time (minutes) 25

26 Conclusions

27 Conclusions on ph range ph range should consider operational needs and corrosion control Accurate measurement is critical for both Must avoid confusing consumers in lab reports Current range of ph can limit optimization of Pb and Cu release (corrosion control), Fe/Mn oxidation and chloramination ph 6.5 may be too low in light of current metal solubility knowledge Upper limit (ph 8.5) needs to be much higher in order to minimize solubility of Pb(II) and Cu(II) 27

28 Acknowledgements/Disclaimer France Lemieux, Head, Materials and Treatment Section. Health Canada Nadia Martinova, Evaluator, Materials and Treatment Section. Health Canada This presentation has been reviewed in accordance with U.S. Environmental Protection Agency (EPA) policy and approved for external presentation. The views expressed are those of the author[s] and do not necessarily represent the views or policies of EPA 28

29 Michael Schock (513) Thank you 29