Cyanuric Acid Stabilizer What is all the fuss about?

Cyanuric Acid Stabilizer What is all the fuss about? Ellen Meyer, Arch Chemicals February 9, 2017 NPC Conference New Orleans LA 1

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 2

Cyanuric Acid Double headed arrow ( ) means reaction can go back and forth Cyanuric Acid Enol tautomer Isocyanuric Acid Keto tautomer 3

Chlorination of Cyanuric Acid + 3 HOCl + H 2 O Isocyanuric Acid Hypochlorous Acid Trichloroisocyanuric Acid 4

Cyanuric Acid Equilibria (O Brien) Cl 3 Cy HCl 2 Cy Cl 2 Cy - H 2 ClCy HClCy - ClCy 2- H 3 Cy H 2 Cy - HCy 2- Cy 3-5

% Species Cyanuric Acid Equilibria with H + (Using O Brien measurements) 100 80 60 40 H 3 Cy pka 6.88 H 2 Cy - pka 11.40 HCy -2 pka 13.5 Cy -3 20 0 0 2 4 6 8 10 12 14 ph 6

%Species Cyanuric Acid Equilibria with Cl (Using O Brien values) 1 ppm AvCl, 20 ppm CYA, ph 7.5, 800 ppm TDS, 85 F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% [H2ClCy] [HClCy ] [ClCy2 ] [HCl2Cy] [Cl2Cy ] [Cl3Cy] [HOCl] [OCl-] 0% 0 2 4 6 8 10 12 14 ph AvCl = Available Chlorine, TDS = Total Dissolved Solids 7

%Species Cyanuric Acid Equilibria with Cl (O Brien) 1 ppm AvCl, 20 ppm CYA, ph 7.5, 800 ppm TDS, 85 F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% [H2ClCy] [HClCy ] [ClCy2 ] [HCl2Cy] [Cl2Cy ] [Cl3Cy] [HOCl] [OCl-] 0% 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 ph 8

So how does this impact pool operation? How much does CYA build up over time? 9

Trichloroisocyanuric acid Molecular weight = 232.41 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% 10

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 11

CYA (ppm) Cyanuric Acid Accumulation Rate Model When using Trichloroisocyanuric acid as Primary Sanitizer 500 400 5 ppm/day 10 ppm/day 300 200 100 0 0 7 14 21 28 35 42 49 56 63 70 77 84 Days 12

How does CYA impact water balance? 13

Water Balance- ph ph = -log [H + ] H 2 O H + + OH - Minimum ph 7.2, Maximum ph 7.8 MAHC 5.7.3.4.1, APSP-11 7.1 14

ph ph 7 = neutral [H + ] = [OH - ] ph <7.2 ph >7.8 Corrosion of plaster, grout and metal Eye irritation Scale, mineral precipitation Eye irritation Chlorine less effective 15

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 NaHSO 4 H + + Na+ + SO 4 2- Carbon dioxide (CO 2 ) CO 2 + H 2 O HCO 3 - + H + To raise ph Bases take away H + to raise ph Soda ash = sodium carbonate (Na 2 CO 3 ) Na 2 CO 3 + H + 2Na + + HCO 3 - HCO 3 - = bicarbonate 16

ph Lowering ph by adding CO 2 CO 2 + H 2 O => HCO - 3 + H + Raising ph by losing CO 2 to the air HCO - 3 + H + => CO 2 + H 2 O ph will drift up when carbonate alkalinity is present Faster in spas High temperatures Aeration of the water 17

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 HCO 3 - + H + H 2 CO 3 When acid is added HCO 3 - + H + H 2 CO 3 When base is added H 2 CO 3 HCO 3 - + H + 18

% Species Carbonate Alkalinity 100 80 Buffers best at ph where two lines cross 60 40 Carbonic Acid H 2 CO 3 Bicarbonate HCO 3 - Carbonate CO 3 2-20 0 0 2 4 6 8 10 12 14 ph 19

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 + 20

% Species Cyanurate Alkalinity 100 80 60 40 Cyanuric acid H 3 Cy Cyanurate H 2 Cy - 20 0 0 2 4 6 8 10 12 14 ph 21

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 22

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 23

Alkalinity Adjusting Alkalinity To lower carbonate alkalinity Muriatic acid (hydrochloric acid, HCl (aq) ) Dry acid (sodium bisulfate, NaHSO 4 ) HCO - 3 + H + CO 2 + H 2 O Other acidic pool chemicals (trichlor, chlorine gas) To raise carbonate alkalinity Sodium bicarbonate (NaHCO 3 ) Soda ash (sodium carbonate, Na 2 CO 3 ) Will raise ph too Other pool chemicals (calcium hypochlorite) 24

How does CYA impact chlorine chemistry? 25

Cyanuric Acid vs. Percentage Free Chlorine Remaining After One Hour CYA, ppm %Loss 0 35% 10 12% 20 5% 30 3% 40 2% Stabiliser (Cyanurate) Use in Outdoor Swimming Pools http://www.health.nsw.gov.au/environment/factsheets/pages/stabiliser-cyanurate.aspx mg/l = ppm 26

Percent HOCl HOCl as a function of ph HOCl OCl - + H + HOCl is the primary active sanitizer in chlorine pools 100 80 60 40 ph %HOCl 5.0 99.7% 7.0 77.5% 7.5 52.2% 8.0 25.7% 9.5 1.1% 20 0 5 6 7 8 9 10 11 ph Dissociation constant from G. C. White, Handbook of Chlorination, Second Edition, Van Nostrand Reinhold Company, New York, 1986 27

%Species Cyanuric Acid Equilibria with Cl (O Brien) 1 ppm AvCl, 20 ppm CYA, ph 7.5, 800 ppm TDS, 85 F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% [H2ClCy] [HClCy ] [ClCy2 ] [HCl2Cy] [Cl2Cy ] [Cl3Cy] [HOCl] [OCl-] 0% 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 ph 28

%HOCl HOCl- Varying Free Chlorine (FC) and CYA ph 7.5, 85 F, 800 ppm TDS 50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 1 ppm FC 2 ppm FC 3 ppm FC 4 ppm FC 10 ppm FC CYA, ppm 0 47% 5 13% 10 7% 20 3% 50 1% %HOCl for 1 ppm FC 0% 0 10 20 30 40 50 CYA, ppm Equilibrium constants from O Brien 1972 29

How does CYA impact chlorine activity? 30

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 31

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 Organism Time E. coli O157:H7 Bacterium <1 minute Hepatitis A Virus Giardia Protozoan Cryptosporidium Protozoan About 16 minutes About 45 minutes About 15,300 minutes (10.6 days) These values will be higher in the presence of CYA 32

CT, ppm min Effect of CYA on Chlorine Kill Rates 9 8 7 6 5 4 Anderson 1965 S. faecalis Fitzgerald 1967 S. faecalis Golaszewski 1994 P. aeruginosa Robinton 1967 E. coli Robinton 1967 S. faecalis Robinton 1967 Staph. aureus 3 2 1 0 0 50 100 150 200 250 300 350 400 CYA, ppm 33

CT with CYA / CT without CYA Effect of CYA on Chlorine Kill CT with CYA / CT without CYA 50 45 40 35 30 25 20 Anderson 1965 S. faecalis Fitzgerald 1967 S. faecalis Golaszewski 1994 P. aeruginosa Robinton 1967 E. coli Robinton 1967 S. faecalis Robinton 1967 Staph. aureus 15 10 5 0 0 50 100 150 200 250 300 350 400 CYA, ppm 34

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 35

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 NH 3 -N, there is enough nitrogen for all of the chlorine to be present as combined chlorine. 36

Study Effect of CYA on Chlorine Kill Rates- In Pool Water 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 37

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 Rakestraw 1994 (Pinellas 1992 study) Favero 1964 12 (3) Low bather load 6 (3) %Pass Unstabilized Stabilized 60 Total 82 88 63 Total 90 98 60 e coli 96 98 63 e coli 89 96 486(396) %Pass Unstabilized Stabilized <500 HPC 86 91 No T Colif 84 92 No F Colif 90 95 No non Colif 41 32 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 38

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 39

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-log 10 inactivation (hr) Average Estimated 3-log 10 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 40

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-log 10 inactivation (hr) Average Estimated 1-log 10 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 41

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-log 10 144 hours (6 days) 1.6-log 10 40 ppm AvCl 24 hours 0.8-log 10 72 hours 1.4-log 10 42

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 43

Remedial Treatment MAHC 6.5 Remedial treatment Use the following CT values for treatment Contaminant Unstabilized Stabilized Formed stool Diarrheal stool 50 ppm min (2 ppm 25 min) 15,300 ppm min (20 ppm 12.75 hours) 100 ppm min (4 ppm 25 min) Vomit 50 ppm min 100 ppm min Blood 0 0 Lower CYA to 15 ppm, and 20 ppm for 28 hours 30 ppm for 18 hours 40 ppm for 8.5 hours 44

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 45

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 46

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 47

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 48

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) 49

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 50

Potential, V 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 + ]) 1.19 1.18 1.17 1.16 1.15 1.14 1.13 1.12 0 20 40 60 80 100 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 51

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 52

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 53

CYA Removal Costs Assume 100 ppm CYA in pool 1 lb Trichlor/10,000 gal/day used Cleveland TN utility rates ($2.21/ft 3, ~0.3 /gal) Trichlor used (lbs/day) CYA residual added (ppm/day) Daily water removal to maintain 100 ppm (gal) Yearly cost in replacement water ($) AvCl used CYA added Pool size (lbs/day) (lbs/day) 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 54

CYA Control- Prevention Control additions of CYA Prudent use of CYA Prudent use of stabilized sanitizers 55

Next Steps Enter the debate Conference for the Model Aquatic Health Code (CMAHC) for MAHC revisions 56

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. 57

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 58

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. Rakestraw, L. F., Nelson, G. D., Flanery, D. M., Pabst, M., Gregos, E., Plumridge, A. M., Vattimo, R. M., A Comprehensive Study on The Microbicidal Properties of Stabilized and Unstabilized Chlorine and The Relationships of Other Chemical and Physical Variables in Public Swimming Pools; A Report of A Study Carried Out in Pinellas County, Florida, Summer/Fall, 1992, published November 1, 1994, available from Occidental Chemical Corporation. Robinton ED et al. An evaluation of the inhibitory influence of cyanuric acid upon swimming pool disinfection. Am J Public Health. 1967 Feb;57(2):301-10. Shields JM, et al. The effect of cyanuric acid on the disinfection rate of Cryptosporidium parvum in 20-ppm free chlorine. J Water Health. 2009 Mar;7(1):109-14. Shields JM, Hill VR, Arrowood MJ, Beach MJ. Inactivation of Cryptosporidium parvum under chlorinated recreational water conditions. J Water Health 2008;6(4):513 20. Warren IC et al. Swimming pool disinfection. Investigations on behalf of the Department of the Environment into the practice of disinfection of swimming pools during 1972 to 1975. Water Research Centre, Henly-on-Thames, England, 35 pp., Oct 1978. Yamashita, T., Sakae, K., Ishihara, Y., Inoue, H., and Isomura, S. 1985. Influence of cyanuric acid on viricidal effect of chlorine and the comparative study in actual swimming pool waters. Kansenshogaku Zasshi, March 3, 1988, 62(3), 200-205. Yamashita, Teruo; Sakae, Kenji; Ishihara, Yuichi; and Isomura, Shin, Microbiological and Chemical Analyses of Indoor Swimming Pools and Virucidal Effect of Chlorine in These Waters, 1990, Jap. J. Publ. Health, 37, 962-966. Yoder, JS et al. 2012, MMWR 61(SS05); 1-12. Zhou, L., Kassa, H., Tischler, M. L., Xiao, L., (2004) Host-adapted Cryptosporidium spp. In Canada Geese (Branta canadensis, App. Envir. Microbiol., 70(7), 4211-4215. 59

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