Battlefield Methane Tanks
From Halopedia, the Halo wiki
Battlefield Methane Tanks are equipment used primarily by Covenant Grunts.
The Grunt's homeworld, Balaho, is a methane planet and as such they breathe methane gas. Thus, when they are in a rich oxygen atmosphere, like that of the Halos or Earth, they need to wear a methane gas tank. When Grunts are in battle, they may run low on their supply of methane, so they must have a ready supply nearby if required. These tanks contain the ready supply of Methane Gas the Unggoy need.
They are short and round in shape, purple in color and have a large blue valve on the top. The Methane Tanks are seen generally where Grunts are operating. They appear in Halo 2[1] and Halo 3.[2]
The tanks are also highly explosive due to their methane content, but it is unknown as to why they are not more armored, if they are on the battlefield.
