Re: First time poster - excited to use BBB method but have a question!

Originally Posted by
CarlD
I was just exploring the rationale for combining Borax and the alum with Tri-chlor. I also don't know much about it chemically.
Let's look at the possible rationale and then blow holes in it, just for fun.
The rationale is to try and keep the pool clear since continued use of Trichlor builds up CYA lowering chlorine effectiveness so algae can start to grow faster than chlorine can kill it. When this happens, the water may not show visible green algae but instead look dull or cloudy. You already mentioned how Borax can help balance the acidity of the Trichlor and building up borates can help prevent algae. The aluminum helps to clarify the water so initial algae gets swept to the filter so if you backwash you get rid of it but if you don't clean the filter then it sits there using up chlorine and possibly continuing to grow.
However, with 80% Trichlor and 10% Borax and 10% aluminum sulfate, for every 10 ppm FC added by Trichlor the relatively small amount of Borax (I assume it's sodium tetraborate pentahydrate, not decahydrate) barely touches the pH and adds 0.2 ppm borates so at 2 ppm FC per day that's only an increase of 1.2 ppm Borates per month. So your assumption about it being more about "filler" may be accurate though there are less expensive fillers than borax. The minimum aluminum sulfate concentration for coagulation (as in DIN 19643-2) is 0.05 g/m^3 expressed as aluminum so for the example I gave there was 51.7 grams aluminum sulfate (4.08 grams aluminum equivalent) in 10,000 gallons or 37.85 cubic meters so this is 0.11 g/m^3. So while the addition of borates is of questionable value, the use of aluminum sulfate does appear to be in the ballpark at least after 10 ppm FC equivalent of chlorine is added (the alum gets caught in the filter so must be replenished).
15.5'x32' rectangle 16K gal IG concrete pool; 12.5% chlorinating liquid by hand; Jandy CL340 cartridge filter; Pentair Intelliflo VF pump; 8hrs; Taylor K-2006 and TFTestkits TF-100; utility water; summer: automatic; winter: automatic; ; PF:7.5
Bookmarks