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Pool pumps, pool filters and the plumbing of
swimming pools. Sand filters, cartridge filters,
fabric filters and alternative filter media.
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Postby JPlunket » Wed 01 Apr, 2009 09:12

I use Taylor drop system, which supposedly tests for both Free and Total Chlorine. Since shocking several days ago with sodium hypochlorite, both measures have been "redder" than the 5-10ppm color, no perceptible difference betwteen the two. No problem with algae. Filter seems to be clogging much more slowly than before.

I'm going to acid wash the filter again, then try some of the things you have advised.

Qs: in the muriatic acid approach to reducing Calcium, what period of time between the muriatic acid gallons and "rebuilding pH" (and with what "pH up" chemical?).


chem geek
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Postby chem geek » Wed 01 Apr, 2009 11:04

You can also use real TSP (tri-sodium phosphate) to clean a cartridge filter as described here. Note that acid washing is only recommended if you have scale deposits and only after degreasing first. In your case, you possibly have scale deposits, but it's good you did a degreasing though dishwashing detergents do not use TSP anymore. Real TSP is more effective at powerful degreasing, though using Cascade is better than nothing.

One key to easier maintenance of cartridge filters is to get one that is oversized for the pool. That way, less frequent cleanings are required. Nevertheless, sand filters are easier to clean, though backwashing does mean using more water.

As for sand filters and use of a Filter Aid, one can also add DE to their sand filter by adding a cup or two to the skimmer (just enough for a 1-2 PSI rise -- you need to wait a bit to see this rise). As was noted with the Filter Aid, you need to add this again after each backwash and it will initially require more frequent backwashing as it catches more particles from the pool water.

For the CYA test, you are supposed to look at the back of the tube, not the front, as shown at the bottom of this link. Halfway between 1.5 and 2 on the front is roughly 80 ppm CYA on the back. You should redo the test to see for sure -- the CYA could be higher as its a logarithmic scale (goes up quickly in that region).

Adding acid will not lower the calcium content in the pool. Adding acid lowers the pH and the Total Alkalinity (TA). The only easy way to lower Calcium Hardness (CH) is with dilution via water replacement with water that is lower in CH. Using rains to overflow is effective for this. Doing a partial drain/refill using fill water only helps if the CH level of the fill water is significantly lower than that of the pool water. Evaporation followed by refill with fill water increases the TA and CH of the pool because evaporation only removes water while the TA and CH in the fill water get added to the pool. Depending on the source of chlorine (i.e. if it's acidic), the TA may be seen to drop over time, but the CH will continue to rise.

With a CH above 500 and a TA of 150 ppm it is true that you possibly have calcium carbonate scale buildup, especially if your pH is also high. You can use The Pool Calculator for calculating the saturation index as well as for dosing quantities. Your high TA is a bit of a puzzle along with the high CYA level. What is your source of chlorine? If it's Trichlor, then that explains how the CYA get high since for every 10 ppm Free Chlorine (FC) added by stabilized chlorine Trichlor pucks/tabs, it also increases CYA by 6 ppm. However, Trichlor is very acidic so usually your TA will be lower and you'll be adding Alkalinity Up (baking soda) to compensate or if the pH is low as well then you'd be adding pH Up (Arm & Hammer Super Washing Soda) which also increases TA. My guess is that you are using Trichlor pucks/tabs and that you are using pH Up to keep the pH up but that increases the TA level.

As for algae, you likely did not keep your FC level higher to compensate for the higher CYA level. In a manually dosed pool, you need to maintain a minimum FC that is 7.5% of the CYA level to prevent algae growth (in an SWG pool, the minimum FC is 4.5% of the CYA level). FC is NOT a measure of the amount of active chlorine (hypochlorous acid) concentration in the water. CYA binds to most of the chlorine so at higher CYA levels you need a proportionately higher FC level to have the same disinfecting, oxidizing, and algae-killing power. This is known science since 1974 as described in the paper in this link. You can read more about managing your pool at the Pool School. If you want to continue to use stabilized chlorine products and not worry about the rising CYA level, then you will need to use a weekly PolyQuat 60 algaecide or a phosphate remover to prevent algae growth, both are at extra cost. Alternatively, you can use unstabilized chlorine, specifically chlorinating liquid or 6% unscented bleach, as your chlorine source and not build up CYA over time, but you will need to add your chlorine every day or two unless you have a pool cover or an automatic dosing system. The only easy way to lower the CYA level is through water dilution (but at least fill water has zero CYA).


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