Jane Weeks, it must be code differences but weren't you allowed the option of a tank + drainfield combination? I've seen locally that more people are being "recommended" by the county to put in pump + mound + drainfield systems in order to protect water quality better.....somehow....in someway that I've not been properly informed. (??) Our local pump rate is $175 USD per tank and the drainfield allows for some extra time before that needs to be done. As Travis noted, anything to get that second tank under some kind of insulation. Around here (south and west of you on the other side of Minnesota) in low snow years, many seem to pile up straw bales on the tank which seems to help. With regard to grey water, we keep the kitchen sink still going to the septic tanks, but remember that any other sink/drain can, at a future time, be "judiciously redirected" as grey water with a little help from a Sawzall and some assisting friends with the know-how.
The water table here is too high for any kind of drainage system. If I had the money for a regular septic system, I'd have to have many, many truckloads of soil brought in (difficult to get to) and have a new well drilled as well, as it would be too close for modern laws. I've piled lots of straw on top of the second tank and I'll keep my fingers crossed when it gets really freezing. I hope that in the future I can redirect the grey water, as you say "with a little help from my friends."
I realize I’m late to this discussion, but since it’s that time of year again...
Trying to thaw pipes is always more hassle than keeping them from freezing in the first place. When the pipe is installed, the trench needs to be well compacted on the bottom. Any different and you could have settling which will create a low spot for water to sit (and freeze) in the pipe. The trench needs to be sloped, the more the better for greywater (NOT true for septic, but that’s a different discussion). The trench should have good drainage, so that water is not pooling outside the pipe, which will conduct heat away from the pipe. Dry gravel will not. Again, it is critical that the bottom of the trench is all on the same plane with no low spots. Also critical to avoid driving over the pipe area with anything heavy, as that can cause a low spot to form.
The deeper the pipe can be buried the better. If you can’t get it below frost line for your area, insulate the sides and top of the pipe, but not underneath. You are trying to trap the earth’s heat, which is below the pipe. (If for some reason [bedrock, etc] you can’t bury the pipe deeply then it needs to be encased in insulation a few inches thick). Rigid pink or blue board insulation should be used. The configuration can either be an upside down ‘U’ or ‘V’, with the main thing being to insulate as wide an area as practical. The deeper the pipe and the wider (and thicker) the insulation, the less likely it is to freeze. Last, it’s good to create a slight mound over the trench, so that ground water is directed away, rather than soaking into the backfill.
If you have freezing pipe issues, it would be well worth excavating and re-burying the pipe properly. And running a heat tape the length of the pipe, ‘just in case’.
My final update on this subject: Last winter I piled lots of straw on top of the holding tanks and everything was fine; no freezing. There's now some more soil on top and I'll add more straw shortly. By next autumn, I think they'll be fine without adding more, but if necessary I will. Thank-you so much for all your suggestions.