(You do realize that there are people who will dump 100mg nicotine down the toilet.....into the wastewater system, right? (I can't iimagine doing this on a septic system eeks.) These are things that concern me greatly since wastewater is piped to a central sewage plant, then discharged into area rivers, lakes and streams. Most municipal systems rely on bacteria or other organisms to decompose the waste, and some pass through the system unchanged and thus pollute the water downstream. )
Per CASAA nicotine is only slightly more toxic than caffeine. In the amounts in normal e-juice, that is probably about like dumping coffee grounds, or unused ground coffee. (And, yes, it is possible to get a fatal caffeine overdose, it's just freakin' difficult if you start with beans....)
Or composting cuttings from an ornamental nicotiana plant. And unlike heavy metals, it is an organic compound that probably breaks down, though if someone knows better please correct me.
It is true that we are supposed to take our used pesticide cans to the toxics recycling center rather than throw them in the trash, but I bet no more than about 40% of households do that, which means if you pour a 10 ml bottle of e-juice into the sewer system, you've maybe done about .1% of the harm as someone down the block probably did last week, and someone else will do next week, and if you put it into the trash (wrapped, like we are supposed to wrap flea collars and empty flea-treatment capsules) it is probably about .001%.
And a study of EXHALED vapor shows no nicotine effects on experimental subjects that were locked in a room with vapers for hours.