As it's near the treatment works, I'd say it's a vented man access shaft to the outfall sewer possibly..
Without looking further into this, which I have no immediate inclination to do so, I'm struggling to come up with A.N Other sensible answer.
That type of vented hard standing feature is usually associated with 'other' utilities. Maybe do a bit more research as to what else runs below ground in the vicinity, it's all part of the game