The theory is that PolyQuat disrupts the algae cell membranes thus bursting (or opening up) the cells and killing nascent algae. However, this is a somewhat slow process so it works well with algae trying to grow (stuff falling into the pool and trying to establish itself), but doesn't work well with large quantities of algae as it would take a lot of polyquat to deal with it (and is much more expensive than chlorine). Also, some types of established algae (and bacteria) form biofilms which are resistent to chemical penetration and would tend to block the PolyQuat from killing the cells underneath the biofilm. High chlorine concentrations will eventually breakdown such biofilms and utlimately get to what's lurking below (for black algae, brushing is also required).

At least that's the theory.

I think Ben's recommendation of using PolyQuat as a last resort is so that one has multiple actions going on at once. However, what we have recently discovered is that the shock levels in Ben's table are not sufficient for mustard/yellow algae, but we do seem to have figured that one out by using higher chlorine levels (about 1.0 ppm disinfecting chlorine).

What I'm trying to figure out is what the proper application of PolyQuat would be when closing a pool since generally one uses a shock amount of chlorine and I am concerned that this would breakdown the PolyQuat defeating the purpose of using it. I'm not suggesting abandoning the use of PolyQuat -- just figuring out when it is most effective, perhaps when chlorine levels are lower (normal) rather than when shocking. So my proposal might be (with real-world verification) to use PolyQuat first at normal chlorine levels, wait a week, then use shock chlorine levels and close the pool, OR vice versa (shock levels of chlorine first, then when the chlorine drops to more normal levels, use PolyQuat and close the pool).

Another option is to use 50 ppm Borates instead of PolyQuat and that seems to have the advantage of inhibiting algae without breaking down from chlorine the way that PolyQuat *may* do. It will be interesting to see if Borate levels are maintained over the winter with those people who are now using it.

Richard