>suddenly trillions of sewer bacteria get exposed to a low dose and suddenly start developing resistance.

I think you're misunderstanding how resistances work.

Bacteria don't "start developing" resistance, if they survive the antibiotic at all they already had resistance by stroke of luck. The surviving, resistant bacteria will subsequently survive and outnumber the dead, not-resistant bacteria and we generally call this "developing resistance", but the resistance itself is down to a simple question of whether it was already there or not.

Resistances do not come to existence after bacteria come across an antibiotic. If resistance is there, the dosage won't matter because the resistant bacteria will survive regardless.