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Both Sides of the Cyanuric Acid
Stabilizer Debate
Thomas C. Kuechler, Ph.D.
Occidental Chemical Corporation
Presented to the
2015 NEHA Expo
Orlando, FL
July 14, 2015
Background

Chlorinated isocyanurates and cyanuric acid (CYA)
were introduced ~1961.

There has been debate ever since over the effect of
CYA on efficacy. Over 100 papers have been published.

There was considerable debate during development of
the Model Aquatic Health Code on the CYA limit:
50 ppm vs. 100 ppm.

Why CYA? – CYA stabilizes free available chlorine
against degradation by UV
Chlorine Stabilization
in Pool Water Exposed to Sunlight
100
90
100 ppm CYA
Percent AvCl Remaining
80
70
25 ppm CYA
60
 t½ ~ 0.5 hr
50
40
30
20
No CYA
10
0
0
0.5
1
1.5
2
2.5
3
3.5
4
T im e ( h r )
The debate is not about banning CYA from outdoor pools.
Background
↔

Free Cl + CYA
“Bound” Cl (very fast reaction)

Less free Cl means lower efficacy (longer killing time).

Critics have proposed several reasons for limiting CYA
levels, but lower efficacy is the chief reason.
Cyanuric acid is
NOT cyanide

Cyanuric acid has nothing to do with cyanide. It is not
made from cyanide and does not produce cyanide.

CYA has very low toxicity and is not a health hazard.
cyanide
CYA
Alleged reason 1 for limiting CYA

Cyanuric acid interferes with the total alkalinity test.

Not true. CYA contributes to total alkalinity.

To calculate the Langelier Saturation Index, need to use
the bicarbonate alkalinity.

Calculate the bicarbonate alkalinity from the measured
total alkalinity and the CYA concentration:
bicarb alkalinity = total alkalinity – ppm CYA * factor
factor = 0.23 to 0.35 at pH 7.0 – 7.8.
Alleged reason 2 for limiting CYA

CYA interferes with the ability of free available chlorine to
destroy amines.

Chlorinated CYA readily transfers chlorine to amines, which
bind available chlorine much stronger than CYA.

A kinetic study* has shown that chlorinated CYA transfers
available chlorine directly to ammonium ion. Chlorinated
CYA does not have to dissociate into HOCl.

CYA has no effect on the reaction rate of available chlorine
with amines.
* D. Reading, J. Morgan, G. Purser, “Cl atom transfer to ammonium ion from
monochlorocyanuric acid, a common agent in swimming pools”, Abstracts, 229th ACS
National Meeting, San Diego, CA, March 13-17, 2005
Alleged reason 3 for limiting CYA

CYA lowers the oxidation potential of the water making it
impossible to control the chlorine feed rate with ORP.

CYA does lower the ORP reading. The whole ORP curve is
shifted.

An ORP controller can still control the chlorine feed rate.
The ORP set point must be changed for each pool.
Calculated ORP Curves at pH 7.4
850
800
750
ORP (mV)
700
650
600
550
500
at 0 ppm CYA
at 25 ppm CYA
450
at 75 ppm CYA
at 175 ppm CYA
400
0
1
2
3
free chlorine (m g/L)
4
5
Alleged reason 4 for limiting CYA

CYA levels > 100 ppm degrade plaster.

Arch claimed* that high CYA (100-500 ppm) degrades
plaster even if the pool water is properly balanced (pH =
7.2 – 7.8, alkalinity = 60 – 120 mg/L).

Also claimed that CYA levels rapidly dropped over time,
indicating a reaction with the plaster.

Note that the alkalinity “correction” for 500 ppm CYA is
about 150 ppm.
* E. Meyer, “High CYA levels & plaster degradation in swimming pools”, Pool & Spa
Marketing" Jan., 2006.
Alleged reason 4 for limiting CYA

Recent work* refutes these results.

Plaster coupons were submerged for 6 months in well
balanced simulated pool water (pH 7.2 – 7.6, bicarbonate
alkalinity 80 – 100 mg/L) with CYA levels up to 250 ppm.

CYA concentrations were stable throughout the test.

There was no measurable surface deterioration by
microscopic imaging and quantitative surface roughness.

CYA levels up to 250 ppm do not degrade plaster if the
water is properly balanced.
* K. Mitchell, “The truth about CYA”, 26th Annual National Plasterers Council, Phoenix,
Arizona, Feb, 2015.
Test Coupon – 250 ppm CYA
4A Pre-immersion 15X
4A Post-immersion 15X
Effect of CYA on kill times

Cyanuric acid increases the time required to kill various
microorganisms in clean water.

We compare efficacy using “CT”.

CT = concentration (ppm)  time (min) to achieve a given
% kill (90%, 99%, 99.9%) of a given microbe

Lower CT = more effective

Different CT’s for different microbes

CT varies with pH and temperature
Effect of CYA on kill times
CT with CYA
CT no CYA

CT Ratio =
(at the same % kill and
same conditions)

The Total AvCl / Free AvCl ratio can be calculated
based on the known equilibrium constants.
Effect of CYA on kill times

The predicted CT Ratio is NOT equal to the ratio
Total AvCl / Free AvCl for a given microbe when the CT is
not a constant.

Published experimental data on the effect of CYA is
scattered – is this experimental error or non-constant CT’s
for some microbes?

Contamination could affect results – small amounts of
ammonia could give erroneous CT’s since ammonia binds
available chlorine to much greater extent than CYA.
Effect of CYA on kill times

Two ways to use available chlorine in swimming pools:

Shock treatment – bring the pool back under control or after
significant contamination (treat for Crypto after a fecal
incident)

Routine treatment – maintain a residual available chlorine
level to keep the pool under control during use
Shock treatment

Cryptosporidium is quite resistant to chlorine. Normal use
levels of free chlorine do not control Crypto. CT99.9 for
Crypto / free chlorine / pH 7.5 = 10,500 ppm-min* (8.75 h at 20
ppm).

Longer kill times at even 50 ppm CYA means that treating
Crypto with chlorine is not practical. Changing the CYA limit
from 100 to 50 ppm does not help much.

Need to consider more effective disinfectants – chlorine
dioxide, UV or ozone.
*
J. Murphy, M. Arrowood, X. Lu, M. Hlavsa, M. Beach, V. Hill, “Effect of CYA on the inactivation of
Cryptosporidium parvum under hyperchlorination conditions”, Environmental Science & Technology,
2015, 49(12) 7348-7355.
Routine treatment

What is a satisfactory level of control?

The Model Aquatic Health Code does not define any bacteria
standards for swimming pools.

 World Health Organization guidelines for recreational water:
HPC < 200 cfu/mL
E. coli < 1 cfu/100 mL
The chemical treatment standards are somewhat arbitrary.

Pool water does not need to be sterile.
Routine treatment

To maintain control, the kill rate must be faster than the
growth rate.

“generation time” = the average time between consecutive cell
divisions. For most bacteria, generation times are 20 – 60
minutes under optimum growth conditions.

Pool conditions are far from optimum for growth – limited
nutrients, temperature, presence of oxidizers and UV light.

It is not necessary to kill bacteria in seconds in order to keep
the bacteria population under control.

UV or ozone require hours for 99% kill throughout the pool.
Two field trials

Two extensive field trials on the effect of
cyanuric acid.

Pinellas County – 1992

Albany, New York – 1999
HPC (Heterotrophic bacteria) vs. CYA
10000
Pools with Free Cl2 > 0 ppm
H
E
T
E
R
O
T
R
O
P
H
I
C
B
A
C
T 1000
E
R
I
A
100
,
C
F
U
/
m
l
10
Satisfactory
Log Average HPC
1
0
10 - 20
21 - 50
51 - 100
101 - 200
CYANURIC ACID, ppm
201 - 400
401 - 800
% HPC Satisfactory Pools vs. CYA
105%
Pools with Free Cl2 > 0 ppm
%
S
A
T
I
S
F
A
C
T
O
R
Y
100%
95%
P
O 90%
O
L 85%
S
80%
75%
70%
0
10 - 20
21 - 50
51 - 100
101 - 200
CYANURIC ACID, ppm
201 - 400
401 - 800
Two field trials

No variation in the log average HPC with CYA.

The percentage of HPC satisfactory pools or COLI
satisfactory pools does not vary with CYA.

CYA up to 200 ppm did not affect ability to control
the pool.

The CYA concentration did not determine whether a
pool was in control or out of control.

CYA makes it easier to control bacteria and algae by
stabilizing the FAC.
Two field trials

Pools could be divided into 3 groups:
1) under control
2) out of control
3) in transition

The most important parameter for controlling bacteria or
algae is the free available chlorine (FAC) concentration.

FAC should be 1 ppm or more to control bacteria,
regardless of the presence of CYA.

FAC should be 3 ppm or more to control algae,
regardless of the presence of CYA.
Conclusions and comments


Many unstabilized pools maintain  1 ppm of free
available chlorine. Trying to maintain higher levels is
difficult and expensive. But there is little reserve in the
pool.
Maintain the free available chlorine in a stabilized pool at
2 – 4 ppm. This is quite easy to maintain and the usage
rate is still low. The chlorine reserve is then in the pool
water, not in a tank in the pump room.
Calculated Chlorine Usage Rate
(due to decomposition by 6 hr/day of sunlight)
30
t1/2 = 30 min
Usage rate (ppm/day)
25
t1/2 = 6 hr
20
Unstabilized chlorine
15
10
5
Stabilized chlorine
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Free chlorine (mg/L)
3.5
4.0
4.5
Reducing CYA levels

Do not use melamine.

New product – Bio-Active CYA
Reducer

Live bacteria & enzymes which
degrade CYA

Use with normal chlorine levels,
but no algaecide or shock

> 65oF, outdoor pools only

Mixed results so far
Thank you