"erik" wrote in message ...
But city water is chlorinated and should be bacteria free.
The carbon prefilter removes the chlorine so the water reaching the
CTA membrane should be chlorine and bacteria free.

Any thoughts? Is my logic incorrect?

Logic would be correct if you did not underestimate bacteria.
Exchange "city water is bacteria free" with "city water has
less bacteria than well water". City water is NOT sterile and with
no chlorine presence bacteria will quickly populate your membrane.
Remember, bacteria multiply much, much faster than rabbits... ;-)
But hey - your father is the expert in this subject so do not listen to me.
If bacteria chew up on your membrane he will grab another for you ;-)
Just check your tds readings every month :-))