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Conference Papers Year : 2015

Chlorination of drinking water biofilms: a time-limited benefit?

Abstract

Despite the best available cleaning-disinfection techniques used in distribution systems, biofilms continue to be one of the most common and significant concerns associated with drinking water distribution. The fact remains that operational practices advocating chemical or mechanical actions or both have some limited success in removing the biofilms. Failure to eradicate biofilms is explained by two main reasons: (i) First, biofilm bacteria escape disinfection treatments with chlorine or other chlorinated oxidant due to their diffusion-interactions with the organic material accumulated on the surface (DeBeer et al., 1994; Lee et al., 2011). As a consequence, some species escape to the treatment which determines a selective pressure on the remaining biofilm population depending on the disinfectant nature and concentration. As an example, Mathieu et al. (2009) reported that -proteobacteria survived better to chlorination than -proteobacteria in drinking water biofilms. Interestingly, as soon as the selection pressure ended, the proportions of -, -, -proteobacteria returned to the initial equilibrium suggesting resilience of the bacterial community. (ii) Second, biofilm bacteria are embedded in an extracellular polymeric matrix (EPS), which give a strong cohesion and viscoelastic properties to the structure (Abe et al., 2012). EPS form a network of polymers (mesh size from 5 nm to 11 nm) with an entanglement rate of nearby 106 contact points/μm3 (Mathieu et al., 2014). Chlorination (up to 5 mg Cl2/l) of such structures led to both a partial disinfection of pipe walls and the erosion of drinking water biofilms (- 75 % of the cells) and a weakening of the EPS matrix by reducing (- 40 %) the entanglement rate between polymers (Mathieu et al., 2014). But biofilms remain at the surface (~106 cells/cm²) after such chlorination in the form of still firmly adherent clusters that could be compared to a “conditioning film” or to “protobiofilm” including viable bacteria. In this study, we questioned the impact of biofilm “residues” remaining after chlorination on the early stages of repopulation by drinking water bacteria. The comparisons were made between high density polyethylene (HDPE) coupons bearing chlorinated-biofilm “residues” and bare HDPE. The key questions we addressed were whether chlorinatedbiofilm surfaces would favour their repopulation and if new equilibria or bacterial selection are determined by chlorination. Material and Methods A two month-old drinking water biofilm formed in a rotating disk reactor was first chlorinated (6.5 ± 1.8 mg Cl2/L) under hydrodynamic shear forces (τw ~10 Pa) during 2.5 h. Repopulation of HDPE surfaces after chlorination was evaluated by quantifying the attached biomass (bacteria cells number and Proteobacteria diversity (FISH)) populating both bare and chlorinated-biofilm surfaces over a 7 days exposure to non-chlorinated drinking water. Roughness and hydrophobicity of these two HDPE surfaces was also characterized by Atomic Force Microscopy and Chemical Force Microscopy, respectively. Results and Conclusions Compared to the strong hydrophobic adhesion forces measured on bare HDPE (8.9 ± 0.2 nN), chlorinated-biofilm HDPE surfaces exhibited lower hydrophobic forces (2.1 ± 1.5 nN) due to the presence of biofilms remaining after chlorination. Macromolecules associated with the chlorinated-biofilm surface were detected over 50.6 % ± 14.4 % of the total surface. Rapid accumulation of cells from the bulk occurred as 4.7 x 106 cells/cm² and 1.8 x 106 cells/cm² were measured after 3 h exposure to drinking water on chlorinated-biofilm surfaces and bare surfaces, respectively. Beyond 24 hours, the cell accumulation increased quasi linearly on the two surfaces, with 3.4 x 104 cells/cm2.h and 5.1 x 104 cells/cm2.h on chlorinated-biofilm and bare surfaces, respectively. At T7days, the biofilm ecosystems on both surfaces reached a similar equilibrium as the total numbers of cells were equivalent (~1 x 107 cells/cm2). This time-limited benefit of the chlorination of HDPE surface and the repopulation dynamic between bare and colonized surfaces questioned the rationale of the chlorination practices during cleaning procedures. The early stages of repopulation of these surfaces were characterized by a succession in the Proteobacteria dominance according to the colonization time. In the first 3 hours, the γ- proteobacteria sub-class was the majority on both surfaces although the drinking water composition was dominated by α-Proteobacteria. At T24h, a shift in the populations’ dominance occurred similarly on bare and chlorinated-biofilm HDPE surfaces as the number of α- and β-proteobacteria increased significantly, while, at the same time, the level of γ- proteobacteria stabilized. Even if repopulation took place rapidly, no apparent selection of bacterial population occurred durably (i.e. within 7 days) between bare and chlorinated surfaces.
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Dates and versions

hal-03749822 , version 1 (22-09-2022)

Identifiers

  • HAL Id : hal-03749822 , version 1

Cite

Laurence Mathieu. Chlorination of drinking water biofilms: a time-limited benefit?. Biofilms in drinking water systems, Aug 2015, Arosa, Switzerland. ⟨hal-03749822⟩
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