"Not sure if there are other factors that make centralized waste treatment systems preferable over individual septic systems"
Capacity is the main thing I can think of. If you get dense enough (e.g. 8-story apartment blocks) you'd need a pretty big septic field right next to the housing. Now that's land you can't use for housing or offices. At some point it's more cost-effective to whisk that waste away to some low-value land to do the processing.
I don't know at what level of density that's true, though. It could be pretty high.
Yeah, density was the main concern that led me to make my original comment in this thread. I was thinking of a more suburban setting when I wrote the comment you're responding to, but you do have a point (unless local zoning laws allow septic drainage fields to be used for other things, but that I know too little about). If you can't build on top of drainage fields or dual-purpose them, then you'll need to sacrifice some building space, which isn't optimal from a make-the-most-of-a-land-parcel view.
Capacity is the main thing I can think of. If you get dense enough (e.g. 8-story apartment blocks) you'd need a pretty big septic field right next to the housing. Now that's land you can't use for housing or offices. At some point it's more cost-effective to whisk that waste away to some low-value land to do the processing.
I don't know at what level of density that's true, though. It could be pretty high.