Well, there's a very small sewage farm about 400 m from the house we just bought! It is really not a problem. I know that the part of the village upwind of it sometimes gets a whiff, but nothing terrible. We've driven and walked past it loads of times now and you honestly only smell a small whiff right outside it. The prevailing wind seems to be in one direction (away from our new house) and I did ask in the village, and also asked the vendor, who seemed genuinely surprised it even came up. Obviously she's not a source of disinterested information...
Anyway, it's not always a nightmare. (I'll let you know in a year's time!)