Sewage disinfection towards protection of drinking water resources.

Wastewater applied in agriculture for irrigation could replace the use of natural drinking-water resources. With respect to high concentrations of human pathogens wastewater has to be disinfected prior to use. This paper introduces disinfection methods with emphasis on UV irradiation.

[Microbial contamination of wash water in automatic washing and disinfection machines]. / Mikrobielle-Kontaminationen von Spülwasser in Reinigungs- und Desinfektionsautomaten.

Samples were taken from the water containing tube system of 7 cleaning and disinfection machines located at the university clinics. The machines were used for the treatment of medical tools. The samples were bacteriologically examined. The results showed systemic contaminations of the tubes of all tested machines. The well known positive effect of common ion-exchangers on bacterial growth was not the only reason for the high degree of microbial contaminations. Partial bacterial growth on the inner sides of the water tubes was detected. The reason was the use of bacteriological objectionable tube materials. Recontamination of the post treated medical tools is possible at any time by the last washing water. Preceding sterile filtration showed as supposed no effect. Consequences for the use of cleaning and disinfection machines are discussed.

Influence of geometrical parameters in different irradiation vessels on UV disinfection rate.

Possible differences in the disinfection rate of two irradiation concepts, e.g. irradiation in cuvettes and petri dishes, were examined and quantified on a UV laboratory irradiation plant unit. In addition the geometrical parameters of the irradiation vessels and mixing of the bacterial suspension during UV irradiation were considered. The results showed differences in disinfection rates of more than 2 log between different sample areas in the vessels. The mixing caused an improvement of the disinfection rates in unfavourable irradiation areas of the cuvettes and in contrast a deterioration in all irradiation areas in the petri dishes.

Photoreactivation of E. coli depending on light intensity after UV irradiation.

Interest in the disinfection of drinking water by UV rays has been growing in recent years. Since the beginning of the 40's it has been known that, by means of certain repair mechanism, microorganisms are able to restore lesions on DNA. These processes are more or less clarified. However there is no secure knowledge about the degree of possible repair. Photoreactivation induced by light of 350-600 nm is one of these repair mechanisms. Thus the effect of light exposure on disinfection results with E. coli after UV disinfection was examined. E. coli ATCC 11229 were cultivated, washed, suspended and irradiated by means of UV rays from a mercury low-pressure lamp. After UV disinfection, the bacterial suspension was illuminated by fluorescent light. The degree of photoreactivation could be influenced by illumination intensity. The speed of reactivation decreased with the distance of the probes to the illumination source and increased with the period of time of illumination. After a 4 log reduction of the colony count (10(6)/ml-10(2)/ml) through UV rays, the reduction rate could be decreased to only 1 log (10(5)/ml) after UV disinfection and illumination. In probes kept in darkness for the same period there was no change in the count of reproductive bacteria. It seems to be necessary to take photoreactivation into consideration with the UV disinfection of drinking water.

[Effect of the reflection of UV rays in the disinfection of drinking water]. / Einfluss der Reflexion von UV-Strahlen bei der Desinfektion von Trinkwasser.

The disinfection of drinking water by means of UV-rays represents an interesting alternative to chemical disinfection. However, several questions still remain to be answered, making the disinfection of drinking water by means of UV-radiation appear unsafe. The regulations concerning the disinfection performance of UV-systems are still presenting difficulties. Within the course of comparative experiments in the laboratory and using a flow-through reactor on the disinfection effectiveness of UV-radiation, it has been shown that the reflection of UV-rays can have a considerable influence on the disinfection effectiveness. In order to be able to make reliable statements on the dose-effect relation it is necessary, therefore, to record exactly the radiation applied, including the reflected proportion.