WordPress trash can be deleted manually by clicking on the Empty Trash button. But you can also automate the process. This can be done using the wp-config.php file. This file can make a decision after what interval the WordPress trash needs to be cleaned. 

By default, WordPress cleans the trash after 30 days. To change this interval, open your wp-config.php file. This file is located in the root directory. 

After you have opened the file, search for the line saying;

define( ‘EMPTY_TRASH_DAYS’, 30 ); ​

You can change the number from 30 to any number. By doing so, the interval will change. For example, you can change it to 5 days by changing the number to 5.

define( ‘EMPTY_TRASH_DAYS’, 5 ); ​

In order to disable the trash, you can change the value to 0. So the code will become:

define( ‘EMPTY_TRASH_DAYS’, 0 ); ​

It is not advisable to change the value to 0 as you can lose your important entries. So change the number to a minimum value, so that the trash gets removed on a regular basis.

That's it. This is how you can delete your WordPress trash automatically.