Many of these ink particles are too big, apposed to white blood cells. The white blood cells are why tattoos fade over time, but won't completely disappear naturally.
Imagine, in comparison, that we as humans are a single white blood cell, trying to engulf an ink particle the size of an elephant.
For the white blood cells to be able to 'eat' at the ink particles, these paticles need to be broken up and made smaller. This is where Valhalla steps in.