A few weeks ago, my boys (9 and 14) attended a class. Part of the 'hands on' was making a water filter with a 2 litre coke bottle. Imagine you cut the top off the bottle, invert the top, place a coffee filter over the opening. Now fill the (inverted) top with sand and then 'gravel'. Place the sand filled top into the 'bottom' of the bottle that you cut it from. Now pour the water into the sand and allow it to filter through. They provided us with the water. It came from an outdoor tank that houses reptilian animals. We took samples home and continued the filtering 'test'. We put the water under the microscope both before and after. I wouldn't put my faith in coffee filters people.....while the filter and sand 'removed' the chunks from the water....they let through just about everything else....I don't remember my microbiology all that much, but...rods, cocci, flagellates.....others I couldn't identify......No gram staining just visualized under a scope.....But the 'after' filtration looked just as bad a the 'before'. I would have thought that the set up would have caught SOMETHING...but no. Evaluate you set up if needed....