slow sand filters in freezing weather: update

All but two of the filters have been frozen solid for 2 weeks (15 days), with the exception of the two filters that purify shallow well water. The water that runs through them has been running continuously and was still running as of last night (Jan 20). These filters are outside where the temps have been 25 degrees F and lower (down to 20 deg. F) for the past 15 days. There has been some warming during the daytime, but not enough to thaw out any of the other filters. The freezing weather started affecting the filters on the 3rd of Jan 2013 – they would freeze at night and run slowly during the day. The two filters (filter 2 and filter 3) that run on shallow well water have been running non-stop, and were not slowed down by the freezing weather (the water underground is about 48 degrees year round). When the freezing weather ends (possibly by next week), we will do a test to determine how well the filters function after being frozen for an extended period of time, after they start flowing again – that may take another week following the end of the freezing weather. As of Jan 25, all of the filters are now thawed out and flowing. The temp got up to 47 degrees F here today.