True, but you can't see bacteria! Fortunately, the amount of chlorine needed to kill algae is higher than the level to kill "easy-to-moderate" bugs (bacteria, viruses) up to and including E. coli. So if your pool water isn't developing algae, then it is more likely than not to be sanitized as well, but this obviously isn't a sure bet since you could just be "lucky" about having no algae (algae growth is dependent on other factors such as food sources including nitrates and phophates). At any rate, most users of SWCG systems seem to be operating below the minimums in Ben's chart (they are at around 3 ppm FC at up to 80 ppm CYA which is 0.015 HOCl level) and are still higher than that needed for disinfection (0.011 HOCl level roughly corresponding to 650 mV ORP) and since they are constantly generating chlorine to maintain their level, they are probably safe.

Nevertheless, I would prefer that the SWCG manufacturers design "longer" salt cells (more surface area so generation rates can be lowered per unit area) or change the internal flow rates and geometry so that they could operate efficiently using a lower CYA level (30 would be great, though even 50 would be much better). They will still generate high chlorine levels for their "shock" effect, but the lower rate per unit area will give the chlorine time to combine with the lower amount of CYA at normal flow rates (into and out of the cell, not inside the cell near the plates, where the flow is slowed down intentionally).

Richard