Finally got our septic mess fixed. I thought I'd do a short post-mortem here in case the info is useful to anyone.
...We have a complex system. The septic tank drains into another tank, which has a sump pump to send the liquid uphill to the drainfield.
Well, it started backing up a few weeks ago -- toilets wouldn't flush, gas bubbled up through drains. ...
The septic tank needed pumping, but the primary problem was a strainer that prevented solids from exiting the septic tank into the separate pump tank. The strainer was totally clogged. Problem was, I didn't even know this device existed, or required maintenance. So the moral is, if you have a complex septic system, find out everything about how it works. This is especially an issue if you buy an existing home that already has such a system (as we did).
...Got it pumped. ... It took us about 10 days to fill up all the tanks again, during which we had exceptionally cold weather. ...
That was pretty stupid of us, since the cold weather was in the forecast.
There is a check-valve at the pump to prevent backflow. So the pipe leading uphill to the drainfield is always completely full of wastewater. We learned today that, in places, it is only 3 inches below the soil surface! (Incompetent installation.)
So, yes, it froze. And when the pump came on, it ran for days against the blockage, until the high-level alarm finally alerted us to the problem. By that time the pump was dead, and had to be replaced.
Which didn't fix things because, unknown to us, the pipe was still frozen. An amazingly skillful plumber came out this afternoon, tried snaking out the line from both ends, discovered the ice clog, thawed it with a torch, and... no, that didn't go smoothly, because the pipe split. But he got the break repaired, and we are back in the civilized world. Yay!
So one obvious lesson is to time repairs so as to avoid leaving standing water in pipes during frigid weather. But the real problem here was that some idiot didn't bury the pipe below the frost line. No easy way to know about that when you buy an existing house, unless you want to do a lot of digging.
Our plumber is contemplating a modification to the system that would allow the pipe to drain back into the pump chamber after the pump shuts off. If he can work out a way to do this that doesn't risk siphoning out the entire drainfield, we'll probably go for it. Plus I'll be plowing a lot of soil over the top of the pipe, for a little frost protection.