You don't build a pretreatment plant if you're only using a fraction of its capacity. You'd instead just ship in pre-treated methane by pipeline or by truck.

But that's not what SpaceX is asking for. SpaceX is asking for their own, private, pretreatment plant. For... some reason. (The underlying theory is: SpaceX seems to be trying to mine their own natural gas)

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EDIT: And if the power plant was expected to be operating at only a minor fraction of the year, you'd expect something in the PEA to note that fact.