Is stabilizer necessary?

And technically the flow rate of circulating water goes to zero at surfaces. Water and chemicals get to surfaces via diffusion while circulation has more to do with the size of that diffusion layer and therefore the net rate of diffusion through that layer. This is why brushing the pool is still important even with proper chlorine levels, especially with plaster or other somewhat "rough" surfaces.

In practice, we don't see pool surfaces in most pools properly maintained getting slimy or having noticeable biofilm with the possible exception of just above the waterline on tile or in spas and above the water in skimmer baskets, etc. That's why the question was asked about circulation and why brushing is still recommended.

"The important thing to understand is: in a stabilized pool, more chlorine is available where it needs to be to destroy pathogens that enter the water."

does this mean that where Cl destroys "dirt" it leaves behind a Cl free void? The Cl is in suspension operating in a liquid? so, if a piece of "dirt" falls from the sky into the pool does it leave a Cl free trail on it's decent to the bottom of the pool? And when it lands does it have a Cl free zone around it because it exhausted the Cl in it's immediate contact? If so, how long does that Cl free zone last? Long enough for algae to develop?

"And technically the flow rate of circulating water goes to zero at surfaces. Water and chemicals get to surfaces via diffusion while circulation has more to do with the size of that diffusion layer and therefore the net rate of diffusion through that layer. This is why brushing the pool is still important even with proper chlorine levels, especially with plaster or other somewhat "rough" surfaces."

Is this considering the sides and floor beneath the water line "surfaces?" The OP indicated his
blooms were in crevices at the base of his stairs and the like, not where surface tension inhibits circulation? I am dense so pardon me, but where the OP indicates he operates at 60-80ppm CYA and 5-7ppm Cl why wouldn't 80-85/90 CYA and 8-11ppm Cl fix that? I almost NEVER shock, just once all of last season after a hurricane, so like never. I must be dumb lucky. I'm counting my blessings and going swimming! Recurring algae blooms would make me nuts. I just knocked wood of course...

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Originally Posted by PBLsQuad450

does this mean that where Cl destroys "dirt" it leaves behind a Cl free void?

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Yes; in a pool without stabilizer. If there is chlorine in the immediate vicinity bound to stabilizer then when the unbound chlorine is used up, chlorine is released by the stabilizer and is available right where it's needed.

Chlorine that is bound to CYA is released very quickly where half can be released in 0.25 seconds. And yes, you are right that the bigger problems tend to be in corners or other areas where circulation is particularly poor (and yes, by surfaces I mean all solid surfaces, walls and floor). If you didn't have any CYA in the water, then the reaction rate of FC will be higher and most likely your FC level will be lower since it's so strong without CYA moderating its strength. So it's much more likely to run out (go towards zero) locally in areas of poor circulation in that case though normally in pools with higher bather load (so that there's more stuff that uses up chlorine in the water). If instead you have CYA in the water and a high enough FC to kill algae, then it is less likely to run out of chlorine locally. Think of the CYA as an active chlorine buffer holding it in reserve for when it is needed.

Now in spite of all that, the fact is that if circulation is really poor in an area, then algae can still grow because if your FC/CYA ratio is close to the algae inhibition limit in most areas of the pool but the FC drops in areas of poor circulation, then algae could grow faster than chlorine can kill it. So while the CYA buffering of chlorine helps prevent chlorine from completely running out, it doesn't necessarily help it from preventing algae completely in areas of very poor circulation. The chlorine/CYA chart guidelines are just that, guidelines. They aren't absolutes and one can readily create situations where those recommended levels won't be good enough, but for MOST pools the recommendations work very well.

Thank you Both! chem geek, that is a great explanation. The sanitizing action is almost counterintuitive for me so pardon the repeat question. I am glad I have a normal pool. I also vac by hand and take a few extra minutes each time to do stair fronts, side walls and and the like. Those areas get done maybe a little less than 2X per month where the pool is vacuumed every week, at least. This is an important caveat in understanding sanitizing. Thanks again!

Paul