IWA Publishing in conjunction with the International Water Association’s Young Water Professionals is happy to announce the newest post spotlighting the work of Young Water Professionals and showing how the work published in IWA Publishing Journals can be useful to those beginning their careers in the water sector.

Our 9th Young Water Professional's Spotlight Blog comes from Marco Gabrielli, who is currently studying for a PhD in Environmental and Infrastructure Engineering at Politecnico di Milano, Italy. Marco is also a member of the steering committee the new for Italian Young Water Professionals (YWP) chapter. You can connect with Marco on LinkedIn or Twitter.

Marco was granted access to our entire journal portfolio for one month in order to select papers that relate to his research interests. He has discussed how his chosen papers relate to his work with the new Italian YWP chapter below. Thank you to Marco for participating, we wish you the best of luck for the future!

Drinking water distribution systems: a land of microbiological variability

Hello! I’m Marco, a Ph.D. student at Politecnico di Milano in Italy. I was fortunate enough to be offered the opportunity to access IWA Publishing’s articles to talk a bit about what drives my research. I would also like to highlight the creation of the Italian Young Water Professionals (YWP) chapter, of which I am a member of the steering committee.

Let’s start with the creation of YWPIT, the Italian YWP chapter. In February 2022, both IWAHQ and Utilitalia (the Italian Governing Member), accepted our Constitution and Workplan which allowed us, a group of young researchers and practitioners, to form a YWP chapter in Italy. After a somewhat slow first month, things have started to speed up. We are currently planning an informal online launch event to anticipate our participation at the Water Festival, organized by Utilitalia in Turin as well as the first official meeting that will be held in June at the 3rd IWA Disinfection and Disinfection By-Products Conference in Milan. We are also commencing with other initiatives, such as the creation of a periodical newsletter, the collaboration with other YWP chapters around Europe and the organization of a webinar series which will touch upon the most interesting topics for Italian YWPs.

Moving on to my research, the first topic I faced in my short research career has been the modelling of contaminants of emerging concern in the context of wastewater treatment and agricultural wastewater reuse. However, during my Ph.D., I shifted focus to the study of drinking water microbiological quality dynamics in distribution systems and the optimization of monitoring strategies.

Historically, drinking water distribution has only been considered a hydraulic problem, as water quality issues would be eliminated during treatment. However, more attention is now being directed towards these systems as they have been found to affect the quality of the water that is delivered to the consumers, as summarized by Prest et al. (2016a). In fact, regarding the microbiological side of water quality, even though these systems are often characterized by very low nutrient concentrations, they present complex microbial communities that are not only able to survive in the presence of disinfectants, but can also form thriving biofilms on the internal pipes surface. While most microorganisms are harmless to human health, some pathogens might still be present in drinking water distribution systems. Furthermore, microorganisms can also be linked to the deterioration of water quality (e.g. nitrification) and operational issues such as biofouling, problems often faced by water utilities. In addition, drinking water distribution systems are both temporally dynamic due to fluctuations in both the environmental conditions and the consumers’ water demands, and also spatially distributed, meaning how they behave in one area may be different to another. These two characteristics are not just limited to hydraulics and chemical water quality, but are also reflected in drinking water microbiology.

Indeed, when looking at microbial communities in these systems, past studies have shown the presence of multiple temporal dynamics occurring at different time scales. For example, Bautista-De Los Santos et al. (2016) and Besmer and Hammes (2016) showed how the bacterial communities  can originate during water treatment and how they presented daily patterns which depended on water consumption. In fact, both biofilm erosion at higher flow rates and stagnation events can affect the microbiological composition of the water that we all use, stressing the need to find proper ways to control their effect. More significant effects can be caused by seasonal changes due to greater swings in water quality and temperature as it was shown by Pinto et al. (2014) and Prest et al. (2016b). As the conditions in the distribution system change throughout the year, not only do the number of microorganisms present in the treated water and the extent of their growth during distribution change, but also the entire microbial community can shift between seasonal clusters.

Besides the temporal dynamics, microorganism concentrations and communities also present spatial variability within the distribution systems. This is partly due to the fact that microbial communities which grow on pipes’ surfaces as biofilms are affected by the material of the distribution pipes on which they grow (Douterelo et al., 2020). Another reason for the spatial variability can be the different water chemistry at different locations. It is interesting to note that these changes might even be due to the microorganisms themselves, as shown by the spatial distribution of the nitrogen cycle within a chloraminated distribution system by Pullerits et al. (2020). In fact, microbiological nutrient limitations can change during distribution, as shown by Nescerecka et al. (2018), diversifying the conditions that microorganisms are exposed to, especially in cases where waters from multiple sources are mixed within a single distribution system.

While most studies focus (or focused) on bacteria, it is important to remember that they are not the only inhabitants of distribution systems. In fact, both archaea, viruses, and micro-eukaryotes, such as fungi and amoebas, can also be found in drinking water (Gomez-Alvarez et al., 2012). However, we still  know very little about them. In fact, I think that the use of new technologies, which can provide more detailed information about the microbiology of the water systems with respect to traditional techniques commonly used by water utilities, is not only important for research, but also for the management of these infrastructures. For example, Van Nevel et al. (2017) compares the use of flow-cytometry and traditional plate count methods concluding that the use of the former not only provides higher reproducibility, faster results, and more detailed information regarding the cells present in water, but is more economical when analyzing large quantities of samples. Another example is provided by Kahlisch et al. (2010) who, through a combination of advanced techniques, are able to identify the taxonomy of the “dead” and live bacteria following drinking water chlorination and distribution. In fact, I believe that with the information provided by these new technologies, one of the future goals of this field is to understand how water quality and several other factors affect the microorganisms living in our distribution systems. This knowledge is key to properly managing their presence and mitigating potential negative effects.

Last, but not least, we should not overlook the importance of selecting sampling locations, sampling times, and sampling techniques properly, depending on the goal of our monitoring campaign. In fact, as argued by Bautista-De Los Santos et al. (2016), one might want to always sample at the same time to try to minimize the effects of short-term variations and focus on longer temporal trends. Alternatively, one can actively seek samples that might present the effect of short-term dynamics and sample the largest set of microbial communities possible. Similarly, the selection of the sampling locations is  critical as it is not possible to estimate the microbiological water quality at all the locations within a distribution system. One possible strategy might be to select locations with different water residence times to catch microorganisms community changes, as done by Pullerits et al. (2020). On the other hand, sampling locations might be chosen close to relevant infrastructures, such as hospitals, to ensure the protection of the more fragile population. Finally, it is worth noticing that both harmless microorganisms and opportunistic pathogens can  show preferences for different niches within a distribution system, for example, concentrating in biofilms or sediments (van der Wielen and Lut, 2016). This fact is very important when sampling, as probing the wrong niche might result in the underestimation of the presence of the microorganisms of interest.

To sum up, drinking water distribution systems harbour a complex microbiota that is altered not only by temporal fluctuations, but also by spatial variations. We still know very little about these changes and, for this reason, it is important to exploit new technologies to understand them and allow for their management.

I hope you have enjoyed reading this blog post and that this has sparked some new ideas regarding drinking water distribution systems. I hope to get the chance to meet you in the future at a workshop or conference. In any case, feel free to reach out, I’ll be happy to hear your thoughts.

Feel free to write to me if you’re interested in YWPIT. Otherwise, you can find our latest news on LinkedIn or Twitter.

In the meanwhile, take care!

Marco Gabrielli

Ph.D. student

Department of Civil and Environmental Engineering, Politecnico di Milano



Bautista-de Los Santos, Q. M., Schroeder, J. L., Blakemore, O., Moses, J., Haffey, M., Sloan, W., & Pinto, A. J. (2016). The impact of sampling, PCR, and sequencing replication on discerning changes in drinking water bacterial community over diurnal time-scales. Water Research, 90, 216-224. https://doi.org/10.1016/j.watres.2015.12.010

Besmer, M. D., & Hammes, F. (2016). Short-term microbial dynamics in a drinking water plant treating groundwater with occasional high microbial loads. Water Research, 107, 11-18. https://doi.org/10.1016/j.watres.2016.10.041

Douterelo, I., Dutilh, B. E., Arkhipova, K., Calero, C., & Husband, S. (2020). Microbial diversity, ecological networks and functional traits associated to materials used in drinking water distribution systems. Water Research, 173, 115586. https://doi.org/10.1016/j.watres.2020.115586

Nescerecka, A., Juhna, T., & Hammes, F. (2018). Identifying the underlying causes of biological instability in a full-scale drinking water supply system. Water Research, 135, 11-21. https://doi.org/10.1016/j.watres.2018.02.006

Gomez-Alvarez, V., Revetta, R. P., & Santo Domingo, J. W. (2012). Metagenomic analyses of drinking water receiving different disinfection treatments. Applied and environmental microbiology, 78(17), 6095-6102. https://doi.org/10.1128/aem.01018-12

Kahlisch, L., Henne, K., Groebe, L., Draheim, J., Höfle, M. G., & Brettar, I. (2010). Molecular analysis of the bacterial drinking water community with respect to live/dead status. Water Science and Technology, 61(1), 9-14. https://doi.org/10.2166/wst.2010.773

Pinto, A. J., Schroeder, J., Lunn, M., Sloan, W., & Raskin, L. (2014). Spatial-temporal survey and occupancy-abundance modeling to predict bacterial community dynamics in the drinking water microbiome. MBio, 5(3), e01135-14. https://doi.org/10.1128/mBio.01135-14

Prest, E. I., Hammes, F., van Loosdrecht, M., & Vrouwenvelder, J. S. (2016a). Biological stability of drinking water: controlling factors, methods, and challenges. Frontiers in microbiology, 7, 45. https://doi.org/10.3389/fmicb.2016.00045

Prest, E. I., Weissbrodt, D. G., Hammes, F., Van Loosdrecht, M. C. M., & Vrouwenvelder, J. S. (2016b). Long-term bacterial dynamics in a full-scale drinking water distribution system. PLoS One, 11(10), e0164445. https://doi.org/10.1371/journal.pone.0164445

Pullerits, K., Chan, S., Ahlinder, J., Keucken, A., Rådström, P., & Paul, C. J. (2020). Impact of coagulation–ultrafiltration on long-term pipe biofilm dynamics in a full-scale chloraminated drinking water distribution system. Environmental Science: Water Research & Technology, 6(11), 3044-3056. https://doi.org/10.1039/D0EW00622J

van der Wielen, P. W., & Lut, M. C. (2016). Distribution of microbial activity and specific microorganisms across sediment size fractions and pipe wall biofilm in a drinking water distribution system. Water Science and Technology: Water Supply, 16(4), 896-904. https://doi.org/10.2166/ws.2016.023

Van Nevel, S., Koetzsch, S., Proctor, C. R., Besmer, M. D., Prest, E. I., Vrouwenvelder, J. S., ... & Hammes, F. (2017). Flow cytometric bacterial cell counts challenge conventional heterotrophic plate counts for routine microbiological drinking water monitoring. Water Research, 113, 191-206. https://doi.org/10.1016/j.watres.2017.01.065

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