why not run SWG pool with NO CYA?

[mgolanlan]

I'm new but my pool is running great:

1.6x5x5 m (40,000 litter)
Jandi Apure 1400 chlorinator (560gram Cl/24 hrs, or about 0.7 FC in one hour)
Salt 3500
TA 90
Ph 7.6
CYA zero
FC 0.7 at midday (100% Chlorination running)
water is sparkling

Having read thru the pool school and details info on Cl/Cya, I'm concluding that my pool is small enough
and my SWG big enough, that I can use this cycle:
1. Run pump for 1 hour at day break, bring FC to 0.7
2. Idle 3 hours, letting FC get destroyed by UV, down to 0.011 minimum safe level (half life 30 minutes)
3. repeat until sundown
4. run pump continuously when pool is being used

Advantages:
1. lower overall FC
2. minimum HOCl at all times (never too low due to CYA eat up all HOCl)
3. very high HOCl (compared to using CYA) at night, ensuring no contaminations/algea can survice

Since this isn't mentioned anywhere, what am I missing?

 

[JasonLion]

Welcome to TFP!

Something like what you describe can be done, but it ends up using way more chlorine and being more expensive than using CYA.

You actually have to run the pump and be constantly adding chlorine the entire time the sun is up for it to work. Chlorine levels below 0.5 don't leave you with enough chlorine reserve in any single spot in the water. Say a leaf falls in some patch of water. The chlorine in that area will all be used up in minutes (at most), and you are left with a patch of water with no chlorine. That is not good.

Running the pump constantly and maintaining a FC level around 0.5 is usually enough to keep up with the local chlorine demand, though some situations may require higher FC levels. With FC maintained at 0.5, you are losing 0.25 every 30 minutes, or 0.5/hour or somewhere around 5 to 8 ppm per day. On the other hand, using CYA you can get your FC usage down below 2 ppm per day.

 

[chem geek]

This link describes the kind of situation you are writing about in terms of not using CYA and having a low FC level instead, though he has an automated dosing system for more continuous control. The minimum chlorine level to prevent algae growth at a pH near 7.5 would be an FC of around 0.05 to 0.08 ppm with no CYA (the 0.01 you are quoting is insufficient to prevent algae growth), but as Jason points out, it's impractical to be able to maintain such a low chlorine level everywhere in the pool without it getting used up locally too easily. CYA not only protects chlorine from breakdown from sunlight, but it also acts as a chlorine buffer, holding chlorine in reserve.

In a very light bather load pool without organic matter getting dumped into it and with a regulated control system, then perhaps this could work out OK. In most pools, it's far easier to use CYA as a chlorine buffer, but of course use an appropriate FC/CYA ratio as well.

Richard

 

[Guest]

You are also overlooking the effect that the cell running at 100% will have on the pH stability of the pool. This thread might be a good starting point for you:
water-balance-tips-for-a-swg-t3663.html

It's not just about the chlorine!

 

[mgolanlan]

Thank you all for the comments.

I should have mentioned that the pool is on a roof at 6th floor some 25m above ground. wood deck all around and no vegetation (zero falling leafs)...

I'm aware of the Ph stability issue (seems like a non-issue, it has been 7.6 stable for over a week).

In regards to chemgeek's Algae and 0.05-0.08 HOCl:

What are my actual algae risk this high above ground?

What happens if the HOCl level drops below 0.05 for an hour 3-5 times a day, but then is 0.5 at night? wouldn't that totally kill any possible algae growth start? From your HOCl/CYA graphs it seems that even the lowest CYA in the water makes fighting Algae with just FC a very boarder line issue, while running without CYA allows even 1.0ppm of FC to have a ton of active HOCl in comparison. You graph (HOcl vs FC at different CYA) seems to imply FC of 8ppm with CYA of 70 (SWG), and this only gives the bare 0.05 minimum to fight algae 24hr/days. I'm aware of the 4.5% recommendation for SWG due to the super-chlorination at the filter, but it still seems better to me to run no CYA and have lots of extra HOCl at night.

This is of course assuming that 0.5 ppm of HOCl in the water is less irritant to my skin than 6 FC with 70 CYA and effectively much lower HOCl. Is this correct?

BTW I do intend to run this via home-automation. Where can I get a decent Ph/ORP sensor? (I can work the A/D and software issues, but would rather have rs232 or usb output)

 

[JohnT]

What are my actual algae risk this high above ground?

Blue-Green Algae, the kind that is most common in pools is a unique bacterium, and it can be spread through the air. The green growth seen on houses and roofs is also a cyanobacteria, so your pool will regularly be exposed to spores.

 

[JasonLion]

Even without leaves, the low levels of chlorine in local patches of water can be an issue. Rain water and blown in dust/spores can cause the same effect.

FC around 6 with CYA around 70 will be gentler on your skin than FC around 0.5 and CYA at 0, though the difference will be small and I doubt you will be able to detect it. This will mostly be because of secondary effects. When CYA is around 0, chlorine reacting with organic contaminates forms more of the irritating forms of CC than it does when CYA is higher.

 

[chem geek]

I think a little history is in order. Pools used to be managed with chlorine alone and no CYA -- even outdoor pools before CYA and stabilized chlorine became widely available. They tried to keep relatively low FC levels, usually below 1 ppm, and in fact today the German norm DIN 19643 standard requires FC levels in pools of 0.3 - 0.6 ppm (0.2 - 0.5 ppm if ozone is additionally used) as that correlates with rather rapid disinfection. They require a 4-log reduction in Pseudomonas aeruginosa in 30 seconds which pretty much translates to around 0.2 - 0.3 ppm FC at a pH of 7.5 so that eliminates the possibility of using CYA at such low FC levels (with CYA, it takes an FC of around 20% of the CYA level to achieve this sort of disinfection). However, many commercial/public pools found it impractical to maintain such low FC levels consistently in the pool. It's not just leaves, pollen, etc., but bather load itself -- the ammonia and urea from sweat and urine. In a commercial/public pool with moderate-to-heavy bather load, most of the chlorine is used up by oxidizing bather waste, not by loss from sunlight. This is why such pools rarely see much benefit of having the CYA level be much above 30 ppm.

So today, the minimum FC in most pool standards is 1 ppm and the "recommended" minimum is 2 ppm. This gives an ample buffer of chlorine to not run out locally and therefore continue to provide protection against person-to-person transmission of disease. What is now missing from those standards is an understanding that these higher FC levels in pools without CYA in them (such as indoor pools and most commercial spas) are essentially over-chlorinating the pool being too harsh on swimsuits, skin, hair, equipment, etc. and also more likely to be producing more irritating disinfection by-products (especially nitrogen trichloride). The standard should require at least a small amount of CYA (20 ppm) in all such pools if the FC is going to be 1-2 ppm minimum.

As noted above, most of the problems with low FC pools have to do with higher organic demands and higher bather loads. I think your biggest challenge is going to be in the automation and measurement. To have any hope of maintaining the chlorine level consistently under varying conditions you'll have to have a feedback system such as ORP calibrated to FC or a more direct chlorine measurement system. This seems like a lot of extra expense and hassle to achieve something that is done chemically when using CYA with an appropriate FC/CYA ratio.

Richard

 

[mgolanlan]

Thank you for the hisotry lesson, it is a critical one to understanding the issue. I think I also understand all the variables for and against this zero CYA option. I should also note that the "Linux pool guy" is actually running a new SWG from 1600-2400 and just doesn't worry about residual FC during the hot hours.

CYA has a benefit of a buffer, but it also limits the actual available HOCl severely. For myself, I'd rather swim in ultra-low FC water and disinfect at night with 0.5ppm of HOCl than manage a barely-enough HOCl at all times.

I've not seen anyone suggesting that 0.5ppm of HOCl for 8 hours/day isn't sufficient to kill all algae.

What I dislike about CYA is the overall FC required based on chem geek's table right here on TFP - not less than 10ppm FC for
the recommended SWG 80 CYA (I am aware of the "unexplained" 4.5% rule for SWG)

zero-CYA has the benefit of less chemicals in the water. It also has the benefit that "super chlorination" can be easily achieved overnight every night, by raising FC to 1ppm. This probably kills all algae every nite much better than having FC 6ppm with CYA, and allows the pool to run clean even when HOCl goes below "algae green line" (0.05) during UV radiation


My pool is not used daily. When it is used, it is primarily me for 30 minutes (endlesspool fastlane), and my baby girl for maybe 15 (and she doesn't pee in the water, really). The mother hardly goes in. Visitors are on a weekly basis only.

Management

My SWG produces 0.6ppm/hour - I've tested this and it matches the specifications.
I also solved the differential equation and came up with 0.5ppm if the SWG is on 100% on during UV and I start with zero.
This was actually pretty neat ... but then I just used excel to solve numerically:
Every minute, add 0.01 and reduce total by 2^(1/34)

Starting at FC 0.04, time in minutes
Time FC
10 0.11
20 0.18
30 0.24
40 0.28
50 0.32
60 0.35

Decay from 0.24ppm (not running) 1.5hours to reach 0.04

I believe I will be fine with this setup:

1. run the SWG if anyone is using the pool daytime (override switch over to the automatic timer, with its own 1hour timer)
2. run after sunset for 1.5 hours, reaching 1ppm for the whole night
3. after daybreak, wait 120 minutes, then run the SWG for 30 minutes followed by a 90 minute break
4. past midnite or bath time, run for 90 minutes to reach 1ppm

Schedule (sunrise 0600)
0800, 1000, 1200, 1400, 1600, 1800 each 30-minutes run
midnite: run for 90 minutes reaching 1ppm.
total run 4.5 hours (or just 3ppm/day, so this IS economical)

I will probably adjust to 70% production and run the pump/heater for 1 hour (also getting to 0.24) to allow the heater enough pump time...schedule
0800 1030 1300 1530 1800 each 60-minutes at 70%
midnite: run for 120-minutes to reach 1ppm
total run 7 hours (still just 3ppm/day)

Certainly, this is overkill since UV isn't 100% the whole day, and ORP (anyone has a source?) would allow a fully automated setup, but its a good start.

 

[JasonLion]

It is clear that you have put a lot of thought into this setup. I have a couple of minor comments.

First, the last time I read through, it appeared that "Linux pool guy" actually had some CYA in the water, though he didn't put it there on purpose. Along with that, his explanation how CYA works is quite misleading because he is basing his description on a paper that has a number of mistakes in it.

Second, there is no question you can prevent algae from growing using the approach you have described. However, you can not reliably prevent person to person transmission of germs using the procedures you have described. Regions of pool water will end up chlorine free at various times under your procedures, opening up windows of possible person to person transmission. That may not be significant in the usage pattern you describe, but it could become significant if you have guests over to swim.

Third, higher CYA levels use less total chlorine over a period of time, even though there is more total chlorine in the water. Perhaps you understand that already and are only focused on the amount of chlorine in the water while you are swimming. If so, I am not disagreeing with you. However, when you talk about it, you make it sound like you will be using less total chlorine, and that simply isn't true.

Fourth, you are taking account of chlorine lost to sunlight, but you don't appear to be taking account of chlorine lost to organic load caused by bathers.

And finally, I suggest you consider one additional design criteria. What happens when some part of your system fails, ie what is your backup plan? Since you are carefully tuning things to run right on the edge of minimum chlorine levels, it doesn't take as much of a disturbance, compared to other approaches, to allow the pool to get out of control.

For the long term, it is nice if the pool remains stable even when there is a 36 hour power failure, or the pump loses prime, or something similar prevents one or two days of SWG operation. Depending on small amounts of additional effort in such a situation is reasonable, as long as you have a plan in advance about what that effort will be.


There are any number of ORP sensors and controllers available on the market. Sensors suitable for continuous use tend to run around $200, for example this one. Complete systems with a sensor, water cell, and controller tend to start around $1000+, for example this one. If you start from an ORP sensor you will need a high quality differential amplifier with noise isolation, which isn't exactly difficult but does require some careful attention to detail. There are some much less expensive ORP sensors, but I think you will find that they fail fairly quickly and end up being more trouble in the long run.

 

[chem geek]

What I dislike about CYA is the overall FC required based on chem geek's table right here on TFP - not less than 10ppm FC for
the recommended SWG 80 CYA (I am aware of the "unexplained" 4.5% rule for SWG)

I agree with Jason's comments, but also wanted to comment on your statement I quote above.

Even in my chart, at 80 ppm CYA the minimum FC for manually dosed pools was 5.8 ppm, not 10, and for SWG pools (and possibly other automatic dosing pools) it's 3.6 ppm. Most people just shoot for 4 ppm FC with 80 ppm CYA in an SWG pool and this works out well.

The 4.5% rule is no more unexplained than the 7.5% manually dosed pool min. FC rule. These are just based on observations and were not derived from any theoretical algae kill rate data. That's not published in the same way that CT data is for bacteria, viruses, etc. The original rules were based on Ben's best guess table from his observations and we tweaked that based on further observations with mustard/yellow algae and with SWG pools.

Richard

 

[mgolanlan]

chem geek, I was referring to the actual graph in this thread, not the table you reference:

pool-water-chemistry-t628.html

Jason, thank you for the issues you've raised. Yes, I wasn't clear enough but I do understand that actual FC use is higher this way, as I let more of it burn in the sun. I did email the Linux guy, he insists he is using no CYA and have no problems. And yes, I've compared CYA notes from his site and TBF, he is treating CYA like poison which is a little absurd.

In regards to actual FC in the water, or rather, HOCl, I wonder what causes skin to "smell like pool" when I get out ... I suspect that with CYA essentially supplying HOCl "on demand" it will be worse than what I'm planning. I've done some swims with very low FC and it felt like bathing afterwords, which is what I'm shooting for....

For person to person transmission, my approach is to turn the SWG on as soon as we go up to the roof. Since it is producing 0.6 FC/hr, I'd expect this to be enough, esp. with the water being pumped and new HOCl being dumped in. Same goes to FC lost to organic matter - either it is only me (and it get cleaned overnite) or other people show up and I turn the SWG on.

My backup plan is to run the SWG non-stop, according to the linux guy, without CYA, this clear up a green pool rather quickly because of very high HOCl.

Once again, thank you. I will follow up in a few months as it heats up...

 

[chem geek]

Ahhh. The graph you refer to used a 0.05 ppm HOCl value as a limit for algae growth though I mentioned in the text that it could be anywhere from 0.02 or less to 0.1 or more. This was in the early days before we had more experience. Ben's chart "Min." column was close to a 0.03 ppm HOCl which is roughly where the FC as 7.5% of CYA rule came from. Then we noticed that SWG pools would work at the 0.02 ppm HOCl level which is about 4.5% FC of CYA and prevent green algae. So as time goes on, we learn more and refine our numbers.

You are correct that the skin "smell like pool" is likely to have more to do with the FC level than the HOCl level. The reaction of chlorine with ammonia from your sweat is very fast even when CYA is present so the total capacity of chlorine, the FC level, is probably more relevant for how much smell there will ultimately be -- at least if it's a monochloramine smell. With CYA, the reaction rate for things like swimsuits and skin will likely be slower -- slow enough to not be a problem if you rinse your swimsuit after use. Of course, you are planning on low FC levels with no CYA so won't be that much different in active chlorine than a higher FC with CYA (that is, not orders-of-magnitude different). If your skin is "clean" with minimal sweat, then the lower active chlorine level from using CYA may smell less (that is, less "clean" chlorine bleach-like smell) since the chlorine will be less volatile and as the water evaporates a solid chlorinated cyanurate would likely result.

Richard

 

[mgolanlan]

Richard, thanks for the skin info. I guess that makes the whole thing worthwhile for me.

I've found an interesting ORP Sensor that comes with USB connection, software, and temp compensation.

http://www.fondriest.com/products/nexsens_wq-orp.htm

Seems like a great product compared to poo-computer interface that all are above $1xxx

However, reading the manual, I'm unable to tell if I can fit this into the pumpl line for continuous measurement.
Presumably, since they recommend wet storage it should be ok.

 

[JasonLion]

Yes, that sensor appears to be suitable for continuous duty. You will want an appropriate flow cell and a sealing gland that fits the cell and the sensor.

 

[mgolanlan]

I clueless as to "flow cell and a sealing gland".
I understand I need some "housing" to place the sensor, and to seal it at high pressure.
Do you mean some DIY PVC pipe that is added in the system like the SWG itself?
Can you link to a picture please?

 

[JasonLion]

If you look at this picture, you will see an acrylic block, the flow cell, that the sensors mount to. Water flows through the channels in the block, providing an ideal situation for the sensor. You don't strictly need a flow cell, but it does help you get more stable readings. The alternative is to put a T in one of the pipes and mount the sensor on the non-straight through leg of the T. The T approach can work, but it is subject to problems if the sensor does not protrude into the water flow by the correct amount or the water flow becomes turbulent.

A sealing gland is a threaded nut with a hole in it. The sensor fits through the hole and the nut is screwed into a threading fitting on the flow cell or pipe. Some sensors have threads built in, but the one you linked to does not. The middle sensor in this picture has the kind of sealing gland you will need on it.

 

[mgolanlan]

I keep looking for ORP measurements (before TFP I looked for FC electronic testing, and couldn't find any) and it seems the "serious aquarium" guys have cheaper probes and computers compared to the pool guys.

What do you think of those?

http://www.aquacave.com/SearchResult.as ... goryID=455

A complete ORP controller with probe and a relay output for $200, not bad ...

specs are here - 2 year warrenty seems to include the prob:
http://www.marinedepot.com/ps_AquariumP ... ation.html

Clearly, to get a fully working system I will have to do some serious programming. A good system should store internal data on the average time the SWG is turned on to increase FC, and self-diagnose when things start to drift. Having the SWG turn on for 48 hours because the prob was "stuck" is not acceptable IMO.

 

[JasonLion]

I am not familiar with that brand. The less expensive probes tend to drift, requiring frequent calibration. I don't know if they have that problem or not. Even the higher end probes need to be replaced ever two years. I would be surprised if they can do all that well at that price, but as I said I have no experience with that probe.

 

[Guest]

I can tell you that I have kept marine aquarium since the '70's and with ORP controllers you do get what you pay for and , as you have seen the controllers do come at various price points. One of the main differences is that aquarium controllers are not trying to maintain a sanitizer level in the water but, rather, are just tying to give you an idea of overall water health. Also,you will NEVER find an aquarium with an ORP as high as that needed in a pool.

Also, while I have used ORP controllers in aquariums I find their use in pools to be overkill and do not use one in my own nor do I need to since my water stays so stable (with 70-80 CYA and 50 ppm borates). As far as chlorine smell on the skin...never noticed it, not even after a long soak in my attached hot tub which has the same CYA and borate levels but a higher FC level than my pool since my SWG automation allowed me to set separate levels for each.

The use of ORP electrodes with CYA is problematic but it's gotten better. If you want to spend the money you can get a direct reading chlorine electrode and controller and then it becomes a non issue but at this present time they are extremely expensive.