I can't say what they do in China but in the US it's not fed or swimming in sewage. That is, if you define sewage in the usual manner as waste water from domestic or industrial sources. Here the waste water is the outlet from an upper pond or ponds that are used to farm fish that need clean, cool water to thrive. The "clean water" fish require a high concentration of dissolved oxygen, and DO goes down as the water becomes contaminated with organic debris (uneaten food and poo) and heat.
Tilapia, carp and your favorite Swai (never heard of it before) thrive in warm water with low DO concentration. Maybe it's because of their habit of gulping air (I don't know). And like all fish they eat just about anything that is floating in the water. So they are farmed in ponds that are downstream of the trout and salmon.