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Cyanuric Acid Stabilizer What is all the fuss about? Ellen Meyer, Arch Chemicals February 9, 2017 NPC Conference New Orleans LA

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Overview

Chemistry of cyanuric acid (CYA) Impact on build up of CYA Impact on water balance Chlorine stabilization with cyanuric acid

The effect of cyanuric acid on chlorine kill rates In the lab In the pool

Recent Crypto data Implications for pool maintenance

Sanitizer residuals Remediation procedures Measurement issues CYA control

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Cyanuric Acid Double headed arrow (↔) means reaction can go back and forth

Cyanuric Acid Enol tautomer

Isocyanuric Acid Keto tautomer

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Chlorination of Cyanuric Acid

Isocyanuric Acid

Trichloroisocyanuric Acid

+ 3 HOCl + H2O

Hypochlorous Acid

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Cyanuric Acid Equilibria (O’Brien)

Cy3-

ClCy2-

Cl2Cy- HCl2Cy

H2ClCy HClCy-

H3Cy H2Cy- HCy2-

Cl3Cy

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Cyanuric Acid Equilibria with H+ (Using O’Brien measurements)

0

20

40

60

80

100

0 2 4 6 8 10 12 14

% S

pe

cie

s

pH

H3Cy pKa 6.88

H2Cy-

pKa 11.40

HCy-2

pKa 13.5

Cy-3

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Cyanuric Acid Equilibria with Cl (Using O’Brien values)

1 ppm AvCl, 20 ppm CYA, pH 7.5, 800 ppm TDS, 85 °F

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12 14

%Sp

eci

es

pH

[H2ClCy]

[HClCy–]

[ClCy2–]

[HCl2Cy]

[Cl2Cy–]

[Cl3Cy]

[HOCl]

[OCl-]

AvCl = Available Chlorine, TDS = Total Dissolved Solids

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Cyanuric Acid Equilibria with Cl (O’Brien)

1 ppm AvCl, 20 ppm CYA, pH 7.5, 800 ppm TDS, 85 °F

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6

%Sp

eci

es

pH

[H2ClCy]

[HClCy–]

[ClCy2–]

[HCl2Cy]

[Cl2Cy–]

[Cl3Cy]

[HOCl]

[OCl-]

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So how does this impact pool operation?

• How much does CYA build up over time?

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Trichloroisocyanuric acid

Atom Number of atoms

Molecular weight, g/mole

Weight %

Carbon (C) 3 3 x 12.01 15.5%

Nitrogen (N) 3 3 x 14.01 18.1%

Oxygen (O) 3 3 x 16.00 20.7%

Total 3C+3N+3O = CYA 9 126.06 54.2%

Chlorine (Cl) 3 3 x 35.45 45.8%

Molecular weight = 232.41

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CYA Products

• Add cyanuric acid independent of sanitizer

– 95-100% granular cyanuric acid

• Add chlorinated cyanuric acid as sanitizer/shock

– Trichloroisocyanuric acid • 54% CYA, so for every 100 lb of trichlor added to a pool, 54 lb of CYA is

added

– Dichloroisocyanuric acid • Hydrated (49% CYA)

• Anhydrous (57% CYA)

– Rough rule of thumb • For every pound of trichlor or dichlor added, you are adding ~½ pound of

CYA

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Cyanuric Acid Accumulation Rate Model When using Trichloroisocyanuric acid as Primary Sanitizer

0

100

200

300

400

500

0 7 14 21 28 35 42 49 56 63 70 77 84

CYA

(p

pm

)

Days

5 ppm/day 10 ppm/day

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How does CYA impact water balance?

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Water Balance- pH

pH = -log [H+]

H2O ↔ H+ + OH-

Minimum pH 7.2, Maximum pH 7.8

MAHC 5.7.3.4.1, APSP-11 7.1

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pH

pH 7 = neutral [H+] = [OH-]

pH <7.2

Corrosion of plaster, grout and metal

Eye irritation

pH >7.8

Scale, mineral precipitation

Eye irritation

Chlorine less effective

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Adjusting pH

pH = -log [H+]

To lower pH Acids contribute H+ to lower pH

Muriatic acid = hydrochloric acid HCl (aq) ↔ H+ + Cl-)

Dry acid = sodium bisulfate NaHSO4 ↔ H+ + Na+ + SO4

2-

Carbon dioxide (CO2) CO2 + H2O ↔ HCO3

- + H+

To raise pH Bases take away H+ to raise pH

Soda ash = sodium carbonate (Na2CO3) Na2CO3 + H+ ↔ 2Na+ + HCO3

-

HCO3- = bicarbonate

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pH

Lowering pH by adding CO2

CO2 + H2O => HCO3- + H+

Raising pH by losing CO2 to the air

HCO3- + H+ => CO2 + H2O

pH will drift up when carbonate alkalinity is present

Faster in spas

High temperatures

Aeration of the water

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Alkalinity

What is Alkalinity ?

Measure of pH buffering capacity

Buffer = something that keeps the pH from going up and down quickly

Something that absorbs H+ when an acid is added

Something the contributes H+ when a base is added

Carbonate

HCO3- + H+ ↔ H2CO3

When acid is added HCO3- + H+ → H2CO3

When base is added H2CO3 → HCO3- + H+

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Carbonate Alkalinity

0

20

40

60

80

100

0 2 4 6 8 10 12 14

% S

pe

cie

s

pH

Carbonic Acid H2CO3

Bicarbonate HCO3

- Carbonate CO3

2-

Buffers best at pH where two lines cross

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Cyanurate Alkalinity

Cyanuric acid (stabilizer) does provide buffer capacity

Cyanuric acid does not gas off and make pH drift like carbonate buffers

Cyanuric acid is measured in alkalinity test

Cyanuric acid does not provide corrosion protection for plaster

You must have carbonate alkalinity to protect plaster

+ H+ ↔

-

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Cyanurate Alkalinity

0

20

40

60

80

100

0 2 4 6 8 10 12 14

% S

pe

cie

s

pH

Cyanuric acid H3Cy

Cyanurate H2Cy-

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Alkalinity

For water balance need carbonate alkalinity

Example :

Total Alkalinity (TA, measured value) = 90

Stabilizer (measured value) = 120

(high, but common near season’s end)

Carbonate Alkalinity

= 90 - 1/3 (120)

= 90 - 40

= 50 (low)

pH Replace 1/3 with

7.9 1/ 2.7

7.7 1/ 2.9

7.5 1/ 3.2

7.3 1/ 3.6

7.1 1/ 4.2

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Alkalinity

Low Carbonate Alkalinity

pH changes abruptly and frequently with small chemical additions

Water may be corrosive in one area of pool and scaling in another

Overall- water will be more corrosive

pH of water drifts with the pH of the sanitizer

High Carbonate Alkalinity

pH changes slowly - stays around 8.0 to 8.4 and returns even after adjustment with acid

pH of water drifts up

Water will cause scaling and may appear cloudy or dull

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Alkalinity

Adjusting Alkalinity

To lower carbonate alkalinity

Muriatic acid (hydrochloric acid, HCl(aq))

Dry acid (sodium bisulfate, NaHSO4)

HCO3- + H+ → CO2 + H2O

Other acidic pool chemicals (trichlor, chlorine gas)

To raise carbonate alkalinity

Sodium bicarbonate (NaHCO3)

Soda ash (sodium carbonate, Na2CO3)

Will raise pH too

Other pool chemicals (calcium hypochlorite)

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How does CYA impact chlorine chemistry?

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Cyanuric Acid vs. Percentage Free Chlorine Remaining After One Hour

Stabiliser (Cyanurate) Use in Outdoor Swimming Pools http://www.health.nsw.gov.au/environment/factsheets/Pages/stabiliser-cyanurate.aspx

CYA, ppm

%Loss

0 35%

10 12%

20 5%

30 3%

40 2%

mg/L = ppm

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HOCl as a function of pH HOCl ↔ OCl- + H+

HOCl is the primary active sanitizer in chlorine pools

Dissociation constant from G. C. White, Handbook of Chlorination, Second Edition, Van Nostrand Reinhold Company, New York, 1986

0

20

40

60

80

100

5 6 7 8 9 10 11

Perc

en

t H

OC

l

pH

pH %HOCl

5.0 99.7%

7.0 77.5%

7.5 52.2%

8.0 25.7%

9.5 1.1%

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Cyanuric Acid Equilibria with Cl (O’Brien)

1 ppm AvCl, 20 ppm CYA, pH 7.5, 800 ppm TDS, 85 °F

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6

%Sp

eci

es

pH

[H2ClCy]

[HClCy–]

[ClCy2–]

[HCl2Cy]

[Cl2Cy–]

[Cl3Cy]

[HOCl]

[OCl-]

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HOCl- Varying Free Chlorine (FC) and CYA pH 7.5, 85 °F, 800 ppm TDS

Equilibrium constants from O’Brien 1972

CYA, ppm

%HOCl for 1 ppm FC

0 47%

5 13%

10 7%

20 3%

50 1%

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

0 10 20 30 40 50

%H

OC

l

CYA, ppm

1 ppm FC

2 ppm FC

3 ppm FC

4 ppm FC

10 ppm FC

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How does CYA impact chlorine activity?

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Disinfection Efficacy CT Values

Concentration x Time = CT

Usually 3 log (99.9%) reduction in ppm∙minutes

Will vary with pathogen strain, temperature, pH, etc.

Assumed to be linear

If CT = 100 ppm minutes Then

It will take 100 minutes to kill the organism with 1 ppm

Or It will take 1 minute to kill the organism with 100 ppm

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CT Values (MAHC A 5.7.3.1.1.2)

Tests conducted with chlorine demand free water with 1 ppm chlorine at pH 7.5, 77° F, no CYA

These values will be higher in the presence of CYA

Organism Time

E. coli O157:H7 Bacterium <1 minute

Hepatitis A Virus About 16 minutes

Giardia Protozoan About 45 minutes

Cryptosporidium Protozoan About 15,300 minutes (10.6 days)

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Effect of CYA on Chlorine Kill Rates

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250 300 350 400

CT,

pp

m m

in

CYA, ppm

Anderson 1965 S. faecalis

Fitzgerald 1967 S. faecalis

Golaszewski 1994 P. aeruginosa

Robinton 1967 E. coli

Robinton 1967 S. faecalis

Robinton 1967 Staph. aureus

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Effect of CYA on Chlorine Kill CT with CYA / CT without CYA

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250 300 350 400

CT

wit

h C

YA /

CT

wit

ho

ut

CYA

CYA, ppm

Anderson 1965 S. faecalis

Fitzgerald 1967 S. faecalis

Golaszewski 1994 P. aeruginosa

Robinton 1967 E. coli

Robinton 1967 S. faecalis

Robinton 1967 Staph. aureus

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Chloramine Comparison Values from EPA LT1ESWTR Disinfection Profiling and Benchmarking, 2003 EPA 816-R-03-004, pH 7-9, 25 °C, 3-log

Pathogen CT Free Chlorine (FC), ppm min

CT Chloramine (CC), ppm min

CT CC / CT FC

Giardia 45 750 17

Viruses 2 497 249

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Effect of CYA on Chlorine Kill Rates • Fitzgerald 1967

──●── No CYA

−−○−− With CYA

– 0.5 ppm AvCl used

– 1:1 molar AvCl:N = 5:1 ppm (by weight)

– Cyanuric acid does not appear to hinder the activity of combined chlorine

With 0.1 ppm NH3-N, there is enough nitrogen for all of the chlorine to be present as combined chlorine.

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Study Total Pools (stabilized)

Results

Yamashita 1988 19(9) Time (minutes) required for inactivation of poliovirus, ~1 ppm AvCl Unstabilized Stabilized Polio 40 sec >3 min

Yamashita 1990 6(3) Time (minutes) required for inactivation of poliovirus, 1 ppm AvCl Unstabilized Stabilized Polio <1 min >2-5 min

Effect of CYA on Chlorine Kill Rates- In Pool Water

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Effect of CYA on Bacterial Counts in Pools Study Total Pools (stabilized) Results- Percent of Pools that Passed the Criteria

Kowalski 1966 15 (7) 1960 138 (7) 1963

%Pass Unstabilized Stabilized ‘60 Total 82 88 ‘63 Total 90 98 ‘60 e coli 96 98 ‘63 e coli 89 96

Rakestraw 1994 (Pinellas 1992 study)

486(396) %Pass Unstabilized Stabilized <500 HPC 86 91 No T Colif 84 92 No F Colif 90 95 No non Colif 41 32

Favero 1964 12 (3) Low bather load 6 (3)

More Pseudomonas in stabilized pools %Pass Unstabilized Stabilized e. Coli 83 72 Staph 97 80 Total count 64 47

LeGuyader 1988 3749 (1055) %Pass Unstabilized Stabilized No Staph 50 40 No Pseud 97 86 No Colif 100 99

Black 1970 83(28) %Pass Unstabilized Stabilized No Colif 82 64

Yamashita 1990 6(3) %Pass Unstabilized Stabilized No Adenovirus 100 100 No Colif 100 92 Total plate counts 92 50

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Implications for Pool Maintenance- Continuous Treatment • Association of Pool and Spa Professionals (APSP)

– APSP-11 • CYA <100 ppm

• Model Aquatic Health Code (MAHC) – MAHC 5.7.3 Disinfection

• FAC – 1.0 ppm no CYA – 2.0 ppm with CYA

• CYA – <90 ppm, most venues – 0 ppm for spas and therapy pools

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Effect of CYA on Cryptosporidium (pH 7.5, 25°C) (Murphy et al. 2015)

Average FC

conc. (mg/L)

Average CYA

conc. (mg/L)

Average Time 3-log10

inactivation (hr)

Average Estimated 3-log10

CT value (mg·min/L)

21.6 0 8.2 10,500

21.1 8 14.1 17,800

19.1 16 27.5 31,500

40.6 0 5.1 12,400

40.9 9 6.2 15,300

38.3 15 8.4 19,400

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Effect of CYA on Cryptosporidium (pH 7.5, 25°C) (Murphy et al. 2015) Did not get 3-log removal with >16 ppm CYA

Average FC

conc. (mg/L)

Average CYA

conc. (mg/L)

Average time 1-log10

inactivation (hr)

Average Estimated 1-log10

CT value (mg·min/L)

21.6 0 2.7 3,500

21.2 48 61.9 76,500

40.6 0 3.7 4,100

38.5 46 17.2 40,000

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Effect of CYA on Cryptosporidium (pH 7.5, 25°C) (Murphy et al. 2015)

• 100 ppm CYA – 20 ppm AvCl

• 72 hours (3 days) 0.8-log10

• 144 hours (6 days) 1.6-log10

– 40 ppm AvCl • 24 hours 0.8-log10

• 72 hours 1.4-log10

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Implications for Pool Treatment- Remedial Treatment

• MAHC 6.5

– Close pool

– Remove fecal material (no vacuum)

– pH ≤7.5, temperature ≥77°F

– Operating filter while maintaining chlorine

– Test for chlorine multiple places

– Use only non-stabilized chlorine for remediation

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Remedial Treatment

• MAHC 6.5 Remedial treatment

– Use the following CT values for treatment

Contaminant Unstabilized Stabilized

Formed stool 50 ppm min (2 ppm 25 min)

100 ppm min (4 ppm 25 min)

Diarrheal stool 15,300 ppm min (20 ppm 12.75 hours)

Lower CYA to ≤15 ppm, and 20 ppm for 28 hours 30 ppm for 18 hours 40 ppm for 8.5 hours

Vomit 50 ppm min 100 ppm min

Blood 0 0

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MAHC 6.5 Remedial treatment

• Other options for diarrheal stool

– Unstabilized • Circulate through secondary disinfection system to achieve 1

oocyst/100 ml

– Stabilized • Circulate through secondary disinfection system to achieve 1

oocyst/100 ml, or

• Drain

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Measurement Issues

• Test Methods

– Melamine precipitation

– Test strips

– Most test methods have 100 ppm maximum • Need to dilute if reading is near maximum

• MAHC set 90 ppm maximum CYA limit due to testing issues >100 ppm

• Effect of CYA on ORP

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Cyanuric Acid Melamine Test

• This test is notoriously inaccurate • Melamine precipitation provides insoluble complex • Turbidity measurements prone to time dependence as

well as interference • Test is influenced by lighting conditions • Results can be operator dependent • If result is near top endpoint of method (i.e. >80 ppm),

the sample should be diluted and run again

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Cyanuric Acid

• Interference • Water temperature

• Effect • High temperatures, above 90 °F, can result in readings as

much as 15 ppm low • Low temperatures, below 60 °F, can result in readings that

are 15 ppm high • How you can tell

• Measure water temperature • What to do

• Warm sample to ideal temperature of 75 °F

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CYA precipitation?

• The previous slide would indicate that cold water reading will be higher than warm water readings

• Then why are winter time CYA readings often lower than summer?

• Temperature of water in the pool vs. temperature of sample when analyzed

– Previous slide has to do with testing interference from temperature of sample when analyzed

– Low CYA readings in winter may not be test interference, they may indicate CYA precipitation at low temperatures in the pool (anecdotal evidence)

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Cyanuric Acid Strips

• Interference • pH

• Effect • Inaccurate results

• How you can tell • Measure pH

• What to do • Adjust pH to the ideal

range of 7.4 to 7.6

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ORP Probes

• Nernst equation can be used to look at theoretical potential vs. CYA concentration

• These values should not be taken as absolute • Many factors will affect an ORP reading and the slope of this line • Nernst equation: E = Eo - (RT/nF) x ln ([Cl-]/[HOCl][H+])

1.12

1.13

1.14

1.15

1.16

1.17

1.18

1.19

0 20 40 60 80 100

Po

ten

tial

, V

CYA, ppm

Constants used: Eo = 1.49 V R = gas constant T = 85 °F n = 2 electrons F = Faraday constant [Cl-] = 100 ppm pH = 7.5 AvCl = 1 ppm TDS = 800 ppm

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ORP Probes

• Interference • Probe fouling from CYA

• Effect • Reading may be low or sluggish to respond

• How you can tell • Clean probe and see if the reading changes

• What to do • Clean probes according to manufacturer’s directions • To prevent contamination, store probes according to

manufacturer’s directions

Two effects from CYA 1. Lowering of ORP due to lowering of HOCl 2. Probe fouling with CYA

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CYA Control- Removal • Drain the pool

– Water restrictions

– Cost (water, treating fill water)

• Activated carbon – Efficiency is low

– Cost

– Possible disposal issues

• Melamine precipitation – Operational issues (staining, solids don’t settle, etc.)

• Unproven technologies

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CYA Removal Costs

Assume

100 ppm CYA in pool

1 lb Trichlor/10,000 gal/day used

Cleveland TN utility rates ($2.21/ft3, ~0.3¢/gal)

Pool size

Trichlor used (lbs/day)

AvCl used (lbs/day)

CYA added (lbs/day)

CYA residual added (ppm/day)

Daily water removal to maintain 100 ppm (gal)

Yearly cost in replacement water ($)

10,000 1.0 0.90 0.56 6.7 665 717

25,000 2.5 2.25 1.39 6.7 1663 1794

50,000 5.0 4.50 2.78 6.7 3326 3587

75,000 7.5 6.75 4.16 6.7 4990 5381

100,000 10.0 9.00 5.55 6.7 6653 7175

1,000,000 100.0 90.00 55.52 6.7 66529 71745

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CYA Control- Prevention

• Control additions of CYA

– Prudent use of CYA

– Prudent use of stabilized sanitizers

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Next Steps

• Enter the debate

• Conference for the Model Aquatic Health Code (CMAHC) for MAHC revisions

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References • Amburgey, J.E., and J.B. Anderson. (2011). Disposable Swim Diaper Retention of Cryptosporidium-sized Particles on

Human Subjects in a Recreational Water Setting. Journal of Water and Health. 9(4): 653-658.

• Amburgey, J.E., Walsh, K.J., Fielding, R.R., and M.J. Arrowood. (2012). Removal of Cryptosporidium and Polystyrene Microspheres from Swimming Pool Water with Sand, Cartridge, and Precoat Filters. Journal of Water and Health. 10(1): 31-42.

• Amburgey, James. E., Jonathan M. Goodman, Olufemi Aborisade, Ping Lu, Caleb L. Peeler, Will H. Shull, Roy R. Fielding, Michael J. Arrowood, Jennifer L. Murphy, and Vincent R. Hill, Are Swimming Pool Filters Really Removing Cryptosporidium?, available from pwtag.org.

• Anderson JR. A study of the influence of cyanuric acid on the bactericidal effectiveness of chlorine. Am J Public Health Nations Health. 1965 Oct;55(10):1629-37.

• Belosevic, FEMS Microbiol Lett. 2001, 204(1) 197-203.

• Black AP, Keirn MA, Smith JJ Jr, Dykes GM Jr, Harlan WE. The disinfection of swimming pool water. II. A field study of the disinfection of public swimming pools, Am J Public Health Nations Health. 1970 Apr; 60(4):740-50.

• Campbell, A.T. et al. 1995. Inactivation of oocysts of Cryptosporidium parvum by Ultraviolet radiation, Water Research, 29(11), 2583.

• Chappell CL, Okhuysen PC, Sterling CR, DuPont HL. Cryptosporidium parvum: intensity of infection and oocyst excretion patterns in healthy volunteers. J Infect Dis 1996;173:232--6.

• Clancy, J.L., Hargy, T. M., Marshall, M. M., Dyksen, J. E. 1997, Inactivation of Cryptosporidium parvum oocysts in water using ultraviolet light, Conference proceedings, AWWA International Symposium on Cryptosporidium and Cryptosporidiosis, Newport Beach, CA.

• Craik, Water Res. 2001, 35(6) 1387-98.

• DuPont HL, Chappell CL, Sterling CR, Okhuysen PC, Rose JB, Jakubowski W. The infectivity of Cryptosporidium parvum in healthy volunteers. N Engl J Med 1995;332:855--9.

• Favero, M. S., C. H. Drake, and G. B. Randall. 1964, Use of staphylococci as indicators of swimming pool pollution. U. S. Public Health Reports, 79:61-70.

• Fitzgerald GP, DerVartanian ME. Factors influencing the effectiveness of swimming pool bactericides. Appl Microbiol. 1967 May;15(3):504-9.

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References • Fitzgerald GP et al. Pseudomonas aeruginosa for the evaluation of swimming pool chlorination and algicides. Appl

Microbiol. 1969 Mar;17(3):415-21.

• Gerba, C.P. Assessment of enteric pathogen shedding by bathers during recreational activity and its impact on water quality, Quantitative Microbiology, 2000, 2, 55-68.

• Golaszewski G et al. The kinetics of the action of chloroisocyanurates on three bacteria: Pseudomonas aeruginosa, Streptococcus faecalis, and Staphylococcus aureus. Water Research 1994;28(1): 207-217.

• Goodgame RW et al. Intensity of infection in AIDS-associated cryptosporidiosis. J Infect Dis. 1993 Mar;167(3):704-9.

• Hijnen, Water Res. 2006, 40(1) 3-22.

• Hlavsa MC et al., 2014, MMWR 63(1), 6-10.

• Jokipii L, Jokipii AMM. Timing of symptoms and oocyst excretion in human cryptosporidiosis. N Engl J Med 1986;315:1643--7.

• Keuten, M.G.A., Schets, F.M., Schijven, J.F., Verberk, J.Q.J.C., Van Dijk, J.C., Definition and quantification of initial anthropogenic pollutant release in swimming pools, Water Research, 2012, 46, 3682-3692.

• Korich DG et al. Effects of ozone, chlorine dioxide, chlorine, and monochloramine on Cryptosporidium parvum oocyst viability. Appl Environ Microbiol. 1990 May;56(5):1423-8.

• Kowalski, X., Hilton, T. B., Comparison of chlorinated cyanurates with other chlorine disinfectants, Public Health Reports, 1966, 81(3), 282-288.

• LeGuyader, M., Grateloup, I., Relative importance of different bacteriological parameters in swimming pool water treated by hypochlorite or chloroisocyanurates, Journal Francais d’Hydrologie, 1988, 19, Fasc 2, 241-250.

• Linden, Water Sci. Tech. 2001, 43(12) 171-4.

• Lu, Ping. (2012). Enhanced removal of cryptosporidium parvum oocysts and cryptosporidium-sized microspheres from recreational water through filtration, Doctoral Dissertation. University of North Carolina at Charlotte.

• Murphy, J. L., Arrowood, M.J., Lu, X., Hlavsa, M.C., Beach, M.J., Hill, V.R., Effect of Cyanuric Acid on the Inactivation of Cryptosporidium parvum under Hyperchlorination Conditions, Environmental Science and Technology, 2015, 49(12), 7348-7355

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References • O’Brien, J. E., Hydrolytic and ionization equilibria of chlorinated isocyanurate in water, Ph.D. Dissertation, Cambridge, MA: Harvard

University, 1972.

• O’Brien, J.E., Morris, J.C., Butler, J.N., Equilibria in aqueous solutions of chlorinated isocyanurate, Chapter 14 in Chemistry of Water Supply, Treatment, and Distribution, Alan J. Rubin editor, Ann Arbor Science, Ann Arbor MI, 1974, ISBN 0-250-4036-7.

• Okhuysen PC, Chappell CL, Crabb JH, Sterling CR, DuPont HL. Virulence of three distinct Cryptosporidium parvum isolates for healthy adults. J Infect Dis 1999;180:1275--81.

• Peeters, Appl. Environ. Microbiol.1989; 55(6): 1519-1522.

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