The option you're looking for is in sandbox settings under the World tab. Relevant options are as follows:
World Item Removal List: delete everything in this field. This field should be completely empty.
Hours for Removal List: set this to whatever time frame you want things to be deleted in (in game hours). For example, if you want everything on the ground to be deleted every 3 game days, you would set this to 72.0
Removal List as Whitelist: enable this option. This means that every item that is not in the "World Item Removal List" will be deleted.
Using the above settings, this would configure your server to delete everything on the ground every 72 game hours. Don't forget to pick up stuff you want to keep, or it will go poof.
Good luck