Schematic view of the TRD gas system. The gas circulates in a closed loop pushed by a compressor. The flow for each supermodule is determined by the pressure set at individual pressure reducers in the inlet distribution modules. The overpressure is regulated with individual pneumatic valves at the return modules. The gas is purified at the surface and, when needed, supply gas is mixed and added to the loop. For the filling and the removing of the expensive xenon, semipermeable membranes are used to separate it from the CO$_2$. The recovered xenon can be treated in a cryogenic plant in order to remove accumulated N$_2$, prior to storage.