Biodosimetry: Model calculations for u.v. water disinfection devices with regard to dose distributions

The increasing importance of disinfection of drinking water by u.v. radiation makes it necessary to determine the u.v. dose which is applied to the water by u.v.-disinfection plants. In Austria a minimum microbicidal 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 as biodosimeter which are added to the inflowing water, and after determination of their survival rate in the plant one can deduce the applied dose from it. This procedure is unproblematic as long as all microorganisms receive the same dose on their way through the reactor. But in some cases, and probably this is the normal case, not all microorganisms receive the same dose. It follows that a dose distribution will exist among the test organisms which have passed through the reactor. The dose (reduction-equivalent dose or RED) which is deduced from the survival 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 of the survival curve of the test organisms. The broader the dose distribution and the higher the u.v.-susceptibility of the microorganisms, the lower is the measured RED. But if the survival curve of the microorganisms used would have a shoulder (Ds > Dm), and the dose distribution would overlap with the shoulder the RED may increase. We investigated by calculations and by experiments the influences on the test results obtained by this method.