Well, I believe I have figured out why the salt cell seems to do a much better job of superchlorinating a portion of the pool water -- both in disinfecting (killing bugs) and in oxidizing (getting rid of combined chlorine) than simple addition of chlorinating liquid or bleach. When I described earlier how adding chlorinating liquid or bleach was similar to what was done in a salt cell in that there are high levels of chlorine as the concentrated chlorine (125,000 ppm for 12.5% chlorinating liquid) gets diluted in the pool water, I didn't account for the large differences in pH between the two methods. The pH of chlorinating liquid or bleach is very high and though this gets reduced as it is diluted, it is still somewhat high until the dilution is quite broad. This means that most of the chlorine is in the form of the hypochlorite ion (OCl-) which is a much less powerful oxidizer and disinfectant than hypochlorous acid (HOCl).

If I run through some dilution calculations (using standard pool values of pH 7.5, TA 100), I find that the disinfecting chlorine level even with no CYA present only reaches a peak at a dilution of 195:1 and a disinfecting chlorine (HOCl) level of 14.2 ppm. With 30 ppm CYA, the peak is reached at a dilution of 112:1 and a disinfecting chlorine level of 7.1 ppm. At 80 ppm CYA, the peak is reached at a dilution of 75:1 and a disinfecting chlorine level of 3.5 ppm.

In other words, the strong alkalinity of chlorinating liquid or bleach severely cuts down its disinfection and oxidation ability. Of course, the hypochlorite ion (OCl-) is very high (my earlier posts' calculations of dilution would apply mostly to this hypochlorite ion concentration), but is at least 30 times less effective an oxidizer and disinfectant (and possibly worse than that).

Contrast this with what goes on in an SWG cell. At the plate where the chlorine gas is generated, the chlorine gas combines with water to form hypochlorous acid and hydrochloric acid so is a very strongly acidic process. This means that almost all of the chlorine is in the form of the powerful oxidizer and disinfectant hypochlorous acid (HOCl). It is only in the area in the middle between the two plates where the alkalinity from the other plate combines with the acidity to have net alkalinity similar to chlorinating liquid. So, near the acid environment of the chlorine generating plate, water will indeed be exposed to very high levels of disinfection and oxidation -- i.e. will be superchlorinated. Of course, this will be a small subset of the total water flow through the cell, but that is still much better than what happens when simply adding chlorinating liquid or bleach to a pool.

So to simulate the salt cell through adding chlorinating liquid or bleach, one would have to add acid to lower the pH of the pool, then add the chlorine, then add base to raise the pH back up. This is obviously impractical and would swing the pH too much and have other side effects. (Also remember that you never want to add concentrated acid to concentrated chlorine as that produces large amounts of chlorine gas which is quite poisonous).

So that explains why the SWG pools seem to require less FC than non-SWG pools, at least for the pool water that gets circulated through the cell. There are still some users' pools that seem to need higher levels of FC (say, 5-6 instead of 3) to prevent mustard/yellow algae, but that may be due to biofilms stuck to pool surfaces. Regular (once a week) brushing might help to force algae and other microorganisms from pool surfaces into the general body of pool water that will eventually go through the superchlorinated portion of the salt cell. However, if growth rates of algae are high, then even regular brushing might not keep up with such growth rates so the higher chlorine levels in the body of pool water would be the only way to keep the algae at bay.

Richard