INFLUENCE OF LAMP INTENSITY AND WATER TRANSMITTANCE ON THE UV DISINFECTION OF WATER

The efficiency of UV disinfection devices depends on flow, lamp intens ity and water transmittance. In practice the flow is controlled by sui table methods, whereas the other two parameters, to date, have not bee n taken into consideration sufficiently. The surveillance of the funct ion of UV disinfection plants is routinely based on a sensor placed on the irradiation chamber wall. This measures changes of UV irradiance without differentiating if the effect is caused by transmittance or la mp intensity so that two different conditions of disinfection may occu r at the same sensor readings. We investigated the influence of transm ittance vs intensity on disinfection at the same sensor readings in a specially designed laboratory flow through UV irradiation system with one single UV lamp as well as in commercially available UV disinfectio n plants with multiple UV lamps. All devices were equipped with a cali brated selective UV detector connected to a UV radiometer. Lamp intens ity was decreased by diminishing the supply voltage. UV transmittance was reduced by pumping aqueous sodium thiosulphate solution into the w ater inflow. The disinfection capacity was determined by measuring the reduction equivalent doses (RED) using a standardised biodosimetric m ethod. We showed that equal sensor readings, either achieved by reduci ng the lamp intensity or by lowering the UV transmittance of the water , resulted in different REDs in one-lamp systems. The diminishing of U V intensity caused a greater decrease of REDs than reducing the water transmittance. However, in the multiple-lamp systems tested, equal sen sor readings yielded equal REDs. (C) 1997 IAWQ. Published by Elsevier Science Ltd.