A connecting device comprises a buffer chamber (2), a first connecting line (3) between a liquid source (4) and an inlet (5) of the buffer chamber, a second connecting line (9) between an outlet (10) of the buffer chamber and the medical treatment apparatus (11), two valves (7, 8) arranged in series on the first connecting line, a pump (12) on the second connecting line, two liquid level sensors (15, 16) operating in the buffer chamber, a vent valve (18) connected to the buffer chamber. The liquid source is a centralised distribution plant of dialysate, while the medical treatment apparatus is a dialysis apparatus. The invention prevents back- contamination of the centralised plant with disinfectant sourced from the dialysis apparatus.
