For the most part, they don't. Virtually all sever systems are populated by rats. Watch any manhole during a hot summer's day from afar, and you won't have to wait long to see a rat.
Some factors like average flow speed and more slippery pipes, toilet cleaning chemicals, but also changes in how sewers are used (e.g. no longer for food waste disposal) have lowered rat population sizes - and Hanoi being a tropical city probably had some effects as well.
Some factors like average flow speed and more slippery pipes, toilet cleaning chemicals, but also changes in how sewers are used (e.g. no longer for food waste disposal) have lowered rat population sizes - and Hanoi being a tropical city probably had some effects as well.