The design, construction and performance of the system capable of purifying 65m3 of liquid argon to sub-ppm level designed for LEGEND – 200 experiment is presented. The quality of the purified liquid argon is monitored in real-time during the purification process, by measuring the argon triplet state lifetime and simultaneous direct measurements of the concentrations of impurities such as water, oxygen, and nitrogen with a sensitivity of 0.1 ppm. The achieved argon triplet lifetime value measured inside the LEGEND cryostat, when filled in 70% of its capacity, was at the level of τ3 = 1.3 μs. If needed, the system may also be used later to purify liquid argon already filled into the LEGEND cryostat in the loop mode.