application_number US 13726980 A description the essential aspect of this invention is as follows the water in a tank is circulated through a system including the water tank uv irradiation tank and at least one purifying tank the uv irradiation tank incorporates a uv lamp that emits uv radiation of short wavelength and the circulating water is first supplied to this tank where it is exposed to a dose of uv radiation that is greater than a certain level as defined by a dose given per pass or per residence time then the water is fed to at least one purifying tank which comprises a support cylinder wrapped with a cylinder or bag of a free permeable synthetic nonwoven fabric of high porosity which serves as a bed on which microorganisms will grow and which allows substantially uniform permeation of the circulating water through its entire surface and the thus purifed water is returned to the water tank according to this invention the purifying tank comprises a container having an inlet and an outlet which contains a bag or cylinder which serves as a microorganism growing bed made of a porous high permeable synthetic resin nonwoven fabric that is installed on a support in such a manner that a space is provided between the bed and the inner walls of.