When I lived in MA many years ago, the condo building we lived in had a septic tank and leaching field. Worked fine until the jackasses at the town hall allowed a builder to fill in the wetlands behind us and build houses back there. After that, there was no place for the water to go during heavy rains, so the back yard of the condo building flooded. As you can guess, if your leaching field and tank are under water, your drains don't work. We lived on the ground floor closest to the septic system, so when it was flooded and people upstairs took showers, that water would bubble up in our bathtub. It was awful. We finally got the building connected to town sewerage.
Our current house has a septic system with two tanks, an aerator, and an outfall pipe. The system works with bacteria in the first tank that is supported by the aerator. It breaks down almost anything. The next tank lets any remaining solids settle out. That tank has a 25' outfall pipe at the top where supposedly clean water then flows out into the vast woods behind our house. I've seen numerous animals like deer and raccoons drinking the water coming out of that pipe, so it can't be too nasty, I guess. We just have to be careful not to use anti-bacterial soap or flush anything that might kill bacteria or clog the aerator.
The other thing to think about with septic vs town sewerage is if the water cycle in your area is an open or closed system. We live above the huge Midwest aquifer, so our water comes out of the ground under us (and several other states). We don't have a well, but the local water utility company has wells that pull water from the aquifer and supply it to us. Because our septic system then returns the water we use to the ground, it eventually finds its way back to the aquifer. That's a closed system and is self-sustaining. We literally can't "waste" water.
If your town or city sewerage system dumps water out into the ocean, like many coastal communities, then that is an open system. Your water comes from a lake or river or aquifer, but once you use it, it's lost out to the ocean. That type of system can experience water shortages if your supply side doesn't keep up with usage. Cape Cod in MA is always walking that tightrope. They literally have a finite supply of water under them and their sewerage system treats the wastewater and pipes it out into the ocean. Luckily they get enough annual precipitation to meet demand. But you can imagine they're always worried about over-development or a nasty event that contaminates the aquifer.
Your community may have a sewerage system, but if there is a river between your house and the sewerage treatment plant, you usually can't connect to the sewerage system for obvious reasons. In some larger cities, they run sewerage lines across rivers via bridging structures. But smaller communities may not be able to afford such infrastructure or there may be environmental reasons preventing it.
TLDR; properly installed septic systems work. The ones that fail are either installed improperly, poorly designed, or have been affected by nearby development. There is a newish (10-15 year old) neighborhood near us that has septic tanks and leaching fields. In the summer you can clearly see where the leaching field pipes are because the grass grows thicker and greener over the pipes. I wouldn't be happy paying $500k for a house where the leaching field was making patterns on my lawn.
That ends today's lesson on septic systems. There will be a quiz later.
