Can't raise Alkalinity

[blidome]

I'm hoping someone here has an idea about our problem. We have a painted concrete outside pool of approx 1,200 m3. The tA was around 46ppm and pH 7.2. I wanted to raise tA to at least 80. Calculations told me I needed around 50kg of bicarb to raise the tA 30ppm but after the first dose the tA had fallen to 36ppm. I tried again yesterday, another 50kg and it now stands at 40ppm. We left the pump off for 4 hours after adding the second lot to the pool and when restarted the pH was around 8 but had fallen to 7.5 after a few hours. It then it rose slowly to 7.6 but then during the day, with little or no HCl from the doser (target set at pH7.5) it has steadily fallen to 7.15.

Can anyone explain what's happening here?

TIA

[Teapot]

It can take days for the TA to raise following dosing. Even longer if you switch the pump off.
Unfortunately you seem to be adopting daft old practices regarding TA and pH, do you have CYA in the water?
Please post a full set of water test results.

[blidome]

Thanks for the reply, I take it the daft old practice was turning the pump off. I only tried it after the fall in alkalinity 24 hours after the initial dosing. As for taking days to respond I have read from wait one turnover to test to wait 4 days to test from supposed professionals.
I have just put another 40 kg of bicarb in pool (leaving the pump on) and brushed it and within 30 mins the tA tested at 69 (40 before I began).
So the current tests came out as:
1.95 free Cl
2.65 total
pH 7.30
CaH 300
Cya 4
tA 69
Temp 20.1 C
Obviously being open air the Cl varies somewhat but we are feeding it around 12kg Calcium hypochlorite per day.
pH control is HCl.

[Denniswiseman]

It seems you have a hydrochloric acid doser which would lower pH and TA actually Yo-Yoing. Turn it off completely
Those numbers seem like pool store ones which isn't the best. Get a decent FAS/DPD test kit
Those numbers don't look to bad apart from your FC and CYA which should be at least 30
I would also stop dosing with Calcium hypochlorite because your CH is quiet reasonable
Use Trichlor until your CYA raisesthen go over to liquid chlorine (sodium hypochlorite or plain bleach)

[blidome]

The acid doser has been off since we put in the second dose of bicarb as I realise it also reduces tA. As we have an autodosing system I did wonder if that might have caused overdosing with the acid and therefore the first dose being 'eaten' as the bicarb circulated. That was part reason behind trying the 'pump off' voodoo I read about (but only for 4 hours after dosing, otherwise it runs 24/7) and the whole reason the acid has been off since that second try. We had the same calcium hypo / hydrochloric acid system last year but when I put bicarb in it worked.

It seems odd to me that I can add a chemical that dissolves straight away and yet is not measurable for days afterwards, how does that work?

As for the measurements, the free, pH and temp all come from a very expensive control system and we use a Lovibond photometer system for the rest of the readings.

I am working on the Cya level but I cannot add it through the skimmers like I did last year as I'm told the Cl sensor can be damaged by too high a concentration. It's taken 10kg to get it to 4 though calculations say that should be 8 already. We are aiming for 25-30 but as the Chlorine pump has just about been managing to keep up this year, after a couple of system modifications, and the pool has only been filled for about a month we're still sort of trying to achieve a balance and it hasn't seemed that important.

I'm interested as to whats wrong with 2ppm Cl? Most say 1 to 3 and the control system is set to try and maintain 2.5 free as the dosing pump can't quite keep up during the day. It usually recovers at night (if the day tank doesn't run dry!) Combined shouldn't be an issue much longer as we're waiting for the new UV system to be installed.

Although we do have some sodium hypochlorite we don't use it for normal Cl dosing due to cost and storage, it's for shock or emergency dosing if needed. We're pumping around 400 litres of calcium hypo in per day and don't have tanks large enough to store the amount of sodium hypo we'd need to switch over altogether so I'm sort of stuck with the chemicals we have, at least until yesterday's delivery is used up!

[Denniswiseman]

So long as your pH is stable don't worry about the TA
Take a look at Recommended Pool Levels and Chlorine / CYA Chart
Also Cal hypo will build up your calcium level
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm
I have never heard that sodium hypochlorite or plain bleach is to expensive to use. Also the only addition is salt to the water which doesn't affect anything else. Bear in mind that it doesn't keep indefinately and using it for shock or emergency you will only be putting in salt water and maybe a reduced amount of chlorine
Your CYA can be hung in a sock in front of a return which will dissipate quiet quickly
I suspect your problems are caused by your expensive dosing system. Stop using it until you have your water under control and then monitor it to see what is happening

[blidome]

TBH it was the pH bouncing all over the place that made me look at increasing the tA. I think I might have achieved it finally looking at the pH levels today. I will go down and check soon and see how it's faring.
As I don't do the ordering chemicals I don't know how much we're paying but I think the other main issue was having no bulk storage tank for sodium hypo. For one tub of calcium hypo we'd need several 20l cans of sodium hypo and we haven't got the storage space as it is. We have talked about switching but it was the storage tank issue that put the final nail in that coffin for the moment. The CaH has stayed pretty constant so far and we consume a fair amount of water backwashing 2 x 2.4m dia. sand filters so that helps keep it down a bit too.
For chlorine concentrations, we read the PWTAG recommendations in:
https://www.alsenvironmental.co.uk/medi ... e_2017.pdf
where it says 0.5ppm to 1ppm but don't go over 3ppm. But, as we are a large, relatively shallow outdoor pool with no cover and regular wildlife visitors, we have aimed to keep it at 2.5 and thought that was relatively high.
Sock in the skimmer was what I did last year for Cya but as the water sample point is just after the pump and pre-filter I don't want the concentration to go too high and damage the sensor. I am intending to make a solution separately and add to the pool directly bit by bit but other issues have taken priority until now.
I have taken pH control back from the auto system for the moment but as we consume Cl at a frightening rate as soon as the sun rises I have to leave that running.
Is there any theory as to why the alkalinity doesn't show up straight away? I have read that in a couple of places but others say there's no need to wait longer than one recirculation. It seems hard to find anything definitive about a lot of this pool chemistry!
Thanks for the advice so far, I appreciate it!

[Denniswiseman]

You are following the dosing instructions for a public pool which I'm not that familiar with. Teapot may be able to help further
Don't put the sock(s) in a skimmer but in front of a return
Continueing using Cal hypo will increase CH way above the recommemded 5 and 150mg/l seeing as it's 300 now
I understand why you are using the automated dosing system now
There is no mention of CYA in that document, so it may not be recommened for public pools however CYA will buffer your FC loss

[Teapot]

For chlorine concentrations, we read the PWTAG recommendations in:
https://www.alsenvironmental.co.uk/medi ... e_2017.pdf
where it says 0.5ppm to 1ppm but don't go over 3ppm. But, as we are a large, relatively shallow outdoor pool with no cover and regular wildlife visitors, we have aimed to keep it at 2.5 and thought that was relatively high.

PWTAG, that's your first big mistake! Don't listen to that bunch of idiots! I and other colleagues have big falling outs with the lack of scientific expertise, despite the board having letters after their names they really are not well informed. One of my friends had to threaten them with legal action.
When I said "daft old practices" I didn't mean you turning off your pump, I really meant the industry and PWTAG are exactly the kind of thing I meant.

Phew, rant over!!

[Teapot]

As Dennis said, turn off the acid doser, as soon as the increase in bicarb hits the sensor it will go into apoplexy. It will get a pH reading of around 8.1-8.3 and try to counter this.

Why do you have to run the pump for a chemical that dissolves instantly, because it doesn't and this take no account of the water circulation in the pool. You may turn over the water in the pool once, but that is only around 63% of the water actually being moved through the filter at best to actually mix the water you'll need closer to 5 turnovers for 99% of the water to pass and fully mix, again dependent on actual flow rate. (Gage & Bidwell law of dilution)

As Dennis said if your pH remains pretty stable leave the TA where it is, most pools are over alkalinity, even at 40ppm you have nearly 7 times more co2 in the water than the air so the co2 off gasses taking the pH upwards. if you have more alkalinity at 80-100ppm you have even more co2 dissolved in the water which means the offgasing rate is even faster leading to quicker pH rises and the need to add more acid to bring it down, pool industry loves this ££££.

An outside pool with no CYA in the water will loose 50% of the chlorine in 35 minutes! the total loss after about 2 hours. your dosing pump will be working overtime to catch up and try to keep chlorine in the pool. Running with CYA will help minimise this loss and therefore reduce by a factor of 5 the amount of chlorine you'll use.

To keep the water sanitary and algae free the chlorine level should be around 5-7.5% 0f the CYA level i.e. 50ppm CYA needs 2.5pp to 3.75ppm of free chlorine. (O'Brien equilibrium chemistry)

bicarbonate buffer in water is around pH 8.1-8.3 and this will try to pull the water towards this figure but with CYA in the water the acid in the CYA also creates a buffer around pH 7.5 so this helps keep the pH where we need it. That said the latest research shows when CYA is present in pool water the pH doesn't matter and has very little effect on chlorine's sanitising ability provided there is sufficient free chlorine available.

Using Cal hypo will eventually cause scale formation in your pool.

[blidome]

Denniswiseman, yes, we are a public pool, now run by a charitable company which took it over to avoid closure. I can't really leave socks full of cyanuric acid hanging with the public about so I may have to turn off the doser's water supply whilst adding the Cya through a hopper system we devised last year.

Teapot, Thank you for that informative post.

We were directed to the PWTAG recommendations by the official UK Government Health and Safety advice on hse.gov.uk and our pool training course followed those guidelines. As we're relatively new to this it's really hard to know who to believe. For example, last year we polled 6 other open air public pools about whether Cya was necessary, 3 said don't waste your time with it, it's more trouble than it's worth and 3 said use it. When researching online we often seem to find 2, equally vehement, opposing schools of thought and never know which to believe.
Problem is we're told to follow PWTAG by official Health and Safety advice which tells us we should stay under 3ppm and that we have to kick the public out if it goes above 5ppm.

As for the alkalinity problem, the pH is not stable. The Acid has been off since before the two most recent bicarb doses (2 days so far), one of 50 kg two days ago and a second one of 40kg yesterday evening, yet the tA reading still only stands at 50 (from 36 before these 2 recent doses) today. The pH rose to 7.45 immediately after the second dose, fell to around 7.35 after 3 hours and stayed steady for 12 hours but then started to drop again as soon as the pool opened at 1pm and we had swimmers. It's now down to 7.2 as I type so it's still not stable. (In case it helps, I have attached a graph of this below, Chlorine is blue, pH green and temperature red, the scales are on the left. The 3 bicarb additions are labelled underneath and the Acid has been off since the 2nd one with the pool pump off for 4 hours)

Apart from the pH another part of the reason for wanting to raise alkalinity is because the water index tells me it's borderline corrosive and I should raise Calcium hardness or Alkalinity. At 300 already I don't want to raise Hardness that so I'm left with alkalinity to try and resolve it.

[Teapot]

I will go on more about PWTAG another time and the HSE, suffice to say PWTAG lobbied the HSE to make them the de facto training etc. This the HSE did not do and quite a strong opposition from pool professionals who actually know. I raised the issue with Janice Calvert PWTAG's chemist as to whether she had read the O'brien report and understood equilibrium chemistry, seems she did not.
The HSE document HSG179 is an ACOP, guidance not rules or law, if you demonstrate a better way then you can use it. We have the scientific data to back up every point from hypochlorous acid concentrations at various concentrations of CYA to hyperchlorous acid concentrations at various pH levels which contradicts both PWTAG and the HSE. Sadly they do not differentiate between indoor and outdoor pools very well thus the 3ppm level of chlorine in an indoor pool without CYA is much stronger than 25ppm chlorine in an outdoor pool with 30ppm CYA. This fact alludes both PWTAG and the HSE. PWTAG could do some real research but seem to prefer to copy the incorrect information from the web. The biggest problem is they cannot now back track so must carry on with their incorrect un-scientific rubbish.

Enough of that.

[Teapot]

If adding CYA is a problem and you can't get the liquid version, I would recommend Dryden Aqua's ACO, this will also protect the chlorine from being burned up by the suns UV but is also a photocatalyst so it doesn't diminish the chlorine efficiency like CYA does but enhances the disinfection power of the water under the sun's UV.
https://www.drydenaqua.com/swimming-pools/products/aco

Baring in mind the severe drop in sales of chlorine the 3 opposed to CYA would suffer, and the possibility they do not understand the chemistry it's hardly surprising. As a pool pro, I am embarrassed at various trade gatherings as to the woeful knowledge some pool pros have and that is Europe wide and the USA.

I think you need time for the chems to thoroughly mix and settle down, your short term peaks could be small pockets of concentrated chems getting to the very sensitive test probes, when I have large adjustments to make, I switch off the valves allowing water to the probes and use my photometer only. I have disconnected my pool acid doser as I do not need to add acid any more with ALK around 40ppm. To counter the borderline corrosive water issue I increase the pH slightly as small increases in pH have a larger effect on the saturation index than adding bicarbonate or calcium with less risk of scale or fighting to maintain alkalinity.

You may enjoy a read about chlorine in the presence of CYA and pH, this absolutely blows a massive hole in what is taught in pool training in the UK and the USA, give the industry 10 years to catch up and we may finally get to the truth. Please note where CYA is not present the pH does matter.

http://www.poolhelp.com/home/onbalance- ... -efficacy/

[Teapot]

Thank you for the graph, you can see when the temperature peaked the sun was probably at it's strongest which destroyed the chlorine very quickly. I would be interested to see some more graphs.
Please note with low alkalinity the pH will adjust very very quickly with less than 50% of the acid required when the alkalinity is 80-120ppm so be very careful, another reason why my acid doser is off, as I run ALK around 40ppm I also found the doser was too aggressive with any additions. It appears the algorithms programmed into the units are based on the standard old ideas and levels, always a concrete tiled or plaster pool and never vinyl or fibreglass. safer to make small manual additions but that said I only have a 1 litre bottle of hydrochloric acid in store but haven't used any of it in a season.

[blidome]

I know how Alice felt now!
Either I keep to what we've been doing without Cya and keep good disinfection or I add Cya and up the Cl above what the independent tester will consider acceptable. I suppose now I have 4ppm Cya it's probably too late and I should add the rest to around 25, aim for 4 Cl and rely on the UV to do the rest when it's installed in the next week or two!
Still need to stop my pH dropping though!