The invention relates to a biocidal substance of type 1 or 2, to be used for human hygiene and for disinfecting private or public spaces, to an installation and to a process for preparing the same. According to the invention, the biocidal substance comprises water with a pH ranging between 0 and 7 and colloidal Ag in concentration of 1...150 ppm, depending on its purpose. The claimed installation comprises a water ionising apparatus (1) provided with electrodes (6 and 7) supplied with a current of 220 W from the rectifier (8), the positive electrode being introduced into a tank (4) of a volume V, and the negative one into a tank (5) of a volume V, by means of which the tap water (3) is separated into acid and alkaline water, respectively, an electrolyser (2) provided with two electrodes (12 and 13) of pure Ag, connected to the direct current source (14) of 9...12 V, two electronic pH-meters (9 and 10) whose sensors are mounted within the tanks (4 and 5) of the ionizing apparatus (1), controlling the pH values in the two tanks, and a semipermeable square-shaped separation membrane (11), which separates the two volumes, made of polyethylene having a porosity of 60%, at the most, an average diameter of pores of 0.1 μm and an electric resistance of 0.7 mΩ/dm, such that it allows the pH to be maintained constant in both tanks (4 and 5), due to the pore distribution. The claimed process consists in separating the tap water into acid water in a tank (4) and alkaline water in another tank (5), respectively, by means of the ionizing apparatus (1), while controlling the pH values within the preset limits, followed by introducing the colloidal silver by means of the electrolyser (2) provided with electrodes (12 and 13) of pure Ag, such that the time of connection to the direct current source (14) is established depending on the necessary amount of Ag, said Ag being passed in water in colloidal form, which does not loose its properties, and the membrane (11) maintaining constan
