After arriving at the scene, police cordoned off the area and told neighbours to evacuate their homes as a precaution.
An X-ray by an explosive ordnance disposal (EOD) team revealed that there was powder inside the grenade.
The device was taken to a field to be safely detonated. “We heard the explosion,” Mr Williams said.
After contacting the supplier, he said they claimed the grenade had sand inside, but the EOD team found no signs that it had ever been opened.
Mr Williams said he was upset about disturbing his neighbours and wanted to reassure them that nothing in his collection poses a risk to them.
Dyfed-Powys Police said it responded to a call over the suspicion that it was “a live military grenade in his possession”.
A spokesman said: “Officers attended with an explosive ordnance disposal team and the device, a small inert hand grenade, was destroyed in a controlled manner.”