ULTRAVIOLET-IRRADIATED WATER CONTAINING HUMIC SUBSTANCES INHIBITS BACTERIAL METABOLISM

Disinfection of drinking water by u.v. irradiation has been observed t o reduce the biofilm formation in the pipes in a pilot plant. An appar ently inhibitory effect that persists in the water after the u.v. trea tment has been studied in the laboratory. Reduced numbers of viable ba cteria and reduced bacterial metabolism were observed when irradiated waters were inoculated with fresh bacteria. Approximately 60% of the h eterotrophic bacteria in the water samples were inactivated within a I h contact time with freshly u.v. disinfected water. The uptake rates o f labelled tracer substances were significantly reduced when the bacte ria were exposed to irradiated water. The inhibitory effect seems to l ast for at least 1 week. High concentrations of organic matter seem to counteract the inhibitory effect. No relationship was found between u .v. dose and effect within the dose range tested. The observed effects may be explained by the action of oxidizing reagents such as hydroxyl radicals, produced in photochemical reactions between u.v. irradiatio n and humic substances in the water.