BIODOSIMETRY - MODEL-CALCULATIONS FOR UV WATER DISINFECTION DEVICES WITH REGARD TO DOSE DISTRIBUTIONS

The increasing importance of disinfection of drinking water by u.v. ra diation makes it necessary to determine the u.v, dose which is applied to the water by u.v.-disinfection plants. In Austria a minimum microb icidal dose of 400 Jm(-2) at a wavelength of 253.7 nm is demanded for drinking water, the control of it shall be assured by type testing. A method was developed in using calibrated spores of Bacillus subtilis a s biodosimeter which are added to the inflowing water, and after deter mination of their survival rate in the plant one can deduce the applie d dose from it. This procedure is unproblematic as long as all microor ganisms receive the same dose on their way through the reactor. But in some cases, and probably this is the normal case, not all microorgani sms receive the same dose. It follows that a dose distribution will ex ist among the test organisms which have passed through the reactor. Th e dose (reduction-equivalent dose or RED) which is deduced from the su rvival rate of the microorganisms passing flow-through systems for u.v .-disinfection of wastewater or drinking water in general is different from the arithmetic mean of the dose distribution. The RED depends on the special form of the dose distribution and on the specific shape o f the survival curve of the test organisms. The broader the dose distr ibution and the higher the u.v.-susceptibility of the microorganisms, the lower is the measured RED. But if the survival curve of the microo rganisms used would have a shoulder (D-s > D-m), and the dose distribu tion would overlap with the shoulder the RED may increase. We investig ated by calculations and by experiments the influences on the test res ults obtained by this method.