  Purification of the 278 tons of liquid scintillator and 889 tons of buffer
shielding for the Borexino solar neutrino detector was performed with a system
that combined distillation, water extraction, gas stripping and filtration. The
purification of the scintillator achieved unprecedented low backgrounds for the
large scale liquid scintillation detector. This paper describes the principles
of operation, design, construction and commissioning of the purification
system, and reviews the requirements and methods to achieve system cleanliness
and leak-tightness.
