Biofilms and disinfection by-products in Mediterranean drinking water networks: effect of temperature on disinfection strategies

Biofilms inhabiting drinking water networks can degrade water quality, leading to disinfectant depletion, pipe corrosion and organoleptic issues. In this study, the use of in situ biofilm sampling devices, which enabled sample collection without interrupting the supply, provided insights into physicochemical and biological interactions within a network in Valencia (Spain). The devices were installed in an operational chlorinated network and monitored over a year to assess biofilm dynamics. Water quality was evaluated using standard methods for faecal indicator bacteria (FIB) and pathogens, alongside physicochemical parameters including disinfection by-products (DBPs), geosmin, and metals. Biofilms devices were sampled every 3 months to examine development and biofilm regrowth under seasonal conditions. Bacterial communities were characterised using 16S rRNA gene amplicon sequencing and analysed in relation to environmental factors. No FIB or pathogens were detected in any water sample, yet DNA analysis in biofilms (particularly in 6- and 9-month-old biofilms) showed the presence of sequences related with potential opportunistic pathogens including Legionella spp. and Staphylococcus spp. DBPs, increased during warmer months coinciding with higher chlorine dosages and iron and manganese concentrations. A core microbiome formed mainly by species of Pseudomonas spp. and Acidovorax spp. was found in water and biofilm samples. A clear temporal succession in biofilm composition was observed, with diversity peaking at mid-developmental stages, yet regrowth was seasonally independent. The installation of biofilm sampling devices in real networks was effective at monitoring microbial dynamics, providing key information for distribution infrastructure management including optimisation of cleaning and disinfection strategies.