They fist sun-dried the garbage for one to three days to bring down the moisture level.

The moisture level was brought down by sun-drying the garbage for one to three days

One to three days of moisture level was brought down by sun-drying the garbage

The moisture level brought down by sun-drying the garbage for one to three days

The garbage was first sun-dried for one to three days to bring down the moisture level

None Of The Above

Active Voice: They fist sun-dried the garbage for one to three days to bring down the moisture level.

General Rules: The garbage is the object here.

The passive sentence should be started from object.

Passive Voice: The garbage was first sun-dried for one to three days to bring down the moisture level