My question is that if I had a pool with high CH, and I brought it down to 150-ish and had the customer maintain a low pH (in the 7.0 range, for example), would enough time allow for the CH build up on the plaster to come back in to solution, and not cause any other damage?
Where I am (S. California) we have extremely hard water and drought conditions. I hate to see drain and refills, and I hate (and always decline) acid washes. Too many people sell the drain and acid wash to make a quick buck, and it does the homeowner no favors, as is just shortens the remaining time until they need to re-plaster. I am diligently trying to find a less intrusive way to solve the CH build up without harsh chemicals and acid washed type side effects for the customer.
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I appreciate all of the input! Thank you guys!
However, it doesn't directly prevent scaling at usual concentrations found in pools. Borates are a mild sequestering agent, but are very weak for sequestering at 50 ppm (they're far stronger when used in laundry applications since they are far more concentrated in that usage).
One can, however lower the TA level as the CH rises. If one has 50 ppm Borates in the water, then one can lower the TA over time even to 50 ppm since the Borates will still provide decent pH buffering. I'm assuming here that a hypochlorite source of chlorine is being used. You shouldn't use this approach if using Trichlor as it is too acidic and requires a higher TA because the carbonate buffer system is far stronger at preventing large drops in pH while the borate buffer system is much stronger at preventing a rise (relative to normal pH levels in pools).
A TA of 50 ppm with 30 ppm CYA and 50 ppm Borates and 7.5 pH and an 85ºF temperature would be in balance (saturation index 0) with a CH of around 700 ppm. Scaling usually doesn't occur until the saturation index is fairly positive, usually +0.7 or higher though as Que mentions it's traditionally considered in balance within 0.3 of zero. With the numbers I just gave, a CH of around 2300 ppm would be at a +0.3 saturation index. So lowering the TA level can certainly give you a lot of leeway in terms of how high the CH can get before scaling significantly occurs. By comparison, with a TA of 100 ppm, balance is with a CH of 250 ppm and +0.3 is with a CH of 500 ppm.
Of course, the TA is usually high in fill water when the CH is high so evaporation and refill will tend to have the TA rise over time. So to maintain a lower TA level as the CH rises, one would have to add acid regularly (and probably provide aeration to drive off carbon dioxide faster).
As Que noted, the more classic approach to this problem is to use metal sequestrants designed to sequester calcium and to inhibit formation of scale.
Yup, too many variables and no "one size fits all" application! Thanks for all of the responses; I appreciate it!
Bruce Wettstein said:
There are too many unanswered parameters for there to be a simple answer to this question. If the goal is to neither actively etch nor actively scale the pool, simply balance the water. If the goal is to dissolve calcium, that can be achieved by lowering pH, alkalinity, or hardness. The degree to which one lowers them is dependent on the degree of aggressive effect desired. If one wants the process to take days vs. weeks vs. months, pick a degree of aggressiveness.
The term "safe" is relative. Experience and ease of control and available time frame are factors.
And in addition, all calcium carbonate rheology is not equal - some crystalline formations of pool scale are relatively softer or harder, more or less soluble, sharper or bumpier, whiter or stained (containing foreign material in the crystalline structure), etc.
Guess all the variables keep stain removal techs in business, eh?
If I do need to remove existing calcium carbonate scale, and if underwater techniques fail the spot test pre-evaluation, I prefer wet-sanding.
All that being said, as a preventative measure, maintaining soft enough (but not too soft) water is optimal.
The ranges you specified (pH around 7, TA around 40, TC around 175 and TDS around 700) would result in a LSI value of around -1.0. Since water with an LSI of less than -0.3 is generally understood to have an increasing tendency to dissolve calcium carbonate, that routine should slowly dissolve pool scale.
Since pool plaster has a paste component which contains calcium carbonate at surface, the above parameters also have the capability of dissolving the plaster itself. But you should have alot more control over the process than with a full-out acid treatment.
Like anything else, technique and care are key...