how do you remove humic acid from city water?

You can acidify the water and humic acid will precipitate out. Then
you'd have to bring the water back to normal pH. This may leave fulvic
acids in the water. http://www.ar.wroc.pl/~weber/kwasy2.htm It can
also be removed with standard filtration techniques (e.g., "reverse
osmosis," etc.).
It should be noted that humic acid is not necessarily a bad thing,
well, at least not to the environment. It chelates toxic metals, it
acts as a buffer system to prevent acidification/basification of
water, etc. http://www.hagroup.neu.edu/abouthafrm.htm