Exactly: all the sewage systems I know of process it enough to dump into a sea or river, where even if a downstream town draws drinking water from it the diluti

Exactly: all the sewage systems I know of process it enough to dump into a sea or river, where even if a downstream town draws drinking water from it the dilution with other water plus additional environmental chemistry reduces the purification problem. In Singapore's case the sea is the obvious sink for treated sewage, which kicks it back to desalination. Except that I believe that Singapore's water comes across the causeway to the mainland in a pipe.