Shut off the supply and drain as best you can. Low spots will freeze but there should be enough expansion space to keep from breaking lnes. Flush toilets then use a plunger to push as much water out of the bowl that you can OR use RV antifreeze. It's non toxic and won't hurt your system. Also make sure you pour a little down the sink traps.
I don't think regular antifreeze will hurt your septic system.
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