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Key Takeaways:

  • Cyanuric Acid (CYA), aka stabilizer or conditioner, significantly reduces the active chlorine (hypochlorous acid) concentration by orders of magnitude.
  • Fortunately, it takes very low levels of active chlorine to kill most pathogens quickly.
  • CYA does not just protect chlorine from breakdown from the UV in sunlight, but significantly moderates chlorine's strength.
  • Higher active chlorine levels are associated with faster reactions rates with chlorine including those producing disinfection by-products, oxidizing skin, hair, swimsuits and bather waste.
  • Most pools without CYA (such as most indoor pools) have at least 10 times the active chlorine level as pools with CYA.

 

The chemical relationship between chlorine (specifically, hypochlorous acid) and Cyanuric Acid (CYA) has been known definitively since at least 1974 as described in this paper.  When CYA is present at the usual concentrations found in pools, most of the chlorine is bound to CYA in a collection of chemical compounds known as chlorinated cyanurates.  With CYA, the hypochlorous acid concentration is lowered by orders-of-magnitude.  A rough rule-of-thumb at typical pool pH is that the equivalent FC with no CYA is roughly equal to the FC/CYA ratio in a pool with CYA.  So a pool with 30 ppm CYA has the hypochlorous acid concentration lowered by roughly a factor of 30 compared to a pool with the same Free Chlorine (FC) level but no CYA.

 

Fortunately, it takes a very low level of hypochlorous acid to kill most planktonic (free-floating) bacteria as well as viruses quickly and it takes a higher, but still relatively low, level to prevent algae growth.  In pools using stabilized chlorine (Trichlor, Dichlor), the CYA level can build up quickly over months and if the FC level is not proportionately raised as the CYA level climbs, then algae can start to grow faster than chlorine can kill it.  At first, this may appear as an unusually high chlorine demand before the water starts to turn dull or cloudy and eventually the green algae becoming visible.  A rough rule-of-thumb is that for manually dosed pools the minimum FC is around 7.5% of the CYA level while for saltwater chlorine generator (SWG) pools, the minimum FC is around 5% of the CYA level.  This is sufficient to prevent algae growth even when there are high nutrient levels of 3000+ ppb phosphates, for example, with a failure rate of less than 1 in 1000 (perhaps 1 in 5000) pools based on tens of thousands of pool owner reports at The PoolForum and Trouble Free Pool as well as service companies each working with thousands of pools.

 

There is no real controversy about the effects of CYA notwithstanding what is said by some with a financial interest in selling more stabilized chlorine products.  The science is clear and the effects in real pools are consistent with the science.  Having a low FC/CYA ratio has a lower disinfection, oxidation, and algae-prevention capability.  Whether this makes a difference in a particular pool depends on other factors such as temperature, sunlight and nutrient levels or the presence of other disinfectants, algaecides or oxidizers.

 

The obscuring of the chlorine/CYA relationship has essentially resulted in the over-chlorination of most indoor pools in the U.S. since CYA is not currently recommended for such pools and the FC level is not allowed to be below 1 ppm.  A pool with 1-2 ppm FC and no CYA has roughly 10-20 times the hypochlorous acid concentration as a pool with 3-4 ppm FC and 30 ppm CYA.  I believe this higher active chlorine level in pools without CYA results in faster oxidation of swimsuits, skin and hair and in a faster creation of disinfection by-products though the latter effect is more complicated by a build-up of precursors that must be removed through other means.  Yes, the oxidation rates and ORP levels are usually much higher without CYA, but I would argue that they are too high and that supplemental non-chlorine oxidation should be used in order to reduce the side effects I noted.

 

In Europe, some countries are following the German DIN 19643 standard that sets a lower target of 0.3-0.6 ppm FC with no CYA when no ozone is used or 0.2-0.5 ppm FC with no CYA when ozone is used, but it is difficult to consistently maintain a low 0.2 ppm FC chlorine level throughout the pool to meet localized demand.  CYA is a hypochlorous acid buffer so can be used to easily achieve 0.2 ppm FC equivalent by having the FC at around 20% of the CYA level.  So 4 ppm FC with 20 ppm CYA has roughly the same hypochlorous acid concentration as 0.2 ppm FC with no CYA, but provides a large 4 ppm FC buffer of chlorine to meet localized demand.  This approach can't be used with 19643 due to the chlorine stripping via activated carbon in that system, but it could be used in the U.S.

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Replies to This Discussion

There is no evidence that the chlorine bound to CYA in a collection of chemicals called chlorinated cyanurates has significant disinfection or oxidation capability.  In this paper, the chlorinated cyanurates are shown to oxidize monochlorodimedone at a rate around 100 times slower than that of hypochorous acid so there is some effect for oxidation.  For example, at 50 ppm CYA this means that the chlorine bound to CYA may oxidize at about the same rate as the hypochlorous acid so overall net reduction is closer to a factor of 25; at 100 ppm CYA it may oxidize at about double the rate of the hypochlorous acid so overall net reduction is closer to a factor of 30.

 

The effect on disinfection rates is less clear, but unlikely to have this same chlorine bound to CYA oxidation effect when the killing is done inside cells as opposed to being limited by cell penetration (such as with protozoan oocysts or slower-to-kill bacteria with more chlorine-resistant cell walls such as Staphylococcus aureus).  The reason is that hypochlorous acid is a neutral molecule that looks a lot like water so more readily penetrates cells while the dominant species of chlorine bound to CYA is a negatively charged larger ion less likely to penetrate cells (the negative charge is also what makes hypochlorite ion a slower disinfectant).  The "Chlorine/CYA Relationship" section of this post gives many references to scientific peer-reviewed papers that show this relationship in kill times for bacteria, viruses, protozoan oocysts, oxidation and ORP.  The one paper on algae does not show the relationship, but I believe that paper is flawed, perhaps having ammonia-like contaminants in the algae growth medium that would have any chlorine form monochloramine that would still kill algae and not be affected by CYA level.

 

One can see the relative amounts of hypochlorous acid, hypochlorite ion, and chlorine bound to CYA in the graphs in this post that also shows the effect of pH.  The traditional HOCl/OCl- graph is often shown in courses, but the graph on the right which occurs when CYA is present is not shown.

I'm going to excerpt some tables I made as suggestions for improving the CPO Handbook (suggestions are in a Word file or PDF document).  First, the traditional hypochlorous acid / hypochlorite ion (HOCl / OCl-) relationship (at 86ºF):

 

% Active
HOCl

pH

% Less
Active OCl-

96

6.0

4

90

6.5

10

73

7.0

27

63

7.2

37

47

7.5

53

30

7.8

70

22

8.0

78

8

8.5

92

3

9.0

97

 

Now for what really happens when Cyanuric Acid (CYA) is present where you can see that most of the chlorine is bound to CYA and that the effect of pH on hypochlorous acid concentration isn't as dramatic when CYA is present (3 ppm FC with 30 ppm CYA at 77ºF):

 

% Active
HOCl

pH

% Less
Active OCl-

% Less Active
CyA-Cl

6.5

6.0

0.2

93.3

3.3

6.5

0.3

96.4

1.9

7.0

0.6

97.5

1.6

7.2

0.8

97.5

1.4

7.5

1.4

97.2

1.3

7.8

2.6

96.1

1.2

8.0

3.9

94.9

1.0

8.5

10.7

88.3

0.8

9.0

25.5

73.7

 

The following table shows the active chlorine level vs. the FC/CYA ratio so you can see why that is a reasonable proxy (at a pH near 7.5, the HOCl level is half the FC level when there is no CYA, hence the FC/CYA ratio is a rough approximation to the FC level with no CYA):

 

FC ppm

CyA ppm

FC as %
of CyA

Active
HOCl ppm

4

20

20

0.098

8

40

20

0.101

3

30

10

0.042

6

60

10

0.043

9

90

10

0.043

1.5

30

5

0.020

3

60

5

0.020

5

100

5

0.020

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