Polyquat algaecides: Mode of Action

[WorBry]

Of late, I've involved myself (foolishly or not) in discussions relating to the value of polyquat algaecides. Believing that 'a little knowledge is a dangerous thing', I've sought to educate myself more completely on the mechanism(s) by which polyquat agents might inhibit algal growth, or, as it is claimed, actually kill algae.

I can accept that, as strong chelating agents, they might act in some way on the algae cell wall, possibly by interfering with cell wall synthesis or otherwise stressing the cells through disruption of cell wall components, but I have difficulty with the explanations cited in some articles and promotional literature that describe polyquats stripping algae (and bacteria, in some accounts) of their walls, leading the poor little denuded blighters to suffer death by lysis. If they were that effective, why are we still using chlorine?

I've read that polyquats are highly effective against the more resistant forms of Mustard and Black algae. I can see how polyquats might aid in disrupting the resistant outer crust of these varieties and possibly loosen their hold on pool surfaces, but then what? Do they actually play a role in killing the cells or merely make them more vulnerable to chlorine?

I can accept that polyquats might stop green algae from getting a foot-hold by preventing slime bio-film formation, possibly even forming a barrier that prevents algae from adhering to pool surfaces. That seems quite plausible.

I can also accept that polyquats might aid in the clearing of chlorine-killed algae by cross-bridging and clumping cells such they fall out of suspension. I am also aware the interaction with chlorine is complicated by the fact that chlorine is itself rapidly oxidized by polyquat, and that, in turn, polyquats are progressively degraded by chlorine.

What I cannot find however is any credible scientific literature that accurately describes the mode of action of polyquat agents on algae and lends support to any of these oft quoted suppositions.

Can anyone point me to such literature?

 

[chem geek]

Poly [ oxyethylene(dimethyliminio) ethylene(dimethyliminio)ethylene dichloride ], aka POD, is a quaternary ammonium cationic polymer (see the chemical structure in this link). Unlike EDTA, it is not a strong chelating agent. It acts more like a surfactant and its strong positive charge interacts with the exterior negative charge of cell walls disrupting their ion channels causing cell leakage ("membrane-active agents that interacted with the cytoplasmic membrane" is how one journal source puts it). There are literally thousands of journal papers on the killing capability of quaternary ammonium compounds mostly on bacteria since that's of more scientific interest (algae is a nuisance, but not a pathogen). A few of the first papers listed in Scirus are this one, this one (which you don't have to pay for) and this one the latter of which also talks about green algae. Though the "quats" kill many bacteria and algae, they do not do much at all for Pseudomonas bacteria (see this paper, for example). They also kill more slowly than required to pass EPA DIS/TSS-12 at concentrations allowed in pools which are limited for health and irritation reasons (with large margins of safety as the EPA is apt to do, typically 100 -- see the Reregistration Eligibility Decision (RED) for Busan 77 which is the same as Polyquat).

I can tell you from personal experience that Polyquat will not completely prevent algae growth, but it will slow it down. In my own pool 9 years ago, I was using Polyquat but only dosing every other week (one is supposed to dose every week because it breaks down from chlorine and can also get filtered out if water is dirty since it is a weak clarifier) and had algae start to grow faster than chlorine could kill it with 3 ppm FC and 150 ppm CYA. Had I used the Polyquat weekly, then I speculate I could have had the CYA get to perhaps 250 ppm but I don't believe that Polyquat alone would prevent algae growth when algae nutrient levels (phosphates, nitrates) are high.

As you had already read in my post about chlorine degrading Polyquat, it is broken into smaller sub-units that are still at least partially effective. Polyquat does not oxidize chlorine; it is the other way around where chlorine oxidizes Polyquat. If you have a high Polyquat and/or chlorine level, then you will notice this chlorine demand. I'm not sure if eventually the small sub-units are also oxidized by chlorine, but one does need to add the Polyquat weekly to maintain their level in the pool, both to maintain their effectiveness and in measurement (say, by the Taylor K-9065 or K-1582) where Polyquat concentration is usually maintained in the [EDIT] 5-13 ppm [END-EDIT] range.

 

[WorBry]

Thanks so much for that information and the links.

I am a Microbiologist by background, primarily Clinical in practice, although my initial education and early experience did include some environmental Microbiology, including water and soil. In clinical and public health labs also I did a fair amount of routine water testing (mostly potable water, therapeutic hydro-pools etc), including some chemical analysis, but largely concerned with bacteriological and protozoal health and safety standards. Algae rarely came into the picture. So, I guess it is the scientist in me that just has to understand the subject more completely.

Well there's plenty to delve into there. Several of your clarifications have already saved me the embarrassment of making more incorrect statements. I had indeed assumed POD, like EDTA, to be a strong chelating agent, largely based on my experience with the use of EDTA in a laboratory setting. Like I said ' a little knowledge is a dangerous thing'.

Thanks again.

 

[WorBry]

Just came across this very interesting study testing the efficiency of various biocides (including polyquat) for algae control in water cooling systems:

http://www.houghton.com/water/algicides ... index.html

In their test system they used a mixed culture of Chorella vulgaris (green algae) and Phormidium inundatum (black algae).

Look at the results in the last Table 4 in particular, where they tested under different levels of Calcium Hardness. At 0ppm CH, polyquat was inhibitory down to 1ppm. At 250ppm CH, it required 10ppm polyquat for complete inhibition and at 500 ppm CH, polyquat showed no inhibition at all at 10ppm, the highest concentration tested. They put this down to decreased electrostatic association between the polyquat and the algal cell in the presence of 'hardness ions'.

Now that surely has potential significance for swimming pools, where 250-500 ppm is oft quoted as the optimal range for CH.

ChemGeek, you're good at calculating the in-use concentration of pool chemicals. What would be the expected ppm range of a typical polyquat-60 product at recommended maintenance dose levels?

Interesting also that they found bromo-chloro-dimethylhydantoin (Bromine Hydrantoin) to inhibit algae more effectively than chlorine and bromine and that this was not affected by CH.

What's more is that they also tested the impact of polyquat on the performance of a polyacrylate-based scale inhibitor, and found that the calcium carbonate deposition control properties of the polyacrylate-based scale inhibitor was reduced by 50% by the addition of 5 ppm active polyquat. They speculated that this was again due to electrostatic interference.

Again of potential significance for people that might be using polyacrylate-based products to treat scale and excessive calcium hardness in their pools.

 

[WorBry]

ChemGeek, you're good at calculating the in-use concentration of pool chemicals. What would be the expected ppm range of a typical polyquat-60 product at recommended maintenance dose levels?

According to this, maybe in the order of 2-5ppm

http://www.trigon-de.com/data/en_polyquat_dmae60.html

Interesting that they state the product to be highly effective (less than 5ppm) against Chorella vulgaris, the (well, the commonest) green algae species in swimming pools, but note the growth inhibition rate - 14 days at 20oC......although I'm not quite sure what that means - it takes 14 days to inhibit growth or it inhibits for up to 14 days ?? probably the latter.

 

[chem geek]

Look at the results in the last Table 4 in particular, where they tested under different levels of Calcium Hardness. At 0ppm CH, polyquat was inhibitory down to 1ppm. At 250ppm CH, it required 10ppm polyquat for complete inhibition and at 500 ppm CH, polyquat showed no inhibition at all at 10ppm, the highest concentration tested. They put this down to decreased electrostatic association between the polyquat and the algal cell in the presence of 'hardness ions'.

Now that surely has potential significance for swimming pools, where 250-500 ppm is oft quoted as the optimal range for CH.

Thank you for that link to an excellent source on comparative algae inhibition. Note that this is a manufacturer test, not a peer-reviewed paper in a respected journal. Terbuthylazine (TBZ) that they were testing is the same as Purocide 325 that is the title of the paper. So take what is written with a grain of salt.

I hadn't seen the negative effect of CH on Polyquat that didn't affect standard linear quats (ADBAC). That's useful to know, though typical CH levels for plaster pools are around 300 ppm (where they show "slight algae growth" at 250 ppm CH), but for high hardness water areas or where people are keeping a lower TA and therefore higher CH it looks like Polyquat isn't as effective. Note that their test was over a 3 week period.

ChemGeek, you're good at calculating the in-use concentration of pool chemicals. What would be the expected ppm range of a typical polyquat-60 product at recommended maintenance dose levels?

According to this, maybe in the order of 2-5ppm

http://www.trigon-de.com/data/en_polyquat_dmae60.html

This is not the same chemical as Polyquat in spite of "polyquat" in the name. Look at the chemical structure (as well as the full name) and note that it doesn't have the oxygen bridge (-O-) for the polymer and it has a hydroxyl -CH-(-OH) instead of hydrogen -CH₂-. So you can't directly compare it to get ppm.

The minimum growth inhibition rate table that had timeframes of 14 days I believe refers to how long the product continued to inhibit, but of course that will depend on its concentration and conditions (i.e. is chlorine present) and mount of material to control.

At an initial application rate (see this link and this MSDS) of around 9 fluid ounces (approximately 266 ml with density around 1.15 g/ml so 306 grams) per 10,000 gallons, which with 60% weight percentage would be that is 184 grams per 37,854 liters or about 5 mg/L (ppm). The subsequent weekly 3 fluid ounces is to maintain the Polyquat level that is presumed to drop by about 1/3rd from chlorine breakdown and removal by filtration (since it is a mild clarifier). However, the EPA RED (linked to earlier) listed initial dosing of 13-24 fluid ounces which would be 7-13 ppm. The EPA used a water concentration of 9.31 mg/L for it's safety models.

Interesting also that they found bromo-chloro-dimethylhydantoin (Bromine Hydrantoin) to inhibit algae more effectively than chlorine and bromine and that this was not affected by CH.

As usual, the details show some issues in their experiments that need clarification. Note in particular that they wrote "Halogen-based biocides were redosed on a weekly basis during the test." That is, they did not MAINTAIN the chlorine or bromine concentration. So they were not really testing for the minimum inhibitory concentration (MIC) because the biocide would instead react both with the algae and algal nutrients introduced in the inoculant. Chlorine and bromine react with a variety of nitrogenous compounds so all they were really testing was the amount of total halogen demand that had to be met before the leftover biocide could then kill the algae (I'm oversimplifying here since some killing goes on in parallel with oxidation/substitution reactions with non-algae chemicals).

They indicated that it took 0.6 ppm chlorine or bromine to completely inhibit algae, but we know that isn't true from the experience of tens of thousands of pool owners and that the real number is somewhere around 0.06 ppm or somewhat less. The fact that the bromine was more effective at lower concentrations with BCDMH indicates that indeed the issue is the rapid reactivity of the halogen with non-algae chemicals in the nutrient broth that prevented killing the algae itself. DMH is like CYA is to chlorine though we don't know the equilibrium constants so don't know how much of a buffering/moderation effect it has. So the BCDMH was more effective than bromine because it more slowly released bromine letting it not get used up so quickly during the week before more was introduced. It also released both bromine AND chlorine so they really should have used DBDMH that would have had only bromine for a more fair comparison against bromine alone. As you could see from the table, this didn't completely eliminate algae growth, but did slow it down. If they had used CYA in the water, they would have seen a similar effect. However, even with a halogen moderator/buffer, it still doesn't make up for not maintaining the halogen level during the week -- it only slows down the rate of decline in active concentration over the week. When we dose our pools, if we see an initial drop in FC due to some chlorine demand, we add more chlorine to make up for it -- that is, we MAINTAIN the FC level whenever possible (and if the demand is excessive or we see other problems, we shock the pool to get the FC high enough to get ahead of the growth rate so that we kill more quickly).

What's more is that they also tested the impact of polyquat on the performance of a polyacrylate-based scale inhibitor, and found that the calcium carbonate deposition control properties of the polyacrylate-based scale inhibitor was reduced by 50% by the addition of 5 ppm active polyquat. They speculated that this was again due to electrostatic interference.

Again of potential significance for people that might be using polyacrylate-based products to treat scale and excessive calcium hardness in their pools.

This doesn't apply to those on this forum since we recommend HEDP-based metal sequestrants and do not normally recommend scale inhibitors. Instead, we recommend lowering the TA level and maintaining a lower pH level that usually allows one to manage CH of up to 1000 ppm for those in high CH fill water areas.

 

[WorBry]

Thanks again for the clarifications. You sure know your stuff.

So there's polyquats and then there's polyquats. Another education.

Yeah, I probably should know better than to take the results of a manufacturer-supported study as de facto. It's been a while since I was in the heady world of rigorous scientific critique. Management and kids will do that to you. :lol:

I should say also that my interest in part stems from trying to decide what to do with the pool come vacation. Right now I'm hovering between what cocktail of treatments could possibly keep it in a reasonable state (ideally with the pump off) for up to 10 days (late summer) - and - what the heck, close it down, put on the mesh cover and deal with the green on return.

I'd pretty much ruled out the use of polyquats in that mix, believing that they don't really do anything, but this new found information is beginning to turn me around a bit. Since John T thought it better to move this thread to 'The Deep End', I'd better keep it scientific, but out of interest, do you think there would any value in maybe shocking the pool (say 2 days prior), dosing with PolyQuat (maybe up to 'treatment' level) and having a couple of floaters with slow-release Monopersulfate oxidizer (as used in winterizing kits), or would that just be delaying the inevitable?

 

[chem geek]

The best thing to do would be to have a mostly opaque pool cover since that would eliminate most of the chlorine loss and let you shock to a higher level and still have chlorine left by the time you return 10 days later. Another option is to have someone come by and dose with more chlorine -- if you shock, then only one more shock dose after 5 days or so should do the trick. If you were gone only for 1 week, then Trichlor tabs in a floating feeder could work, but 10 days is a stretch for having them last that long (even piling them up in the feeder) and for sure 2 weeks would be too long.

As for using algicide, that would mostly be to prevent algae when the chlorine level went too low. There are several options there. Since you have a vinyl liner pool I presume your CH level is low so Polyquat should work just fine. See this post for winterizing instructions from Buckman Labs, the makers/patent-owners of Polyquat. I would add to that you should check the chlorine level a couple of days after adding the Polyquat and if it's lower than around 20% of the CYA level, raise it as your last step before you leave (I wouldn't go to shock level, which is about double that, since it'll just react even more quickly with the Polyquat). I would bet that between chlorine and Polyquat that one could go 2 weeks without a problem, but longer than that could be an issue since the Polyquat doesn't last.

After the above methods (going down a scale of practicality and reasonableness), are others that have various side effects or extra costs that are likely to be higher. These include phosphate removers, copper-based algicide, and 50 ppm Borates. The Borates don't have side effects, but probably don't completely prevent algae either but would slow down such growth and would be a larger initial one-time cost. Copper, of course, has potential metal staining and greenish blond hair issues. Phosphate removers are expensive (especially if you have high phosphate levels) and won't completely work if you've got organic phosphates in the water since algae can still slowly use those and if the chlorine level gets to zero and bacteria grow, then bacteria can convert organic phosphates to inorganic orthophosphate which algae take up readily.

[EDIT] See this post where shocking with chlorine and using a bunch of Trichlor pucks in a chlorinator worked to keep the pool in good shape for 10 days. [END-EDIT]

 

[WorBry]

Thanks again. I thought you'd be telling me I must be joking.

The best thing to do would be to have a mostly opaque pool cover since that would eliminate most of the chlorine loss and let you shock to a higher level and still have chlorine left by the time you return 10 days later.

Well the mesh cover I use is one of these.
http://www.toilesoleil.com/documents/to ... ernale.pdf
It's supposed to provide 88% vertical shade. Not sure how effective it is in limiting chlorine loss, but I'd put it on anyway to keep the bugs, leaves and seeds out. It's supported by an elastic system that keeps it relatively dry above the water line.

Another option is to have someone come by and dose with more chlorine -- if you shock, then only one more shock dose after 5 days or so should do the trick.

Not really an option. The neighbour shocks his and then crams the skimmer (not just the basket) with pucks (with the pump on timer) so I wouldn't be asking him, as nice as he is.

Since you have a vinyl liner pool I presume your CH level is low so Polyquat should work just fine.

It was at 90ppm (as per tap water source) but in a momentary lapse of sanity I bumped it up to 140ppm. Still, there's time to bring it down a bit with back-washes and maybe a partial drain.

See this post for winterizing instructions from Buckman Labs, the makers/patent-owners of Polyquat. I would add to that you should check the chlorine level a couple of days after adding the Polyquat and if it's lower than around 20% of the CYA level, raise it as your last step before you leave ..... I would bet that between chlorine and Polyquat that one could go 2 weeks without a problem, but longer than that could be an issue since the Polyquat doesn't last.

Sounds hopeful and would seem the best option.

Thanks.

 

[WorBry]

A couple of quick questions about the Buckman 'closing' recommendation:

The pool should be "shocked" with about 5-10 ppm chlorine to destroy any lingering oxidant demand, to remove any residual ammonia or chloramine, etc. After a couple of days, adjust the pH back to 7.2-7.6 and add Polyquat 60 at the maximum rate allowed by the label. This approach allows you to have the maximum benefit from both chemicals......after the addition of Polyquat 60, be sure to keep the circulation pump on for at least 24 hours to make sure that the Polyquat 60 is thoroughly distributed throughout the pool.

The Polyquat I have left over from opening is 40%. The label guarantees it is Poly [(oxyethylene dimethyliminio) ethylene-(dimethyliminio) ethylene dichloride].

Firstly, can one assume that there is no difference between Polyquat 40 and 60, other than the concentration i.e. adding 1.5x the recommended dose for the 40% would be equivalent to that of Polyquat 60?

Secondly, the directions on the Polyquat 40 label state, for treating visible algae, to add the routine maintenance dose (35ml/10,000ml) 24 hours after super-chlorination or else 150ml/10,000L when super-chlorination is not employed.

Should I assume that 'maximum rate' (as per the Buckman procedure) would be the latter, even though super-chlorination is employed? In which case the dose for my (32,000L) pool would be:

150 x 1.5 x 3.2 = 720ml

 

[JasonLion]

Firstly, can one assume that there is no difference between Polyquat 40 and 60, other than the concentration i.e. adding 1.5x the recommended dose for the 40% would be equivalent to that of Polyquat 60?

Yes, the same except for concentration.

Our experience is that polyquat is not particularly helpful when fighting visible algae.

 

[WorBry]

Our experience is that polyquat is not particularly helpful when fighting visible algae.

Appreciate that. My inquiry was only how best to interpret the polyquat (preventative) dosing in the Buckman 'closing' procedure that ChemGeek cited.

But just following on from your comment. What I find strange (objectionable, actually) is that many (if not all) of these Polyquat pool products (both 60% and 40%) give a specified dosage recommendation for 'treating visible algae' when prior chlorine shock treatment is not applied; typically around 5 times higher than the preventative/maintenance dose. This, to my mind, is gross misrepresentation as it leads the consumer to believe that this treatment alone will eradicate, and by implication destroy/kill the algae, without the inconvenience of having to chlorine shock the pool. Of course that has appeal.

As ChemGeek indicated in his first post, while polyquats are capable of killing algae, they do so more slowly than required to pass EPA regulations at concentrations allowed in pools. It is perhaps therefore not surprising therefore that 'minimum algae-cidal concentration' data (i.e. laboratory tested against isolated or reference cultures of the common pool algae) do not appear in the product literature. The few MSDS sheets I have come that actually cite supportive laboratory data give only growth inhibition (i.e. algae-static) rates.

I find it incredulous that the pool industry can be allowed to describe and label these products 'algaecide', let alone promoting the notion that they preventative at one concentration and curative ('destructive') at other. Classic example:

http://www.clubpiscine.ca/search.html?r ... =algaecide

If there is a rationale basis for the higher 'treatment' dosing (other than selling more of the product) I think it is in the hope that the Polyquat will persist longer in the water at an inhibitory level, giving the pool chlorine more opportunity to gradually kill off the algae at normal (non-shock) levels, and possibly aid clearance of the killed algae by virtue of it's clarifying (flocculating) properties, albeit weak - in other words, it serves to limit the algal load. Whether that actually works in practice is another matter....especially when inadequacy of chlorine is most likely the cause of algal outgrowth in the first place.

Anyhow, it is with that hope that I am willing to give ChemGeek's suggestion a try. Unless advised otherwise, I'll assume the 'maximum dose allowed on the label' corresponds to the 'treatment, without shocking, dose'. If it doesn't work, and I come back to a swamp then I'm no worse off - other than $15 worth of Polyquat down the drain.

 

[chem geek]

A few clarifications. The EPA kill standard in DIS/TSS-12 is for bacteria, not for algae. Algae are far harder to kill than most heterotrophic bacteria. Polyquat actually kills bacteria as well, though not quite fast enough to pass DIS/TSS-12. DIS/TSS-12 must be passed to make "kills bacteria" or "disinfectant" claims for use in swimming pools, but is not required for an algaecide. Nevertheless, as seen on the GLB Algimycin™ 600 algaecide product page, it says "destroys and prevents the regrowth of green algae". So there must have been lab results that demonstrated killing algae (though perhaps not at higher CH levels).

I don't think the Polyquat products are misrepresenting themselves. No one here is saying that they don't work at all with existing algae blooms, but that they don't work as well as chlorine, both in speed of eradication and in cost. With a little visible algae, Polyquat may work just fine though is more expensive than shocking with chlorine. However, with a swamp, it would take a very large amount of Polyquat and would be quite a bit more expensive than using chlorine (the bottle instructions don't cover "swamp-like" conditions). The growth inhibition just means that the rate of kill is slow. If the algaecide did not kill at least 50% of the alage faster than their generation rate (doubling in population), then there would be no inhibition. Killing faster than this threshold means that the algae population will get reduced, though it may be somewhat slow.

Note that when Polyquat is used for closing a pool, what makes it work better is that it lasts longer in colder water, mostly because chlorine isn't breaking it down as quickly and the filtration system is off so it isn't getting caught in the filter via its clarifier properties. This isn't the same as going on vacation, but Polyquat at higher doses should last effectively for at least 2 weeks. So I suspect that their closing procedure will work for at least that amount of time even if the chlorine level gets to zero at some point.

The most disconcerting info is what you found regarding higher CH levels preventing Polyquat from working. I've written to Buckman Labs about that and they are looking into it. I must say that they are the best company in terms of honesty and responsiveness of any of the chemical and pools/spa product manufacturers I have worked with. Next would be Dow Chemical regarding MPS. Much lower down is Arch Chemicals and at the bottom is Chemtura (Biolab) who are the primary promoters of the "CYA doesn't matter; only FC matters" mantra. Well, that's not quite true since the bottom would really be some of the "alternative" folks like Ecosmarte and Pristine Blue in terms of some of the ridiculous science claims. See this post for a decoder ring of product brands to the gobbled up chain of companies.

 

[WorBry]

Well, just reporting back on the "mid-summer vacation pool closing routine".

Needs required that we took the 2 weeks vacation a month earlier than first planned. All very rushed and leaving me less than 24 hours to prepare and close the pool.

Dosed the FC to 8.0 the evening before; the water was clean and the CC 0 so I didn't want to hyper chlorinate unnecessarily. First thing in the morning, removed the steps, added the Polyquat 40 and let that circulate for 6 hours. Topped up the FC to 8 again (20% of the CYA as ChemGeek advised) and let that mix for 2 hours while I put on the mesh cover and finally switched off the pump.

Left hoping for the best, but with weather reports of record highs for most of the 2 weeks (unlike the holiday destination where it peed down for the duration!!) I was sure that I would be facing a heaving swamp on return. So I was more than surprised to find the pool water only moderately cloudy and just faintly green. The FC was of course 0.

I'm shocking it right now and at the current rate of progress I reckon it should be ready within a couple of days. Amazing.

I must confess I was quite skeptical, but this exercise has pretty much convinced me that Polyquat does retard algae rather well. Thanks for your advice ChemGeek.

Glad I installed the cover. The day before returning, a heavy hailstorm hit. Only lasted 5 minutes apparently but the hailstones were gigantic and sharp. Had I not put the cover on, the exposed liner could well have been damaged as I had lowered the water level below the skimmer. Amazingly though just a few small holes around the perimeter of the cover where it was raised above the water level. Unfortunately the deck awning was torn to shreds and the cars in the drive-way were badly dented. But the pool survived.....whoopee !

 

[chem geek]

Thanks for reporting back. I'm so sorry to hear about the damage from the rainstorm, but am glad the pool mostly worked out. Generally, the Polyquat is designed to inhibit algae growth, but doesn't completely prevent it if the chlorine level gets to zero, and it only lasts at near full concentration for a week so the second week had lower levels. So it pretty much behaved as expected. Not preventing algae completely, but slowing it down enough so that you had an easier situation to deal with when you got back. Clearly, this wouldn't work for very long trips of 3-4 weeks or more, but sounds fine for 1-2 weeks.

 

[WorBry]

..... I'm so sorry to hear about the damage from the rainstorm,

Yep, the insurance inspectors will be busy this week, not to mention the car body work shops.

 

[WorBry]

BTW, one other thing I did notice is that the CYA was significantly depleted over the course of the 2 weeks.

Before leaving it was definitely 40ppm. On returning, first thing I did was to top up the water level (which I had dropped a few inches before leaving), test the FC (0) and shock. When I came to check the CYA the next morning and again after two days of shock treatment (before any backwash) it was <30ppm, using the normal Taylor test method. Tested again, mixing the sample 50:50 with CYA standard solution, which allows reading down to 10ppm (taylor-cya-test-view-tube-t47329.html) and it gave less than 10ppm.

There is no way that the little water replacement could account for that >30 ppm drop by dilution alone and so one assumes that that the CYA must have been degraded in some way. I've since read that depletion of CYA, under zero FC conditions, is not an uncommon observation and is attributed to degradation by bacteria:

it-can-happen-to-anyone-zero-chlorine-cya-ammonia-t10974.html

degradation-of-cyanuric-acid-cya-t8880.html

Like I said, the pool was not exactly stagnant on my return, but I'm wondering if maybe there was sufficient bacterial growth (at the lower pool level) to degrade the CYA over the two week period. Otherwise, how could it be explained? From what I've read, chlorine also degrades CYA, but far too slowly to suggest that the two days of shock treatment might be responsible.

Is there any known action of Polyquat on CYA?

 

[JasonLion]

Polyquat doesn't interact with CYA, at least not like that.

Did the FC level fall to zero while you were away? If so, bacteria is possible, though not especially likely.

 

[WorBry]

Yes, the FC was 8.0 when I left and 0 (<0.5) ppm on return.

I might add also that the CC on return (before any shock treatment) was 0 (<0.5) ppm. Just thinking, if there had been bacterial/fungal degradation of CYA to yield ammonia, wouldn't that have shown up as some residual CC? Edit: Thinking again - probably not, as there wouldn't have been much, if any, FC around at the point bacteria started growing to react with the ammonia and if there were nitrifying (ammonia-oxidizing) soil/water micro-organisms present in the mix, the ammonia would be mopped up.

Interesting to read also that CYA actually stimulates the growth of some nitrogen-fixing bacteria that are capable of degrading CYA.

Shame I no longer have direct access to a Microbiology lab to see what might have been growing in the water. I suspect Pseudomas sp might be the first to appear - generally higher chlorine and polquat resistance and including species capable of CYA degradation.

 

[chem geek]

The thread you linked to describing the degradation of CYA talks about how different bacteria can create different results. You can end up with ammonia or you can end up with nitrates and nitrogen gas. The former would show up as ammonia in test kits and as CC after you add chlorine while the latter wouldn't show up as CC and wouldn't be a problem. Bacteria can also partially breakdown the CYA which wouldn't show up as ammonia but would still create a chlorine demand -- that's probably what happened to me in the other link you gave. Though I had some ammonia and measured CC, it wasn't enough to explain the full amount of chlorine demand I had.