I'm wondering what happens if starter gets in the septic tank. Do any
of you think it's doing anything in there?

no it will die.

Septic tank bacteria is anaerobic i.e.no oxygen. The sourdough starter
contains yeast and bacteria that are aerobic organisms, requires oxygen.

The pleasant smell of a well nutritioned sourdough starter is the opposite
of the unpleasant smell of biogas from anaerobic bacterial fermentation.

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CK