Kishu is a finely crafted form of activated charcoal sometimes called activated carbon. By heating wood to an extremely high temperature in an oven or kiln and slowly depriving it of oxygen, the wood becomes carbonized. 

Activated charcoal naturally bonds with toxins, particularly metals like lead or mercury. This is why it is used, in some form, in almost every water filtration system in the world. In municipal water systems, various additives (chlorine, for example) are also included in the treatment of water. And some municipal systems are not as rigorous as conscientious consumers would prefer. It is because of this that consumers are wary of their water source and want to add another layer of filtration - and many want their filters to be plastic-free.