Septic tanks in NYC? Yuck, unless perhaps they're in the nether parts of Staten Island.
Actually, I'm surprised that they're exempt from the sewer fee. Around here upstate, any property close enough to a sewer line to hook up pays the sewer fee whether they hook up or not, on the theory that there's a public benefit to getting everyone's poo out of the groundwater.
We don't have any people with wells and sewer service, but some of the other villages nearby do, and they have meters on their own wells to compute their sewer rate.
R's,
John