The main background for the XENON1T experiment are the intrinsic contaminants krypton and radon in the xenon gas. Instead of purifying the xenon once before starting the science run we were able to operate our distillation column in a closed loop with the XENON1T detector system running during its commissioning phase. This resulted into reducing the krypton concentration quickly below 1 ppt (parts per trillion, 1 ppt = 10^(-12) mol/mol) without emptying and refilling of the detector.
In addition, the column was operated in the same closed loop in inverse mode in order to reduce Rn-222 by about 20% during the first science run.
This so-called online removal for both noble gases along with the working principle of the distillation system are presented within this talk.