Your question is actually based on a misunderstanding of how deleting files works (not a criticism - many people get confused by this because the process can be different based on OS and the type of storage device).
In a nutshell, if your only concern is that you want to free up space on your startup disk, then emptying the Trash will have already done this (regardless of how you emptied it). This is because emptying the Trash means the OS no longer has a record of the data you emptied, and this means wherever the data was stored is now free for the OS to overwrite with new data. In other words, that space is now available (free) for you to use.
A bit more detail
Your comment that you emptied the trash "non-securely" actually has no bearing on free disk space. The option to securely erase data basically means that the data is actually overwritten with random data. This makes it a lot harder to recover the original data (hence the term secure) but it's still writing data to that space. However, once complete, as with a normal empty trash the OS has no record of that space being used and therefore it is now free space.
Trash keyboard shortcuts
The following keyboard shortcuts may be of interest to you:
- CommandDelete - moves any selected item(s) to the Trash. (If you've selected an item already in the trash it'll then put them back to their original location.)
- ShiftCommandDelete - Empties the Trash.
- OptionShiftCommandDelete - Empties the Trash as above, but without a confirmation dialog box.