Very few cities <i>don&#x27;t</i> &#x27;just dump the water back&#x27; - that&#x27;s actually the way it works outside of a very very small handful of places.  The reason for these treatment plants is so that the water that gets dumped into the environment is clean.  Well, pretty clean - there are still things that aren&#x27;t removed today with even the most advanced treatments, such as hormones&#x2F;contraceptives...