I bought this product about a month ago to replace another we had. I need lots of water per day as I make no saliva so carry a bottle/container with fresh water with me all the time! I wash this bottle every other day I use 4 cotton buds to clean the straw which is attached to the mouth piece and these cotton buds pick up "black residue" I assume - that comes from the filter into my water so how clean is my water? I now use water straight from the tap use the filtered water for my kettle?! My bottle is now residue free can you explain this to me please?