Disk Utility (Yosemite): Erase Free Space Options dialog
When you empty the Trash, OS X deletes the information used to access the files but doesn’t actually erase the information on the disk. Using the Erase Free Space Options, you can completely erase space used by deleted files so they can’t be recovered by a using a disk recovery app.
Writing over the space three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the space seven times meets the U.S. Department of Defense 5220-22-M standard.
Overwriting the free space several times can take some time. After the process begins, you can interrupt it at any time without harm.