Re, flocculants: I ran into a study on various flocculants looking at how well they precipitated heavy metals (specifically, the study was about arsenic -- I could probably dig it back up). The candidates ranged from 40-90% of arsenic removed -- not perfect, but certainly better than nothing. Boiling doesn't remove any heavy metals, and filters (even activated carbon) usually don't remove much. Flocculants do remove a significant percentage of microbes, although you obviously don't get the 99.99...% that you get with a good filter. I seem to recall the numbers that I've seen being something like 80-90%.

Re, ph: I ran into another study on clostridium (heh, it was all of these studies that got me curious) -- the genus that includes the sources of botulism, gangrene, colitis, tetanus, and others -- which mentioned that they're very sensitive to low pH (I think it was a pH of 5.0 that was lethal to them), and lowering the pH is a good way to waterborne clostridium species. I've ran into pH treatments for other species as well, some high, some low -- don't remember which species were which, though. Certainly, of course, some diseases are hardier than others. I'd imagine that most cystic species, such as giardia, would probably be resistant to pH changes.

Thanks for all the feedback Don't plan on actually using any of these techniques any time soon, but just trying to flesh them out in case I ever do.