Why should the free space on a hard disk be wiped?
When any sort of file is creating in the system, a certain hard drive space is assigned to that file. That memory space is unique to that particular file. When the file is deleted, it would be transferred to Trash. When the Trash is emptied, we get rid of the file effectively.
The hard drive space that was once assigned to the deleted file remains in existence. As long as this still exists, it can be virtually recovered by skilled technicians or hackers. This is one of the principles of data recovery software. If that information is sent to Trash to be effectively removed forever, wipe free space software should be used.
CCleaner currently is able to do 3 types of wiping: Zero Out, 7-pass erase and 35-pass erase. Any of those methods will generally make impossible recovery using commercial grade software recovery. The number of erasing passes is relevant just for very sensitive data that could be hardware recovered by specialized laboratories in a controlled environment using direct plate reading even from defective hard drivers. But in this case, it is better to use dedicated erasing software that include DoD and other sophisticated algorithms.
It can be installed like any usual application, with a simple drag to application folder.
In the tools tab Erase Free Space button is found, as seen in this screenshot.
Choose the hard drive volume of which you want to wipe the free space. After pressing the Erase Free Space button. Depends on the size of the disk, could run for few minutes or for hours.