What's killing my alkalinity?

[yukonerdave]

Hello

I am one of the operators of a commercial facility, with a lap pool holding about 250,000 gallons. The source water has 150ppm total alkalinity, but our pool consistently hovers around 40-50ppm TA. This is with regular addition of sodium bicarbonate (over the last month we added a total of 150kg sodium bicarb, which did nothing but hold the alkalinity steady at 40ppm). We feed gas chlorine and keep it around 1.0ppm free chl2 (~1.2ppm total), and supplement with a slipstream O3 system. Our pH holds quite steady between 7.4-7.6, and the temperature is 27.5 C. We very rarely add small quantities of muriatic acid to lower pH, and feed soda ash automatically to maintain a 7.4 set point. No other chemicals are added.

My question is why our alkalinity is so low, and why it is so difficult to raise it. Also, I wonder if there are any issues with just leaving it at that level, given that our pH is stable. Any ideas?

Dave

 

[JVTrain]

The addition of chlorine gas is by nature acidic. This will lower TA constantly.

I would think it's OK to leave it where it likes to live, even though it may SEEM to be too low. As long as your pH isn't fluctuating wildly, no sense trying to fight it.

 

[onBalance]

I strongly suggest that you be very careful to not let the alkalinity to become any lower. If the TA is allowed to go lower (and it might given that gas chlorine consumes alkalinity, just like acid does), the pH would likely drop drastically and quickly due to the lack of sufficient buffering.

Also make sure there is sufficient calcium to keep the CSI within proper balance.

 

[Patrick_B]

Well, I'm too late to add anything much. You might consider increasing the TA by upping the BiCarb dose. If you switched to 12.5% bleach, the ratio will increase about 10:1 in pounds per day use. Which is likely the reason you all still use gas.

Handled properly as I'm certain you do, there is nothing better than gas. In my opinion.

 

[yukonerdave]

Wow you guys are on the ball - thank you for the quick replies.

Are there other commercial pool operators on the forum? I'd be interested to hear how much bicarb others go through for a similar-sized tank with a gas chlorine system. Anyway, I'm glad to hear that low alkalinity isn't a big problem so long as the pH is stable.

 

[Patrick_B]

Not many commercial operators here, since we focus on residential pools. That said all are welcome. Anyhow, the same chemistry principals apply. I think someone signed up last year having issues with a 250K + pool. If you do stick around, enjoy the forums!