how to take adapter off

I am trying to see if anyone has any idea of how to take this adapter of ?
is there a special wrench for this adapter or is it treaded ?

Looks to me as though you need a C-Spanner. There should have been a suitable one with the machine.
Failing that, a large adjustable plumbers pipe wrench could be used on the knurled bit. Unscrew anti-clockwise I THINK - assuming the coupling comes off upwards.
Good things, pipe wrenches.

Cheers
Roger