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Postby PaulG » Sat 11 Jun, 2011 13:00


I have volunteered to run the community swimming pool in a hot and sunny place for members of the UK Armed Forces, the problem I have is how to know if I am doing the right things with the chemicals, so advice would be appreciated:

1. The pool is 20mx 10m x 2m - how much chlorine should in put in daily/weekly
2. I am using 200g tablets in the filter is that ok?
3. Should I use tricolora or diclora
4. What other treatment (Antealgae)

Any advice would be appreciated.

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Postby chem geek » Sun 12 Jun, 2011 14:52

20m x 10m x 2m is 400 cubic meters or 400,000 liters (106,000 gallons). This is a large pool and as a community swimming pool it must follow the laws and regulations for such pools, including those regarding sanitation. So you should look those up first and follow them. They may describe restrictions in terms of the type of chlorine that is used, the Cyanuric Acid (CYA) level, and other water chemistry parameters.

You can use The Pool Calculator to calculate dosages. 1 ppm FC in this large pool would be 437 grams of Trichlor or 722 grams of Dichlor or 620 grams of 65% Cal-Hypo or 3111 ml of 12.5% chlorinating liquid (bleach; Javel), etc.

Just note the following chemical rules that are independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it will also increase Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it will also increase CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it will also increase Calcium Hardness (CH) by at least 7 ppm.

The amount of chlorine usage depends primarily on two factors. First is the loss due to sunlight which depends on the CYA level where the loss is a percentage of FC that is lower at higher CYA levels. Second is the amount of bather load in the pool. The typical loss from sunlight is from 1.5 to 2.5 ppm FC per day. As for bather load, in a pool it's roughly 4 grams of Trichlor or 7 grams of Dichlor or 6 grams of 65% Cal-Hypo or 31 ml of 12.5% chlorinating liquid for every person-hour in the pool (though this is about doubled for competitive swimming vs. more typical bathing).

Note that even with 2 ppm FC per day chlorine usage, continued use of Trichlor would increase CYA by over 35 ppm per month if there were no water dilution. You no doubt will be diluting water per regulations, possibly at the rate of 30 liters per bather (amount of time not specified, but probably not more than one hour so figure person-hours). However, even for high bather-load pools, the amount of dilution will not keep the CYA level in check. As the CYA level increases, chlorine effectiveness drops because the active chlorine (hypochlorous acid) level is roughly proportional to the FC/CYA ratio.

So clearly you should not be using Dichlor and if you HAVE to use stabilized chlorine then you would use Trichlor but must keep a check on the CYA level. You can prevent algae growth using chlorine alone if you maintain the proper FC/CYA ratio, but if you use stabilized chlorine causing the CYA to rise then you would need a supplemental algaecide at extra expense to prevent algae growth. Your best bet would be to initially get the CYA to an appropriate level for your regulations, though no more than 30 ppm is needed in high bather-load pools since most of the chlorine demand will be from bathers, not from sunlight. For example, if you had one bather per 4000 liters, so 100 bathers in your pool, and this was for 8 hours, then that would probably require 8 ppm FC in addition to the perhaps 2-3 ppm FC lost by sunlight if 30 ppm CYA were used (assuming 4 ppm FC was the maintained chlorine level at that CYA level). At this bather-load, you would dilute with 240 liters, but the 10 ppm FC would increase CYA by 6 ppm so the final CYA even accounting for dilution would be roughly (30+6)*(4000-240)/4000 = 33.8 ppm or a rise of nearly 4 ppm CYA in one day (over 25 ppm per month even with appropriate dilution for this bather load).

Do not use Trichlor tabs in the filter basket or skimmer basket. It is very acidic. There are specially designed inline chlorinators for that purpose with one-way valves to prevent the acidic water from migrating to the pump or gas heater causing corrosion. However, as noted above, Trichlor will increase CYA over time so are probably not a good choice. They are also not the least expensive when you account for all of the pH Up product you need to use to compensate for pH and TA.

You should seriously consider either getting a peristaltic pump system to inject chlorinating liquid / bleach as your chlorine source or you should consider getting a commercial-grade saltwater chlorine generator system. Read the Pool School for more info, but remember that you will be constrained by your local regulations.
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Postby Swimerpool » Thu 16 Jun, 2011 11:07

3. 437 grams of Trichlor or 722 grams of Dichlor for your 20m x 10m x 2m swimming pool
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