There is only a small number of ways to ensure that you computer are properly recycled or more specifically there is only a small number of ways that your confidential data can be securely destroyed.
There is the over writing of the devices and this will allow you to use the machines again or possibly give them to charity. This is quite common with computers that are used for something that is not considered as needing to be highly secure, for example running car parking systems or livestock tracking.
The more secure way to deal with redundant IT is to have it degaussed – effectively this will use a strong magnetic current to scramble the magnetic storage at a basic level. This is considered to be secure and is often used on highly confidential data carrying devices. However the device will no longer be of any use as a computer or data storage device.
The third way is to have the hard drives or the other storage devices shredded with industrial shredders. These machines will grind the metal hard drives into granules and this will put it beyond use or any type of recovery. Some security forces will use degaussing and then industrial shredding. This is a double secure system and gives total peace of mind.