If you're using the exact same universe, then it's all pretty easy to explain since you can basically handwave it the same way K6BD handwaves it - the demons have a functional society based on their own systems, so society doesn't just implode on itself.
K6BD gets away with it as long as there's action going on, so people don't necessarily question how demons, angels, and humans have lasted this long, they're basically hitched to the ride.
But let's say we put aside the K6BD foundation for example and just look at population logistics for a bit. In a standard city, waste is either diverted away from the city, or destroyed at a cost. Naturally for things like water and such there's details like filtration and recycling and settling ponds and so forth, but the idea of "what do we do with waste" remains the same - if you can't recycle it, remove it, if there's nowhere to remove it to, destroy it.
In a very dense city like Hong Kong for example, landfilling in the outer territories is a pretty standard go to. Having the sea nearby makes water wastes relatively easy to handle, provided the treatment facilities are functional and the factories don't just end up dumping waste into the sea again.
But there's eventually a threshold you hit where either your population produces waste at a rate that your facilities can't handle, or you run out of space to handle that waste - essentially a breaking point.
Sci-fi solutions generally tend to lean towards technological advancements to handle this, fantasy has magic to fall back on or just relatively small populations, it's plausible in K6BD that Throne simply diverts waste into somewhere else in the multiverse.
The "duality" of factions in K6BD is something that, since this is a setting with beings that aren't wholly human (and plenty of immortals), makes sense in setting. On the other hand, dual occupations aren't too uncommon either. In most cases it's a matter of mindset.