The primary function of the sterile Whatman is to remove bacteria and other crap that shouldn't be in the solution. It is a mechanical sterilization method.

Now, if something DOES manage to get through the .45u pores, there is yet another level of sterilization to catch it, being the Benzyl Alcohol. While the BA does assist in dissolving, it is mainly to help kill of potential organic material that you don't want.

That is why I chose 0.45u, it does the job. Filtering stays fairly easy and it catches almost everything.

The 0.22u filter will just get some slightly smaller stuff, but everything that you don't want in the solution will get caught by the 0.45u.