The Great Tetraborate Experiment!

[chem geek]

I ran into this little tidbit

Borates are used to lower the pool's requirement for chlorine, by depriving algae of abundant quantities of dissolved carbon dioxide. The lowering of the carbon dioxide levels will slow algae growth and reduce the chlorine consumption.

from this webpage and if it is true that algae depend on abundant quantities of dissolved carbon dioxide, then this could explain reduced chlorine usage IF you had algae in your pool using up your chlorine. I suppose that there may always be some algae dropping into your pool, but I didn't think it was that much nor that it ate up that much chlorine (except during a bloom which you then have to shock).

Also, you have to lower your carbonate alkalinity to get this benefit. This would also lower the tendency in the pH to rise (beyond the buffering effect) and would reduce acid demand. Essentially, the borates let you use a non-carbonate pH buffer in your pool so you can run at a lower carbonate level that apparently helps prevent algae and certainly lowers the outgassing of carbon dioxide with its resultant pH rise and acid demand.

So though the website claims that borates reduce the amount of dissolved carbon dioxide in the water, I believe this is only true if you let the carbonate alkalinity get lower which you might "naturally" do by lowering the TA after its rise from adding so much Borax. HOWEVER, as far as dissolved carbon dioxide being reduced, you would have to have your carbonate alkalinity reduced a whole lot more, I would think, since cutting TA in half will only cut the carbonate alkalinity by a little more than half (with water balance turning more corrosive, though probably still in the OK range). It's still an intriguing thought.

[EDIT] After thinking more about it, I don't think the "reduced dissolved carbon dioxide" theory is the answer. Consider ponds and lakes that have algae in them. They are probably closer in balance with the air for dissolved carbon dioxide and that means they have quite a bit less than pool water unless the water is extremely alkaline. It seems to be common wisdom that Boric Acid suppresses algae in pools and that it interrupts photosynthesis in plants (probably does the same to algae, since the kinds of algae found in pools are plants). This WHO document gives more details. Interestingly, smaller amounts of borates (10 ppm) actual stimulated certain kinds of blue-green algae growth, but growth was inhibited at 50 ppm or above with 100 ppm inhibiting more growth (so perhaps this is a more effective level to use??? -- but these tests were done in simulated sea water which is not the same as pool water). [END-EDIT]
Richard