Abstract: Apparatus for disinfecting water to make it virtually microbe-free and maintaining it in a continuous microbe-free condition for use at a center for dental instruments and the other apparatus by ozonized water. Water is ozonized and provided on both the dentist's side and dental assistant's side of a dental unit which includes a variety of dental instruments such as drills, syringes and an expectoration bowl. At least one water reservoir is connected through fluid conduits with such instruments. An ozone producing device is connected with the water reservoir and the ozone level in the fluid conduits between the water reservoir and the instrument center and apparatuses is controlled so that the ozone level at the water discharge openings at the dental instruments and other apparatus is zero or substantially zero.