Sewage treatment plants are not generally known as water works. I hope this is a reporting error.
The stoppage happens before the waste water enters the treatment plant.
Usually at the confluence of several main or intermediate lines.
Usually in an area with older lines that has been built up with apartments, condos, or restaurants.
Many years ago I was involved in developing equipment used by the epa to detect contamination, we went to the pier and tested the water, the nitrates and nitrites were through the roof, traced the water flow back to a new sewage treatment plant which crossed the pipes and was dumping sewage in the water system..how subtle