Small, warm indoor pool - suggestions?

[DavidJW]

Hi all,

I've been lurking for a while, enjoying the common sense approach espoused in this forum. With a background in academic chemistry until my health collapsed in 1993 due to chemical exposure from working in an organic lab with defective fume extraction, I've particularly enjoyed some of the deeper threads here.

It is probably little surprise that with chemical exposure related health problems and an indoor pool, I won't add chemicals to the pool without wearing a 3M respirator with ABEK1 (organic gas, inorganic gas, acid gas and ammonia) filters stacked with P3 particulate filters. Obviously, my interest is really in the inorganic gas, acid gas and particulate protection when working with the pool. I encourage everyone to consider respiratory protection when working with pool chemicals - modern half masks such as the 3M 7500 series are pretty comfortable and inexpensive, though non-Europeans will have different coding for the filters as the ABEKP system is a European standard. The mask really does make a difference - you notice the smell from the recently treated pool when you take the mask off!


I have a small indoor fibreglass pool with a counter-current swim unit - there's some details of the equipment in my signature. This pool is kept very warm (nearly 31^oC - that's 87.5^oF) due to the arthritis of one of the users, also the water volume is very small for a swimming pool at 9000 litres (2400 US gallons). When not in use, the pool is kept covered to keep evaporation under control and to minimise heat loss, which are both especially important due to the eye-watering electricity prices in the UK. Though the pool is heated using supposedly cheap off-peak electricity, it still costs us well over £1000 a year in electricity bills, making an air source heat pump a likely upgrade during 2012. You can see a picture of the installation on the manufacturer's data sheet for the pool shell (click) - ours is the pool pictured at the bottom right, though that picture was taken very shortly after installation when the plaster on the conservatory walls hadn't been painted and the protective tape is still on the plastic skirting board.

Because of the high temperature and small water volume, in some respects the pool is more akin to a spa when it comes to thinking about water chemistry, despite being used for serious exercise. The key differences I believe there are compared to the outdoor pools more commonly discussed at TFP are:

• limited opportunity to outgas because the pool is usually covered[/*:m:2f9ayivs]
• no UV destruction of free chlorine[/*:m:2f9ayivs]
• significant agitation from the counter-current unit (though the pump can turn the entire pool volume over in about 45 minutes - a 0.33HP unit might be a better match to the pool size and save power)[/*:m:2f9ayivs]
• lots of aeration when the air injector is enabled on the counter-current unit[/*:m:2f9ayivs]
• the small volume means that an exercising bather represents a substantial bather load[/*:m:2f9ayivs]

As I have a colour vision defect (I can't visually read phenol red reliably, nor, for that matter, many of the other indicators used in quality test kits) and test 'gadgets' aimed at the residential market tend to be unreliable, I have a commercial colorimeter and associated chemistry from Palintest. The initial 'sticker shock' of such a setup is horrendous, but over time it doesn't work out too bad as the bulk reagents are reasonably priced, the shelf life is long (being tablet based) and you order each reagent individually. This means that I can test free chlorine, total chlorine and pH frequently, whilst only testing the other parameters every few weeks.

The test results below use the Palintest setup.


Pool

FC 1.64 ppm Cl₂
TC 2.10 ppm Cl₂
pH 7.15
Alk 75 ppm CaCO₃
CH 335 ppm CaCO₃
Sulfate 180 ppm SO₄^2-
Salt 4500 ppm NaCl

No bromine products, borates or cyanurates have ever been added to the pool. By way of confirmation, a cyanuric acid test has no visible cloudiness and reads as 1 ppm CYA in the colorimeter (which is merely a small zeroing error).


Fill water

FC 0.13 ppm Cl₂
TC 0.65 ppm Cl₂
pH 7.55
Alk 200 ppm CaCO₃
CH 219 ppm CaCO₃
Sulfate 114 ppm SO₄^2-
Salt - I don't have an up-to-date test result, but it is going to be very low


From this I deduce:

• the pool pH is on the low side at present, as I try not to drop it below 7.2. This time, the pH dropped from 7.35 to 7.0 when using sodium thiosulfate following a recent chlorine shock. Because of the SWCG, the pH will soon drift up to 7.35 or 7.4 where it typically sits[/*:m:2f9ayivs]
• the combined chlorine level of 0.46 ppm is not unusual for a SWCG based pool with limited opportunity to outgas - if the combined chlorine level gets above 0.5 ppm and the persulfate level is almost 0, the pool gets shocked using potassium monopersulfate. The use of a chlorine shock is rare as the free chlorine level takes forever to drop afterwards, necessitating the use of sodium thiosulfate to bring the free chlorine level down. (N.B. this total chlorine test was carried out with Palintest's DPD Oxystop reagent added, so there's no monopersulfate interference)[/*:m:2f9ayivs]
• the total alkalinity is fine as it is[/*:m:2f9ayivs]
• the calcium hardness is getting a little high, but with relatively hard fill water, it is going to be hard to lower the calcium hardness significantly, unless I import fill water or find a way to collect rainwater, add a touch of sodium hypochlorite bleach, and add that to the pool. The pool system is pretty much closed, with splashout and condensate from the dehumidifier all directed back into the pool[/*:m:2f9ayivs]
• the CSI is acceptable (-0.53 according to the pool calculator) - indicating the calcium hardness really isn't an issue unless it climbs much higher[/*:m:2f9ayivs]
• the sulfate level is fine - as the structure of the pool is fibreglass, the sulfate level isn't going to be a huge issue anyway. The only cement based products near the pool are the mixture the coping is bedded down on and the reinforced concrete under the rubber flooring. Again, with the relatively high sulfate level in the fill water, I have limited scope to reduce the current level. I only test the sulfate level infrequently in order to get an idea of how much water I'm turning over, bearing in mind that most of the chemicals I use add sulfate (pH control is with sodium bisulfate as hydrochloric acid isn't easy to get in the UK and the amounts of concentrated hydrochloric acid I would be adding would be tiny in any event, shock is with potassium monopersulfate, chlorine reduction after a chlorine shock is with sodium thiosulfate) and the filter needs very little backwashing[/*:m:2f9ayivs]
• the salt level is fine - though it's getting towards the bottom of the acceptable range for the current SWCG unit (4000-7000 ppm NaCl), the cell is leaking and the entire SWCG installation is likely to be replaced with a more modern unit that needs a lower chloride level (and uses polarity reversal so it won't require acid descaling several times a year), so I don't want to add salt to increase the level only to struggle to dilute it back down in a few months[/*:m:2f9ayivs]

So, in many ways, all is well with the pool.


However, I would appreciate people's views on:

1. Is there anything I have written so far that appears to be wrong?[/*:m:2f9ayivs]
2. Is a free chlorine level of ~1.75 ppm Cl₂ really acceptable, bearing in mind that the pool, if unused, loses less than 0.1 ppm in 18 hours and I have never seen it lose more than 1.00 ppm on the heaviest used normal days? My fear is that on exceptional days and/or in some places within the water volume, the free chlorine level is dropping too low for safety. Should I target a slightly higher figure instead, such as 2.50 ppm?[/*:m:2f9ayivs]
3. Bearing in mind the small water volume and consequent heavy bather load whenever the pool is used, is it worth considering an ORP controlled SWCG when I go shopping for the replacement unit? I know these units have significant drawbacks - keeping the ORP probe in good order may well turn out to be an expensive nuisance and potassium monopersulfate shock has to be used with extreme caution in an ORP controlled setup as it raises ORP but is not a sanitiser. However, there is the possibility of minimising the swings in free chlorine level using an ORP controlled SWCG unit. (If only someone made an amperometric controlled SWCG, but I guess the expense would be prohibitive).[/*:m:2f9ayivs]
4. Is it worth considering a small cyanurate level, say 20 ppm of cyanuric acid? It is very unusual to use cyanurates in an indoor pool in Europe, but it may be an answer to my concerns about free chlorine dropping too low.[/*:m:2f9ayivs]
5. Does anyone want to suggest target values for my pool, bearing in mind the given pool and fill water test results. I am aware of the usual TFP values, but am unsure whether they are directly applicable to my small indoor pool.[/*:m:2f9ayivs]

The only chemicals I use are:

• salt - for the SWCG[/*:m:2f9ayivs]
• sodium bisulfate - to reduce pH when necessary[/*:m:2f9ayivs]
• sodium bicarbonate - to increase total alkalinity when necessary[/*:m:2f9ayivs]
• potassium monopersulfate - non-chlorine shock[/*:m:2f9ayivs]
• sodium thiosulfate - to reduce free chlorine levels following on the rare occasions that I use a chlorine shock[/*:m:2f9ayivs]
• United Chemical's Pool Stain Treat (in other words, oxalic acid) - control of metal staining[/*:m:2f9ayivs]

I may well switch to ascorbic acid once I've used up the oxalic acid. Overall, my strategy is to add as few chemicals as possible to the pool, and, with the exception of the Pool Stain Treat, I avoid proprietary products.

If anyone has any advice that does not directly relate to the questions I've asked, I'm listening!


My apologies for such a long post. I felt it was more useful to explain where I was and my questions in one structured post rather than a long meandering thread.


With best wishes,



David

 

[chem geek]

Welcome to TFP!

Because your pool is covered, you'll tend to build up some of the volatile disinfection by-products, especially chloroform. My pool is covered except for 1-2 hours every day and even though it is outside, that tends to have chloroform (a trihalomethane, THM) build up unless I aerate the pool. Fortunately, the bromide level is very low in the fill water so the much worse brominated THMs (which are really the ones to worry about) are kept low. You might take a look at the bromide level in your fill water from a water quality report.

The higher combined chlorine level is most likely chlorourea since there is no UV in sunlight to help control it (i.e. no hydroxyl free radicals from breakdown of chlorine in sunlight). The non-chlorine shock (MPS) is only marginally effective at keeping the CC down, but it is better than nothing. Far better would be a UV system and that's the most common approach taken for indoor pools (not counting the DIN 19643 approach in Europe that uses granulated carbon and sometimes ozone). If you were going to spend money on equipment, I'd get a UV system over an ORP controller.

The high active chlorine level in the pool, due to not having any CYA in the water, will make it harsher on swimsuits, skin and hair and will tend to produce more nitrogen trichloride. If you used 4 ppm FC with 20 ppm CYA, then this would be roughly equivalent to 0.2 ppm FC with no CYA. This will be less harsh. You may find that chlorine loss rate will be somewhat higher with no bather load due to a slow oxidation of CYA by chlorine, but it won't be very much -- perhaps up to 0.2 ppm FC per day at the most. With the higher FC level, you'll have a much bigger buffer of chlorine so far less likely to run out when there is bather load.

 

[gsilloway]

I would second chem geek's suggestion to look into using a non-chemical secondary sanitizer, especially since there is abundant research that shows the disinfection byproducts of chlorine in an indoor pool can cause health problems (like asthma). In addition to the UV he mentions, look into an ozone system, recognizing that either of these alternatives only reduces, but does not eliminate, the need for chlorine or some other residual sanitizer. You should be able to maintain .5 to 1 ppm FAC with one of these approaches.

 

[PoolGuyNJ]

Once biggie I didn't see mentioned was your ventilation system. Gassed off DBPs need to be vented at regular, periodic intervals, every day.

Does the current generator have a filter system?

That is a high salt level requirement.

You have a typo for the Alk. You listed calcium Carbonate when you meant Sodium BiCarb.

You might want to consider raising the pH. Your eyes will like that. a 50 ppm level of Borates can help lock it in. Using Borax with get you there inexpensively.

Don't sweat the calcium level. That's really only for cementacious pools.

With as low a pool volume as you have, changing the water periodically is not a bad idea.

Rather than a medium pressure UV chamber based system, a few UV bulbs in the room's light fixtures might do a better job of dealing with the CCs in the water. Combined with good ventilation, airborn DBPs will be minimal.

There are some significant concerns that have recently been discussed on some other forums, specifically on Linked In, I don't think it was open to the general public and i can't find the discussion again (sigh). When I find them, I will post the relevant concerns.

FWIW, chem geek was mentioned a couple times there so I wouldn't be surprised if he knows what and where the discussion is.

Scott

 

[chem geek]

I think it was the Swimming pool water treatment subgroup of Dryden Aqua Ltd. on LinkedIn, but I don't think one gets access to it without signing up. Thank you for reminding us about good air circulation as that is very important for indoor pools. Also, using UV lamps to simulate sunlight may work if the pool is uncovered, though ideally one would only have those turned on when not in the pool area (you don't want to get UV-burned). I don't know if such bulbs produce nearly enough power to have a decent effect -- I sort of doubt they do but I don't know for sure.

 

[PoolGuyNJ]

I did find an additional article I think is pretty germane here.

http://www.poolspanews.com/2011/091/091tech.html

Pretty good stuff.

Scott

 

[chem geek]

In addition to a UV/ozone combo, one can generate hydroxyl radicals directly from electrolysis by using boron-doped diamond (BDD) electrodes as in the Oxineo® system from Adamant Technologies (sold in the U.S. by Maytronics). Something similar is found in the HotSpring® Spa ACE® salt water sanitizing system. These systems are not cheap. The ACE® system for the spa is around $1000 or so. Pool-sized systems would be far more expensive. This is why you usually only seem them in commercial/public pools or if ozonators are used in residential pools they are often too weak.

Note that ozone will oxidize chlorine so if one gets too powerful an ozone system, then one can end up using much more chlorine. This is typically seen in spas with ozonators where the chlorine demand is doubled or higher if one is not using the spa regularly (if one uses the spa frequently, then chlorine demand is cut in half or so).