Inactivation of faecal indicator bacteria in a roof-captured rainwater system under ambient meteorological conditions
In this study, faecal indicator bacteria (FIB) namely Escherichia coli and Enterococcus spp. were seeded into slurries of possum faeces and placed on the roof and in the gutter of a roof-captured rainwater (RCR) system. The persistence of FIB in these circumstances was determined under ambient climatic conditions. FIB persistence was also determined under in situ conditions in tank water using diffusion chambers.
Methods and Results
The numbers of surviving FIB at different time intervals were enumerated using culture-based methods. Both FIB were rapidly inactivated on the roof under sunlight conditions (T90 = 2 h) compared with shade conditions (T90 = 9–53 h). Significant differences were observed between sunlight and shade conditions on the roof for both T90 values of E. coli (P < 0·001) and Enterococcus spp. (P < 0·001). E. coli showed biphasic inactivation patterns under both clean and unclean gutter conditions. Enterococcus spp., however, showed rapid inactivation (T90 = 2 h for the clean gutter and T90 = 6 h for the unclean gutter) compared with E. coli (T90 = 22 h for the clean gutter and T90 = 20 h for the unclean gutter). Significant differences were also observed between the T90 values of E. coli and Enterococcus spp. for both clean (P < 0·001) and unclean (P < 0·001) gutters. Both E. coli and Enterococcus spp. showed nonlinear biphasic inactivation in tank water. Significant difference was observed between the T90 value of E. coli inactivation compared with Enterococcus spp. (P < 0·001) in the tank water.
In this study, FIB were observed to survive longer (T90 = 9–53 h) on the roof under shade conditions compared with sunlight conditions (T90 = 2 h). If there is a rainfall event within two to three days after the deposition of faecal maters on the roof, it is highly likely that FIB would be transported to the tank water. When introduced into the tank, a relatively slow inactivation process may take place (T90 = 38–72 h).
Significance and Impact of the Study
The presence of FIB in water indicates faecal pollution and potential presence of enteric pathogens. Therefore, the information on the resilience of FIB, as obtained in this study, can be used for indirect assessment of health risks associated with using roof-captured rainwater for potable and nonpotable purposes.