SWG and chlorine levels

[doriec]

Hi Sean,
I have read many of your interesting posts. Your information about SWG's helped us choose to buy one. Here are my readings from Ben's test kit:
FC 20
CC 0
PH 7.4
CYA 70
salt level 3600--I have an aquarite for a pool up to 40,000 gal; my pool is 27,000 gal
CH 250
TA 140-I am working on this.
I had been running the pool 12 hours a day at 50 %, and have been maintaining a residual FC of 3.5. I suspect a large pool party and swimming suits worn in a nearby lake were the culprits.
Since then I have little chunks of algae on the bottom of the pool and extremely tingly water. I have been shocking for 2 days now with bleach, bleach, and more bleach. I have also been running the SWG for 24 hours at 70%. I have been checking the FC every few hours to see if I have to add more bleach . The FC level is still dropping slowly each time. this morning I still had the chunks of algae and clear bubbles on the sides of the pool. I have been brushing regularly. IS patience the key? How high should I run my FC after I get rid of the algae? I have had the pool three years and NEVER had green algae. I don't think phosphates are the problem. Thanks, Dorie

[PatL34]

Dorie,

Patience is the Key. I assume you have a plaster pool to let you go up to 20 ppm Cl.

If so keep up with the bleach to help the cell as much as possible. Is there any other source of algae, like fill water that might be problem.

Are you vacuuming to waste after you brush. This can help.

Pat

[doriec]

Pat, Thanks for the help. I have a vinyl pool. I have not noticed bleaching of the liner at the high chlorine level. I have not vacuumed to waste. I backwash to waste. My pool drains very quickly if I vaccuum to waste, and I live in Kansas and am under severe water restrictions. Today at noon my pool was at 20 FC. Five hours later it dropped to 15 FC. How quick a drop in FC is acceptable. I was in full sun today, and I see no algae now. Also, during the day, can I leave my solar cover on to preserve chemicals and shield them from the sun? Dorie

[PatL34]

With the SWCG on, it is better not to cover the are of the return jets, so as to allow the hydrogen generated by the cell to dissipate. The CYA at 70 ppm should be enough to keep chlorine without the pool cover.

By all means backwash to waste. You appear to be winning the algae battle, and as the FC starts to drop less and less, then is the time bring the FC level down gradually, at about 5 ppm per week. If you start to see algae again, bring it back up to 15 ppm and hold it until it goes. Again patience and perseverance is needed.

Pat

[doriec]

Pat,
Thanks for the encouragement. I worry about running my SWG too much. Is it better to run it more hours at a lower %? Right now I am running it for 12 hours at 50%. That doesn't seem to be doing the job. Would you recommend more time or a higher % to maintain a FC of 5? Thanks, Dorie

[PatL34]

You need to check in the SWCG manual regarding the running times needed to get two reversals in polarity for the cell. That would be the minimum running time needed. You can then adjust the % output for the ppm Cl you want.

Some units do not have reversals, so it means checking the cell for scale buildup. Hopefully Poolsean will help on this one.

Pat

[Poolsean]

Doric,

Run your normal filter pump run time, but as Pat recommended, you'll need to consider the reversal rate of your salt system (which I believe is every 100 minutes). Adjust the Purifier % output to maintain your desired chlorine level and see where it lands on.

The overall cell life will equalize either way. If you run more hours, you can reduce the Purifer %. If you leave the pump hours, you will need to increase the Purifier %. About the same either way...

[chem geek]

With the SWCG on, it is better not to cover the are of the return jets, so as to allow the hydrogen generated by the cell to dissipate. The CYA at 70 ppm should be enough to keep chlorine without the pool cover.
Pat

With an FC of 3.5, going from a CYA of 30 to a CYA of 70 only increases the half-life of chlorine (in full sun) from 6 hours to a little over 7 hours. You can see from this graph that the bulk of the benefit of protection from CYA comes at relatively low levels of even 20 ppm. Using a pool cover over part of the pool will definitely help preserve the chlorine during the day.

The key is to see if the chlorine level drops significantly overnight. If it does, then algae is still being killed. If the chlorine level holds (or drops slightly), then the shocking program is near completion. I suspect that even higher levels of chlorine above 20 ppm may be required to quickly kill the algae. As for damaging the vinyl liner, I doubt that even these levels will harm it since it is equivalent in oxidation and disinfection capability to only 0.35 ppm FC with no CYA.

Richard

[doriec]

Thanks Pat, Poolsean, and Richard. I think I've got the algae licked. The FC fell from only 17.5 from 20 FC last night, no algae is visible, the "bubble colonies" clinging to the sides are gone. and the pool is crystal clear. I am running the SWG 2 more hours each day to see if that helps. My goal is 5 FC. My question is, how can people get by with a FC level of 3.5 when the CYA chart says to run it a 5 FC level minimum? Dorie

[chem geek]

Dorie,

That's a question of hot debate at this long thread. The general consensus is that an SWCG does a better job than manual dosing chlorine because it is doing so more continuously and also has a region in the salt cell at very high chlorine concentrations "shocking" the water that passes through it. There is anecdotal evidence that more people with SWCG systems have less problems with algae than those that don't, though interestingly a couple of people have been reporting algae problems in their SWCG pools just this week.

What is not clear is whether it is OK to operate at a lower FC level. Certainly the continual dosing would allow one to operate at the "min" of the range since the range is there partly to allow for adding chlorine that gets used up each day. The SWCG is better able to maintain a constant chlorine level.

As for the shocking effect, it is also not clear that this is very much different than what happens when you add liquid chlorine (or bleach) to your pool since it first hits the water at extraordinarily high concentration and then diffuses. So again, it is the continual shocking (albeit of a small volume of water) that is probably the main difference.

I certainly would not operate in the recommended range of 1-3 ppm at 70-80 ppm -- at least not anything below the 3 ppm of that range. It's safer to go with 5 ppm, but we simply don't have enough data to make a definitive choice. It's too bad that the SWCG manufacturers have not made their cells operate at lower CYA levels since that would allow for the best of both worlds -- low FC with higher disinfection and algae prevention capability.

Richard