Each ozonization loop (1) which includes a compressor (2), an ozonizer (3), and a heat exchanger (8) for cooling the oxygen-ozone mixture, is associated with three cylinders (4,5,6) filled with silica gel. Each cylinder operates in succession in a cocurrent adsorption, in countercurrent desorption by a substitution gas, and in scavenging by taking off a part of the flow circulating in the loop. The scavenging flow is recovered and combined with the substitution gas. The substitution gas may be in particular impure nitrogen supplied by an air distillation apparatus producing the necessary oxygen.