SWG and higher FC levels

[agogley]

First of all, an SWG can't remain algae free with 3 ppm FC with 80 ppm CYA unless the pool is poor in algae nutrients. We've had plenty of examples of a 3 ppm FC target that simply doesn't work -- it's too close to the edge with high chlorine demand even if algae isn't yet visible. 4 ppm FC is pretty much the minimum required with 80 ppm CYA and as a minimum that's a number to be measured in the morning after any overnight loss.

Also, remember that the SWG is maintaining the FC level whenever the pump is running so if things are set up correctly where that is mostly during the day when FC loss is greatest, then the FC level is fairly constant. A manually dosed pool will fluctuate in FC up and down so targeting an "average" 4 ppm FC with 80 ppm CYA may have the FC get below that over long enough periods of time that algae can grow. In theory, an automated dosing system, such as a peristaltic pump, maintaining 4 ppm FC with 80 ppm CYA might work, but we don't have enough experience with those to know for sure.

Also, there is some superchlorination in the SWG itself that might help kill off floating algae though obviously would do nothing against any algae on pool surfaces. The superchlorination in the SWG is different than that from manual dosing using hypochlorite chlorine because the chlorine generated in the SWG at the anode is in very acidic conditions, at least until it meets the water that is very basic/alkaline from the cathode.

So you are recommending that SWG with high CYA have an 4.0 to 5.0 chlorine level? Why don't the manufacturers specify this? And how do we maintain that ratio in view of the plaster/pebble tec warranties?

 

[chem geek]

Re: Water Balance for SWGs

I am not aware of any plaster or pebble-tek warranties requiring you to not have more than 3 ppm FC in the pool. Commercial/public pool regulations in several states allow higher than 4 ppm FC -- it's up to 10 ppm in Florida, for example (see this post for more examples). The manufacturers say 1-3 ppm FC with 60-80 ppm CYA because they don't understand the chlorine/CYA relationship even though it has been known since at least 1974. Also, no EPA-registered product or NSF Standard 50 device will say anything higher than 4 ppm FC because that is the EPA drinking water limit, but again such limits do not consider CYA in the water (and as just noted, it's state/county regulations that determine what is allowed in commercial/public pools).

 

[agogley]

Re: Water Balance for SWGs

I am not aware of any plaster or pebble-tek warranties requiring you to not have more than 3 ppm FC in the pool. Commercial/public pool regulations in several states allow higher than 4 ppm FC -- it's up to 10 ppm in Florida, for example (see this post for more examples). The manufacturers say 1-3 ppm FC with 60-80 ppm CYA because they don't understand the chlorine/CYA relationship even though it has been known since at least 1974. Also, no EPA-registered product or NSF Standard 50 device will say anything higher than 4 ppm FC because that is the EPA drinking water limit, but again such limits do not consider CYA in the water (and as just noted, it's state/county regulations that determine what is allowed in commercial/public pools).

Yeah, I'm not absolutely certain about pebble-tec's warranty but their website recommends using the langlier index. It provides ranges for TA, CH, and PH but not Chlorine. The stonescapes warranty does list these ranges and says anything above 4.0 is corrosive. I'm guessing constant FC above 4.0 would cause warranty issues, but I can't be certain, obviously. I think it sucks, but they stick exactly to the Association of Pool & Spa Professionals (APSP) standards. This is why I tend to keep the CYA somewhere around 70 (IMHO, you are guessing at least +/- 10 points just due to how this test is performed). I wonder if I can keep it at 4 but convince the pool test high schoolers to put down 3 (because you have to have your printouts to prove pool chemistry).