There is an environmental downside to septics too. Where I grew up there was a sandy aquifer that caused nitrogen loading from septic run off in ponds and marshes. This attributed to algea growth, oxygen depletion and marine species die-off. There is now an extensive effort to sewer the region.
It's not quite that stark. I live in a population ~7K ex-urban town. I have town water. I do have a septic tank although I believe a portion of the town near the center has sewer hookups.
