Rainwater harvesting : the impact of residential-scale treatment and physicochemical conditions in the cistern on microbiological water quality
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Rainwater harvesting (RWH) at an individual residence is an alternative method of water supply for potable and non-potable uses. However, raw harvested rainwater and household-treated rainwater frequently contain a substantial number of unidentified microorganisms, some of which might be human pathogens. The objectives of this study were to understand the microbiological quality of harvested rainwater at residential RWH systems and to understand temporal changes in the rainwater cistern microbiome. To achieve these objectives, physicochemical/microbiological water quality parameters and the harvested rainwater microbiome were analyzed at the cistern and finished cold-water taps of residential RWH systems over the period of one year. Additionally, the impact of physicochemical conditions in the cistern on microbiological water quality was studied in bench-scale cisterns over a 28-day period. In the residential RWH systems, potential human pathogens (Mycobacterium avium, Mycobacterium intracellulare, Aspergillus flavus, Aspergillus fumigatus, and Aspergillus niger) were found frequently in cisterns and in treated rainwater delivered at the tap; Legionella pneumophila was not detected as frequently, but it persisted in a system after its first detection. The dissolved organic carbon (DOC) concentration was positively rank-correlated with heterotrophic plate counts (HPC) and L. pneumophila in non-chlorinated cisterns. The harvested rainwater microbiome was diverse and distinct between RWH sites. The diversity of the rainwater microbiome was correlated with HPC and DOC concentrations. The non-chlorinated cisterns had very stable microbiomes over the period of a year, suggesting that fresh rainfall does not change the cistern microbiome substantially. Filtration/ultraviolet-treatment changed the composition of the harvested rainwater microbiome, but DNA from two genera that contain potential human pathogens (Mycobacterium and Legionella) still were found in most samples. The bench-scale cistern experiments showed that the cistern microbiome proceeded towards its pre-disturbance state after an influx of fresh roof-harvested rainwater. The L. pneumophila concentration decreased over time in all the cisterns, even though HPC in the cisterns were stable over 28 days. Chlorination effectively inactivated L. pneumophila in the cistern but only temporarily impacted HPC and the relative abundance of operational taxonomic units in the Firmicutes (e.g., Clostridium spp.).