terry wrote:

Someone wrote?

Yes, but a water powered pump is still reliant on an external power
source.

Not clear on that! But not familiar with water powered pumps! What are
they; some sort of venturi jet?

Yes. Something like a 1:2 or 1:3 ratio of water pumped to water
consumed.

Our municipal water supply, is gravity fed.
The only thing that 'might' fail is the chlorine injection at the main
control station near the lake that supplies the town.

Gravity feed only lasts as long as the pumps filling the tank are
operational. Those tanks don't last very long if they aren't being
refilled.

In other words we never lose water pressure? And the main lines are
sized for fire fighting hydrants.
So would it not be true, in our case anyway; a water powered pump
would not be reliant on external power?
Curious.

Water mains have been known to break, be damaged by construction work
and otherwise fail. When that happens, if you are downstream of the
problem you don't have water, even if the rest of the town still does.
The duration of the outage is usually pretty long due to the need to
isolate / shutoff the water upstream of the damage, pump out the area to
get to the damaged main, repair the damaged main and then turn the water
back on and go around to nearly every fire hydrant purging the
contaminated water out. In some cases the flooding from a broken water
main can also take out underground power utilities.

So, yes, the water powered pump is indeed dependent of an external power
source, and it is also possible to loose both water and power at the
same time. Though the probability of both utilities failing at the same
time is very low in most areas, it is not zero.

Regardless of the type of sump pump and backup sump pump you have, if
you are dependent on the sump pump to keep your basement from flooding
i.e. the sump pump operates regularly you should plan "C" prepared. Plan
"C" should consist of at minimum a battery powered flood alarm with the
sensor located in the sump pit an inch or two above the backup pump
operating level, and a stockpile of wood 4x4s, milk crates or similar
that can be used to rapidly elevate valuable items above the soon to be
flooded floor while you work to move things to higher ground.

A flooding basement is a known, predictable hazard and you should have a
good plan to deal with it, just the same as a house fire, a blizzard, a
tornado or a hurricane as applicable for your area.