gravel, sand, powdered wood charcoal, sand again, gravel again, all layers separated by fabric (like an old tshirt) and then you need to boil the water. first run is undrinkable. let it run for some time till the water gets clear. then after every use clean/exchange everything. as long as water is flowing through this, it shouldnt accumulate too many bacteria or funghi, just dont stop the water flowing through it!