Effect of swabbing cleaning method on biofilm communities of a drinking water distribution system in Madrid (Spain)

Abstract

Drinking Water Distribution Systems (DWDS) are complex engineering networks where it has been shown that the microbial presence plays a key role for the drinking water quality and safety. Most of microbial biomass in these systems is found attached to the inner-pipe surface forming biofilms, which have been associated with several water quality problems including discolouration events, taste and odour problems, degradation and corrosion of pipes, and the presence of opportunistic pathogens. The implementation of biofilm control strategies is thus vital to guarantee the supply of good-quality drinking water. Water flushing is the most common and longest applied management strategy for cleaning the pipes worldwide. However, the amount of water resource required for the cleaning work is favouring the emergence of alternative methods such as swabbing. Nevertheless, there is a lack in the knowledge about the effectiveness or influence on microbial dynamics of this method. This study aimed to investigate the effect of swabbing in the biofilm communities of a real chloramitated DWDS in Madrid (Spain). Biofilm samples from real pipes were taken before and after swabbing cleaning. Then, the biofilm re-growth over a two-years after the cleaning was evaluated thanks to a coupon device that allowed in situ biofilm sampling without interrupting the supply. Coupons samples were taken every 6 months to study the microbial re-growth and the biofilm succession over a two-year period after the swabbing cleaning. Inorganic fraction of biofilm samples was determined by means of Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Flow cytometry (FCM) analysis were carried to estimate the total cells counts (TCC) and the 16s-rRNA bacterial gene sequencing was performed for the taxonomic characterization of biofilm samples. ICP-MS results showed that swabbing did not affect the inorganic content of the biofilm and suggests that it is more influenced by the pipe material over other factors. Regarding the biofilm quantity, FCM results suggested that although swabbing can reduce the number of cells in the biofilm, it is not completely effective removing the biofilm deposits from the pipe walls, as it happens with other cleaning methods such as flushing. Taxonomic analysis showed that swabbing affected the structure and composition of biofilm bacterial community. Important bacterial genera for water quality such as Nitrosomonas highly reduced their relative abundance after the swabbing. Moreover, during the biofilm regrowth over 2 years, taxonomical changes were also observed in the bacterial composition. From this study it can be concluded that swabbing modifies the biofilm communities of DWDS systems. Although this method cannot completely remove the biofilm deposits, the results showed that swabbing can be an effective tool aimed to minimise the presence and thus the risk associated with the presence of certain bacterial genera. This is a first approach to improve the management of DWDS, but more research is needed that includes other cleaning methods to design effective biofilm control strategies that ensure biological safety of the water that reaches consumers' taps.