Are we refering to the hole just below our wash-basin or the hole in the toilet ? Anyway both are drainage systems pipes that carries only waste water from basins and bathrooms.

These waste water goes down to the manholes underground. I belief PUB collects the waste water and treat them for use as newater for industrial use. ( Not sure if they are bottled for drinking as newater ).
Probably those insects crawling in and out of those pipes are actually living in the piping systems running from the ground to the 17th floor ( for insects that can live in water ). Really eradicating them may only be temporary, I guess.