According to the University of Rhode Island, coffee filters have a pore size of about a hundred microns. (http://geo.uri.edu/skydust/faqs.aspx)

Depending on what you mean by big contaminants, a coffee filter might work. If I ever have to use my ceramic filters to purify water, I certainly will pre-filter the water that goes into the reservoir for the ceramic filters. I'm not sure I'd use a coffee filter, but I'll use a strainer of some kind.