I've started rendering the animation frames of the sewage flow, and so far it's nowhere near as bad as I expected. It takes less than 8 hours to render a frame (but it's just the sewage flow being rendered there). The whole picture (which makes the base layer) will take much longer to render - probably 2 to 3 days. I'm using 3000 samples. Usually that number is overkill, but in my case it is necessary because a/more samples are needed when you have materials using "glass" type of shaders (and the sewage flow is just like that) b/ more samples are needed in scenes where the light is dim. My case is a happy combination of both situations. After 3000 samples, I don't even have a noiseless image of the flow, but in our case it won't be really noticeable because it'll be animated.