Betterments can be for many things. However, once a betterment is "attached" to a property, the properties owner is responsible for paying it. So if Sewer comes down your street, and there will be a betterment of $5000 if you don't hook up, and $6000 if you do, you're on the hook either way. I will say this though: Seriously consider hooking up.
1) Remove the risk of having a septic failure and those future costs.
2) Your home will likely increase in value hooked up to sewer. Not much, but savvy buyers pay more.
3) Betterments can be borrowed at great rates, typically very low interest, supported by the town. Oftentimes, that option doesn't exist for septic failure.