Is _more_ better?
It seems like so many people go crazy over a filter that has a smaller and smaller pore size. You might want to take a step back and decide what it is in the water that you are attempting to filter out. For example, in some third-world countries, water can have all sorts of viral contaminants. Been there, done that. In other areas, it is bacterial contamination that you want to avoid. Each contaminant has a different minimum size, so it is good to know what you are trying to hit. Personally, I do most of my wilderness travel in the Sierra Nevada Range, and to a very great extent, the only contaminant that I am seeking to avoid is Giardia Lamblia. At the largest, they go around 25-35 microns. In the egg form, they are more like 2-3 microns. Therefore, if I use a filter that is 2-3 microns or smaller, I am successful. When you try to filter 10 times or 100 times smaller than that, you will need some help to get the raw water through the membrane. That might mean pumping, or it might mean gravity pressure.