Art Todesco wrote:

I agree fully, however, treatment plants primarily remove solids.

?????

Actually, they remove biological waste as well as solids. All those
dissolved nutrients that end up killing things. Solids removal is only
a tiny fraction of the pollutants they remove, usually done in a
small-ish tank or filter at the head of the plant, right where the main
line comes in.

I wonder what they do about acids?

pH neutralization is part of the process.

I have seen the local treatment plant put out a clear
liquid into the
muddy river. It looks good, but I really can't tell what's in that real
clear output.

Ask for the DMRs; they're available under the FOIA. (And if you don't
know what a DMR is, educate yourself on the regulatory process for WWTPs.)