You are? You’re surprised they wouldn’t take a film crew and multi-million dollar talent to a landfill to shoot a scene where they roll around in the trash? For one, production insurance would never cover it - too many potential hazards. Real landfills are full of disgusting, wet, rotting shit, sharp objects, possibly harmful chemicals (if improperly disposed of), etc. Just not a good idea at all. Plus I bet the art department had fun making all the “faux garbage”.
Well, presumably they’d put the faux-garbage at the real landfill, so you could roll around in it and still have a realistic background.
You know that’s not a real office, right. . .? 
That’s kinda what I meant. I’m sure they could just toss a load of cardboard, and old sink, some dummied up garbage bags and a big wooden spool at the edge of the landfill. It wasn’t an insignificant amount of time they spent there. It was long enough that plenty of people noticed it.
Did you know that Reno 911! uses a real (actively in use) police station. . .?
good episode, great compared to most of what this season has given us.
Favorite bit was Michael soaking his pants with rain water from the sink in the dump.
Wasn’t a lot of the garbage dump stuff (at least the Andy and Erin part) set during sunset? Because that would explain the green screen. Shooting during a real sunset is very impractical. You only get a brief period of pretty sunset light, generally not enough to get all the takes the director wants from a single angle, let alone all the shots he wants. If you don’t want to shoot the scene over the course of several days (which is very time-consuming and expensive) you have to go with green screen (which looks fake but at least you can take all day if you need to).
Did you know that it’s shot in L.A., not in Reno? 
That’s basically what they did with the green screen - I’m assuming the background image was an actual landfill. It’s just a lot easier to shoot at home base when possible than send the whole crew out to location (besides the logistics, shooting on location can be several times more expensive than shooting on soundstage). Not to mention a landfill stinks like hell.
Dwight (to Michael): “The acorn becomes the oak.”
“And sometimes the acorn just stays an acorn. Just check my gutters.”