IWA Biofilms 2020 Virtual Conference
December 7-10, 2020
Characterizing the development of biofilm in PE pipes through 1.5 years in the non-chlorinated Danish drinking water distribution system
Søborg, DA.*, Skovhus, TL.*, Højris, B. **, Andreasen***, JO and Brinkmann, K.***
*VIA University College, Chr. M. Østergaardsvej, 8700, Horsens, Denmark
**GRUNDFOS Holding A/S, Poul Due Jensens Vej 7, 8850 Bjerringbro, Denmark
***Aarhus Vand A/S, Gunnar Clausens Vej 34, 8260 Viby J, Denmark
Keywords: Drinking water distribution system; PE pipes; 16S rRNA gene sequencing
Summary of key findings
Biofilm development was followed in polyethylene (PE) pipe setups at two locations in the Danish full-scale drinking water distribution system (DWDS). Community composition was assessed
regularly through 1.5 years. The study identified few dominant bacterial genera in young biofilms and differences in community compositions of mature biofilm at the two locations. In addition, it was shown that biofilm development affected the microbiological water quality.
Background and relevance
Biofilms in DWDS have been associated with different water quality and operational problems such as aesthetic changes (deterioration of taste, odour, colour), bacterial regrowth (Kooij 2000; Prest et al.
2016), biofouling and biocorrosion (Emde et al. 1992; Pizarro & Vargas 2016), and persistence of pathogens (Wingender & Flemming 2011). On the other hand, maintenance of a healthy biofilm may benefit the microbiological water quality. To explore such effects closer, it is critical to understand how biofilms develop in newly installed pipes. Which community differences are found between young and mature biofilms? How does the young biofilm community effect the water quality in newly commissioned pipes? Which factors shape an maintain a healthy mature biofilm and microbiological water quality in the long-term.
Young biofilms from the two different locations (at a waterworks close to the groundwater source, TBR and in the middle of the distribution system, BUS) were both dominated by genera of Comamonadaceae and Caulobacteraceae. The community composition of the mature biofilm, however, differed between the two locations. As seen from the principal component analysis (PCA) plot in Figure 1.1, the samples I-T and 11-20, respectively, clustered together, which showed that a mature biofilm was reached at BUS after 8 months and at TBR after 10 months.
Figure 1.1 Principal-component analysis based on 16S rRNA gene sequencing of biofilm from PE-pipes at TBR (1-20) and BUS (A-T). Each dot represents the full diversity in a sample at a given time from one of the two locations. Sample 1+2 (and A+B) are true replicates (and so on). After 1.5 years, samples from each location clustered separately along PC1, suggesting that the PC1 axis explains variations based on location (effect of upstream factors). Samples distributed along the PC2 axis in relation to the time of sampling (maturation of the biofilm).
The cell number of the mature biofilm was higher at BUS (106 cells/cm2)than TBR (105 cells/cm2).
Similarly the alpha-diversity index as seen in Figure 1.2 was higher for the mature biofilm located in the middle of the distribution system (Shannon Index of approx. 5 compared to approx. 3). An increase in diversity was seen for biofilm at BUS about 8 months after commissioning (Figure 1.2).
Figure 1.2 Shannon Index of biofilm from PE-pipes at TBR (1-20) and BUS (A-T). Sample 1+2 (and A+B) are true replicates (and so on).
Results of water samples showed the importance of reaching a mature biofilm for the biological stability of the water. Within the first approx. 20 days after commissioning of the newly installed PE pipes (while dominance of Comamonadaceae and Caulobacteraceae was seen in the young biofilm), high heterotrophic plate counts (HPC) was observed in the outlet water. After this time, the numbers dropped to below 25 CFU/mL well under the quality criterion (<200 CFU/mL at the users tab).
Another significant drop was seen after approx. 10 months for TBR and 8 months for BUS to < 5 CFU/mL and 2-8 CFU/mL, respectively.
The study documented highly diverse communities of mature biofilm in drinking water systems with cell concentrations of 105-106 cells/cm2 which support previous investigations of drinking water biofilms in non-chlorinated systems (Procter and Hammes, 2015; Martiny et al, 2003). Spatial variation of biofilm was observed. It was found that the biofilm in the middle of the distribution system had higher cell numbers and was more diverse than the biofilm at a waterworks close to the groundwater source. This difference can be explained by upstream factors such as water quality, pipe material, the existing biofilm upstream the new pipe section, flow velocity, etc.
The increase in HPC during the first approx. 20 days of commisoning was expected and also abserved in other newly installed PE-pipes. On the other hand, the drop in HPC after 8 and 10 months in BUS and TBR, respectively, was surprising and could be related to the biofilm reaching the mature state.
This indicate that reaching and maintaining a mature biofilm in DWDS may be beneficial for the microbiological drinking water quality and thereby the consumers.
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Researcher and Associate Professor VIA University College, Denmark
Is the presenting author an IWA Young Water Professional? Y/N N
Bio: Ditte A. Søborg is engineer in biotechnology and has a PhD in molecular microbiology. Since 2014, Ditte worked with numerous research projects in the Research Centre for Built Environment, Energy, Water and Climate at VIA University College, Denmark. Dittes has specialty in microbiological processes in drinking water production, including sand filters and the distribution system.