You can wipe your drive with:
dd if=/dev/zero of=/dev/[device to be wiped]
dd if=/dev/urandom of=/dev/[device to be wiped]
repeat 7 times per device if you want the DoD standard for wiping a drive. If you would like to save time/trouble drop the sequence in a script. Plenty of examples on the web. One pass is usually good for most ppl tbh. Though I did do a recovery once that rebuilt the last 3 OS installs on the server... but if you trust the person you are giving them too not to care then one pass should do.