The influence of disinfection processes on biofilm formation in water distribution systems

Results of a 20 months pilot plant study of microbial regrowth after the action on humic water of three oxidative disinfection processes (chlorination, u.v. irradiation and ozonation) are reported. Regrowth was detected as biofilm produced in 2.6 m lengths of PEH pipes. No biofilm 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 ozonated water, closely followed by the control (microstrained) water. The u.v. irradiated water showed considerably less production of biofilm. This may be the effect of a residual disinfectant, the presence of which was also demonstrated by reduced biological oxygen demand compared to the control. A higher oxygen demand in ozonated water indicated an increase in the content of easily degradable organic molecules, which was confirmed by high performance size exclusion chromatography. The chromatograms showed a rise in low molecular fractions in the ozonated but not the u.v. irradiated water. Examination by a scanning electron microscope of biofilm on pieces of mica inserted in the pipe systems, revealed a higher content and variety of bacteria in the ozonated than in the control system, indicating a lower microbial activity in the latter, which also showed the highest content of inorganic material. The rate of biofilm formation was closely related to the seasonal fluctuations in water temperature. The bactericidal effect of the u.v. irradiated water was less pronounced during the last part of the experimental period. The residual free chlorine in the chlorinated water was, however, sufficient to prevent biofilm formation during the whole experimental period.