CYA Draining?

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CYA Draining?
If I drain my pool at a rate of 6 inches per day and my CYA is at 100. How many days will it take to reach around 40 CYA level in a 25000 gal pool 32 x 16? I'm afraid to drain it anymore then this because I had pillows of standing water behind my liner and wrinkles are there. Is it possible to figure this out?

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CYA Draining?
25000 gallons is 3342 cubic feet. This means your average pool depth is 3342/(16*32) = 6.5 feet which is a bit higher than expected. If your shallow end were 3 feet and your deep end were 8 feet, then that would be a rough average of 5.5 feet. Most 8 foot deepend pools that are 16x32 in area are around 21000 gallons.
Anyway, if I assume your measurements are correct, then 6" out of 6.5 feet is (6/12)/6.5 = 7.7% of the water volume so every day that reduces the CYA level to (10.077) = 0.923 or 92.3% of it's original value. To get to 40 ppm you want to find "d" in 100 * (0.923)^d = 40 so 0.923^d = (40/100) or d*log(0.923)=log(40/100) so d = 11.4 days.
If instead of doing 6" drain followed by refill you were to do continuous drain/refill, then it would take replacing ln(40/100) = 91.6% of the water volume so if you were doing this at the same rate as the 6" per day, then that would be 91.6/7.7 = 11.9 days or roughly the same amount of time. That is, doing the small 6" drain/refill is about the same as doing a continuous drain/refill (i.e. taking water from one end of the pool while adding it to the other, with thorough mixing with the pump running).
A more waterefficient way would be to replace 60% of the water in one fell swoop, but as you point out you don't want to damage your liner. One can use the sheet method which uses a very large plastic sheet put over the pool and then you drain water from underneath while adding water on top. The sheet drops, but there is always a full amount of water in the pool. When you have added 60% of the pool water volume, you remove the sheet and have the water mix. This same approach also works using large silage bags. Of course, these techniques are more cumbersome than a simple continuous or multiple drain/refill.
Richard
Anyway, if I assume your measurements are correct, then 6" out of 6.5 feet is (6/12)/6.5 = 7.7% of the water volume so every day that reduces the CYA level to (10.077) = 0.923 or 92.3% of it's original value. To get to 40 ppm you want to find "d" in 100 * (0.923)^d = 40 so 0.923^d = (40/100) or d*log(0.923)=log(40/100) so d = 11.4 days.
If instead of doing 6" drain followed by refill you were to do continuous drain/refill, then it would take replacing ln(40/100) = 91.6% of the water volume so if you were doing this at the same rate as the 6" per day, then that would be 91.6/7.7 = 11.9 days or roughly the same amount of time. That is, doing the small 6" drain/refill is about the same as doing a continuous drain/refill (i.e. taking water from one end of the pool while adding it to the other, with thorough mixing with the pump running).
A more waterefficient way would be to replace 60% of the water in one fell swoop, but as you point out you don't want to damage your liner. One can use the sheet method which uses a very large plastic sheet put over the pool and then you drain water from underneath while adding water on top. The sheet drops, but there is always a full amount of water in the pool. When you have added 60% of the pool water volume, you remove the sheet and have the water mix. This same approach also works using large silage bags. Of course, these techniques are more cumbersome than a simple continuous or multiple drain/refill.
Richard
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