The impact of disinfection processes on biofilm formation in potable water distribution systems

Abstract

In this study, surface and ground water were used to evaluate the impact of disinfection processes (chlorination, chloramination, ozonation, UV irradiation and hydrogen peroxide) on bacterial regrowth and bioftlm formation in potable water distribution systems using laboratoryscale units. Disinfection was carried out using disinfectant concentrations which were as close as possible to those used in practise. The parameters that were used to evaluate the bactericidal effectiveness of each disinfectant were coliform bacteria, heterotrophic plate count and total bacteria. Membrane filter and standard spread plate procedures were used to enumerate coliform and heterotrophic plate count bacteria respectively. The epifluorescence direct count involving DAPI was used to enumerate total bacteria. The scanning electron microscopy technique was used to visualize biofilm formation on stainless steel and cement coupons. The microbial disinfection efficacy of all disinfectants was found to be equal in the elimination of coliform bacteria in surface water although recovery of coliforms occurred 24 h after disinfection in all treated water systems with the exception of chloraminated water systems. For the groundwater, all disinfectants were found to remove coliform bacteria within the first hours of disinfection with the exception of hydrogen peroxide. Complete removal of coliform bacteria by hydrogen peroxide occurred only 48 h after disinfection. In surface water systems, more than 99 % (average kill percentage) of heterotrophic bacteria were removed by all the disinfection processes. Chloramination and hydrogen peroxide disinfection, however, resulted in a higher effective disinfection (average 99.99 %) although a longer period of time (between 24 and 72 h for chloramination and 72 h for hydrogen peroxide) was required to achieve this kill percentage. In groundwater systems, more than 99.90 % (average kill percentage) heterotrophic bacteria were removed by all the disinfectants with the exception of hydrogen peroxide (99.88 % average kill percentage). However, ozone was highly effective within the first 2 has shown by the average kill percentage of 99.999 % of heterotrophic bacteria. These results led to the conclusion that the microbial disinfection efficacy was greater when using groundw~ter than when using surface water. The phenomenon of bacterial regrowth was linked to the absence of concentrations of disinfectant residuals. In both water sources, bacterial regrowth occurred earlier in chlorinated, ozonated and UV treated water than in chloraminated and hydrogen peroxide treated water. Significantly higher heterotrophic bacteria counts were noted in chlorine, ozone and UV treated waters than either in chloramine or hydrogen peroxide treated waters. The greater persistence of monochloramine and hydrogen peroxide residuals was found to inhibit bacterial regrowth in both test waters.