  The Jiangmen Underground Neutrino Observatory(JUNO) is a state-of-the-art
liquid scintillator-based neutrino physics experiment under construction in
South China. To reduce the background from external radioactivities, a water
Cherenkov detector composed of 35~kton ultra-pure water and 2,400 20-inch
photomultiplier tubes is developed. Even after specialized treatment,
ultra-pure water still contains trace levels of radioactive elements that can
contribute to the detector background. Among which $^{222}$Rn is particularly
significant. To address this, an online radon removal system based on the JUNO
prototype has been developed. By integrating micro-bubble generators to enhance
degasser's radon removal efficiency, the radon concentration in water can be
reduced to 1~mBq/m$^{3}$ level, meeting the stringent requirements of JUNO.
Additionally, a highly sensitive online radon concentration measurement system
capable of detecting concentrations $\sim$1~mBq/m$^3$ has been developed to
monitor the radon concentration in water. In this paper, the details regarding
both systems will be presented.
