Sterilising apparatus (50) comprising: an autoclave for sterilisation or steam disinfector (20), comprising a sterilisation chamber (21), an inner tank (22) for water, inside said autoclave for sterilisation (20) and heating means (23) of water, an outer tank (1) outside the autoclave for sterilisation (20) wherein the outer tank (1) comprises a purification filter (10) suitable to purify water contained therein by the mere passage of said water through filtering layers.