Antimatter waste

From Memory Alpha, the free Star Trek reference.

Antimatter waste (also known as contaminated antimatter) is a byproduct of inefficient or primitive antimatter reaction techniques. As such, antimatter waste is nonreactive with normal matter and emits highly hazardous theta radiation, necessitating disposal as a toxic waste.

Dejaren, a Serosian HD25 Isomorphic Projection, was assigned to cleaning the reactor core and ejecting the antimatter waste from his ship. (VOY: "Revulsion")

By the 24th century, Federation science had developed radiometric converter technology which can recycle antimatter waste, absorbing the theta radiation and converting it into energy and purifying the antimatter back into a usable reactant.

The Malon under the strong influence of the waste export industry still produce large quantities of antimatter waste, which is then transported – along with other industrial byproducts – out of their territory by Malon export vessels, to be dumped elsewhere. (VOY: "Night", "Juggernaut")