I really don't understand what the debate is about here. When I read the passage from Dr. Clark's book, it is clear that when she says 'moderately effective' she is referring to the 5Hz frequency which is what you get when you tap the naked battery with your damp finger. 'Moderately effective' has nothing to do with the results you get when you actually use a constructed zapper which has a frequency of 30KHz. With a zapper you eliminate the parasites, viruses, and bacteria which are not in a protective area. Positive voltage, from a DC battery, and not specific frequency is what matters. And reviewing the process of her research makes that clear.
Now if you like certain parasites, viruses, and bacteria, and don't want then to die when you zap, well, then you may want to find the exact frequency for your target germ and zap away at it with a non-DC operated zapper. Otherwise, the standard design should do nicely.