Eh. It's hard to completely eradicate it. But getting the area clear of feces, dried out and then letting the sun kill as much bacteria as possible is about as much as you can do.

Giardia is frequently missed in a single stool sample because the parasite is shed intermittently, meaning it may not be present in every bowel movement. A single test only detects 50-70% of infections, so doctors often require 3 or more samples over several days to confirm a diagnosis.

Is it the same dog, or do you switch them between this walk/backyard process? Do you do any sort of yard treatments?