purifying water using shredded horseradish-how?

[Brian Ford]

But you do drink it! 

Water from sand-filter beds has been purified through the growth of 
ciliates which sweep up pathogens. My guess is that the horse-radish 
)like the straw) provides the substrate and food trigger which does this 
in contaminated bodies of water. The clear water of a mountain stream is 
kept that way by ciliates, too.

Brian J Ford.