THE INFLUENCE OF DISINFECTION PROCESSES ON BIOFILM FORMATION IN WATERDISTRIBUTION-SYSTEMS

Results of a 20 months pilot plant study of microbial regrowth after t he action on humic water of three oxidative disinfection processes (ch lorination, u.v. irradiation and ozonation) are reported. Regrowth was detected as biofilm produced in 2.6 m lengths of PEH pipes. No biofil m was formed from the chlorinated water containing a residual of 0.04- 0.05 mg/l free chlorine. The greatest production was found from the oz onated water, closely followed by the control (microstrained) water. T he u.v. irradiated water showed considerably less production of biofil m. This may be the effect of a residual disinfectant, the presence of which was also demonstrated by reduced biological oxygen demand compar ed to the control. A higher oxygen demand in ozonated water indicated an increase in the content of easily degradable organic molecules, whi ch was confirmed by high performance size exclusion chromatography. Th e chromatograms showed a rise in low molecular fractions in the ozonat ed but not the u.v. irradiated water. Examination by a scanning electr on microscope of biofilm on pieces of mica inserted in the pipe system s, revealed a higher content and variety of bacteria in the ozonated t han in the control system, indicating a lower microbial activity in th e latter, which also showed the highest content of inorganic material. The rate of biofilm formation was closely related to the seasonal flu ctuations in water temperature. The bactericidal effect of the u.v. ir radiated water was less pronounced during the last part of the experim ental period. The residual free chlorine in the chlorinated water was, however, sufficient to prevent biofilm formation during the whole exp erimental period.