I'm curious why this would be worthwhile: since Singapore is an island conventional desalination would seem easier than either coping with the witches' brew of sewage (random amounts of random chemicals) or the expense of plumbing urine separately.
But it's not just sewage though, it's chemicals as well, e.g. from medication, plus whatever else gets flushed.
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.