Modern septic systems use pumps. They pump the excess liquid out into the drain field and the solids are what settle and have to be pumped out of the first tank every so often. We have a large system and our first tank is nearly 1300 gallons. Eventually, it will be full.
City systems should be similar. At some point, no electricity is going to mean back ups.
We always use the porta potty during floods and electric outages. Having had a major backup, it is better safe than sorry. It is a hell of a clean up job and then, you need somewhere to dispose of the contaminated clean up water.
We had an outhouse, once. We could do it again.
That explains alot. In my area, we use no such power assists for our septic systems. We only use pumps to charge the systems, gravity and rid-ex take care of evacuating them.
I thought everyone did it that way.