Alkalinity in Spas with high aeration and pH automation

[haroldjamiroquai]

I work at a high end hotel with several indoor spas that have both waterfalls and jet pumps. The controllers are Chemtrol boxes with both sanitizer Bromine(solenoid) and pH feeders(stenner pump). Both feeders are set up on proportional feed.

The spas are just under 2500 gallons. The issue that I am having is that in order to keep the alkalinity at ~80 I am having to add over 1kilo of NaHCO3 daily. This causes my TDS to rise over 1000points in a matter of about 10 days. Typically, I drain and refill at about 14 days or 1500TDS. Start TDS is ~400. This 4 day decrease has caused a substantial rise in water costs though.

I have taken to adding 500g each day to reduce this TDS rise and the alkalinity levels off consistently to about 40-50 at the end of the day.

So, my question is, "Is it necessary to maintain the recommended TA(80-120) with my chemical feed setup?"

My conclusion is no, as it seems that TA is largely a pH buffer that with proportional feed control is not quite as critical. I would like to get some feedback from some others though as it seems that I have begun to trod uncovered ground.

 

[linen]

Why are you keeping your TA so high? Typically for spas, we recommend closer to 50 ppm for TA.

 

[haroldjamiroquai]

Because I didn't know any better Cpo cert was a joke and none of my coworkers have the slightest idea what they are doing. I'm trying to get a handle on these spas essentially on my own.
Sorry for the rant and thanks for the response.

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[linen]

If you havent already found this take a look at this : http://www.troublefreepool.com/certified-pool-operator-cpo-training-what-is-not-taught-t18432.html

 

[chem geek]

Because your TA target has been too high, that has caused the pH to rise from carbon dioxide outgassing. Your acid feeder (stenner pump) then added acid to lower the pH, but this lowers the TA. You then add baking soda to raise the TA so this is a vicious cycle. By lowering your TA target, the water will be less over-carbonated so with less carbon dioxide outgassing the acid pump won't need to add as much acid and the TA will drop more slowly.

Also, you should target a higher pH of 7.7 or 7.8 instead of 7.5 or below. That will also help to reduce the rate of pH rise from carbon dioxide outgassing.

You do need to be careful, however, as you don't want your acid feeder to exhaust your TA causing a pH crash. So after you get the TA lower and set a higher pH target, closely monitor the TA to get an idea of how quickly it is dropping. It should be slower, but you should verify to make sure. Then, just add enough baking soda to raise it back to 50 ppm and not higher.

 

[haroldjamiroquai]

Thanks for the link linen.

Thanks chem geek. I have just added these acid feeders and have been fiddling with TA and pH trying to find a good balance. Just drained them yesterday so will try your suggestions out. Amazes me how far behind pool chemistry is considering the prevalence of pools in the US.
My insane amount of aeration doesn't help much either.

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[TomBorg]

I've taken over the maintenance of the "semi-public" or "limited-use" pool and spa at my condo complex. Being semi-public, I have to follow certain state and county laws. A hotel would have to adhere to even stricter laws.

In the Code of the King County Board of Health (website), Title R14 (pdf), it states the following about a spa (paraphrased):

Total alkalinity must be a minimum of 80ppm (R14.08.260 Paragraph D)
Manual addition of chlorine is allowed only in emergencies. That is, an automated feeder is required. (R14.08.180 Paragraph C)
Spa must be drained, cleaned, and re-filled with fresh water, every 30 days max. (R14.08.260 Paragraph D)

These laws appear to be designed by the manufacturers of dichlor pucks. :-( Or at least designed around the assumption that you are using an acidic source of chlorine that causes other chemicals to build up rapidly.

Anyway, I don't claim to be an expert, but be sure to check the state and county health codes before changing your TA to a level that is more logical but less typical. Health code violations might be frowned upon by your employers.

 

[haroldjamiroquai]

Good point. State regs say 70 (in CO). Will have to check county.

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