The county health department says every 2 to 4 years, which seems
like an awfully long time to me. ...

We pump ours every 3 years (2 people). Every 3 months we add two cakes (or
packets) of yeast in one cup of sugar water, The latter keeps the "beasties"
happy. This all is per the recommendation of the installer and pumper.

I used to get industrial plant trade journals
and was amused by the ads for custom-modified bacteria for waste treatment
and how "our bugs are better than theirs".
I'm unsure if they're available for home use.

I eat yogurt with active cultures to keep my "personal bacteria"
healthy and replenished, so it sounds reasonable to me to
aide/maintain the septic tank's bacteria too.

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-- mejeep deMeep ferret!