It is a very common symptom that once you install i3 window manager in Ubuntu, the Unity notification is getting replaced by i3 one which is called
dunst. It is daemon service which starts at system booting time.
This is very unpleasant at least for me. It looks ugly in Unity and besides notification of volume changes will not be shown anymore.
The best and easiest approach is to purge dunst from the system. To remove it, run this command,
$ sudo apt purge dunst
After that, need to kill dunst process.
$ killall dunst
Last step is to verify that dunst is deleted,
$ notify-send “hello”
After executing the above command, you should be able to see that notification is shown using Unity notifier. If not, something is not right.
P.S. There should a more difficult approach to deactivate dunst daemon service as well, however, that’s a hassle and I personally prefer not go through the pain to do that. For those who need notification in i3, this might not be the best approach.