[Microbial emission, immission and changes in the germ count in the cooling water during operation of wet cooling towers III. communication: laboratory tests for the determination of the reduction kinetics of Escherichia coli in cooling tower plumes (author's transl)]

Abstract

The present paper reports on laboratory tests for the determination of the reduction rate of micro-organisms. The investigations were carried out with a pure culture of E. coli, a germ type from the enterobacterial group. By a suitable variation of the thermodynamic parameters it was possible to simulate the conditions through which the cooling tower plume passes inside the chimney of the cooling tower and when it mixes with ambient air. The results allow the following changes in the germ counts in the plume of a wet cooling tower to be expected: 1. As long as the cooling tower plume contains droplets, the capacity of the carried germs to form colonies does not change materially. 2. In the case of unsaturated plumes we must distinguish between two situations: a) The tests in which the germ suspension was sprayed into unsaturated air without any admixture of secondary air, did not demonstrate any dependence of the reduction rate on the relative humidity. b) In contrast to this, the tests in which the germ-carrying droplets were evaporated by admixing a stream of unsaturated secondary air, showed that the capacity to form colonies clearly decreased with diminishing relative air humidity. Case b) is of importance for the dissemination and immission of germs from cooling towers, because here, too, the visible plume initially laden with drops is dried by the admixture of unsaturated ambient air.