This has been a studied and talked about topic within our county over the past few years.  St. Louis County, after the first recent season of septic system freezing problems conducted a study to determine the causes of the freeze ups.  It was found that one of the major reasons a septic system freezes, particularly a newer system, is due to poor construction practices.  Another reason is due to lack of insulation in areas that are shallow and or prone to freezing due to any type of traffic (foot or vehicle).  I have heard people claim that mound systems don't work because they freeze in the winter.  The reality is is that if the entire system is designed and constructed properly, it is very unlikely that there will be any problems at all.