Microbial drinking water quality is of great importance to human health. Drinking water distribution systems (DWDSs) are designed as the final barriers for delivering and maintaining the biosafety of drinking water. Though the drinking water produced is usually safe and clean, it is common that the water quality deteriorates during the distribution. Such deteriorations can be linked to the establishment of biofilm in DWDSs, where the majority of the biomass is residing (> 95%). The formed biofilms are reportedly leading causes of the undesired taste, odor, and color of the drinking water, corrosion of the pipes, decay of the disinfectants, and proliferation of pathogenic microbes, giving rise to public health concerns.
In the Netherlands, the control of the biofilm growth in DWDSs is achieved by producing bio-stable drinking water with extremely low nutrients (e.g., AOC < 10 µg C/l). On the other hand, water utilities in many countries usually apply chemical disinfectants (e.g., free chlorine, monochloramine) to control the biofilm growth in DWDSs. Nevertheless, biofilm formation is inevitable, regardless of the strategy. Additionally, there is no standard method to monitor the biofilm growth in DWDSs, which makes the understanding and management of DWDS biofilms more challenging. Efforts have been made to explore the biofilm formation and structure through pilot studies. However, most of these investigations have been conducted in a short time frame (e.g., within weeks to a max of 84 days), where the developed biofilms were far from mature and significantly different from those in the real DWDSs. To uncover how biofilm develops and what roles disinfectants play during the biofilm development, a newly-built pilot system was followed for a 64-weeks period under different disinfection regimes: no disinfectants (NC), free chlorine (FC), and monochloramine (MC) (Chapter 2). The results showed that residual disinfectants presented intensive suppression of the biofilm growth and shaped the biofilm communities. Specifically, MC exhibited stronger suppression of the biofilm activity (i.e., ATP), whereas FC expressed intense selection pressure on the microbes and established more homogenous and less complex biofilm community, with Proteobacteria comprising on average 82% of the relative abundance. The temporal trends highlighted the essential developmental stages in biofilm formation from initial colonization to accumulation and selection and stabilization, which occurred at different rates under each of the conditions, and were associated with significant dynamic changes in biofilm bacterial communities. Reaching stabilization took longest in the MC condition (> 64 weeks), followed by the NC (~ 36 weeks) and FC (~ 19 weeks) conditions. Holistically, the early stages in the biofilm formation in the NC condition were primarily dominated by stochastic processes where colonizers originating from treated water randomly attached to and settled on the pipes, while deterministic processes progressively increased in their relative contributions at the end of the accumulation stage and became predominant at the later stages. In the MC condition, the biofilm succession was governed by stochastic processes during the entire test, even though some deterministic processes occurred during the accumulation stage. Conversely, in the FC condition the biofilm succession was driven by deterministic processes already from the initial development stage.
DWDSs are highly dynamic ecosystems, where the liquid (i.e., bulk water, suspended particles) and solid (i.e., biofilm, loose deposits) phases interact intensively during transport of the water from treatment to consumer. The cells and/or particles that were introduced with the treated water may attach to and/or settle on the pipes, forming biofilm/loose deposits when the hydraulic forces are weak. Conversely, the biofilm/loose deposits might release cells/particles to the bulk water during hydraulic disturbances, affecting the drinking water quality negatively. The hydraulic conditions in DWDSs are very complex and dynamic. They exhibit daily patterns, with high flow rates at high water demand periods (e.g., morning and/or evening hours) and long stagnancy or low flows during the night. However, most monitoring occurs using grab samples at one point in time. Thus, continuous online sampling is required to obtain a representative image of the particles and microbes in drinking water. In Chapter 3, a novel online monitoring and sampling system (OMSS) was developed to investigate the spatiotemporal variations of the planktonic and particle-associated bacteria in an unchlorinated DWDSs. The 16S rRNA gene sequencing combined with SourceTracker2 was used to trace and reveal the origin of the changes in the planktonic and particle-associated bacteria, assigning sampled biofilm and loose deposits as sources. The results showed that, spatially, the particle loads significantly increased from treatment plant within distribution networks, while the trend in the quantity of the particle-associated bacteria was the opposite. Similar to the trend of particle loads, the number of the observed OTUs in both planktonic and particle-associated bacteria increased from the treatment plant within the distribution network. The spatial results implied a dominant role of sedimentation of particles entering the DWDS from the treatment plant, while the observed increases in particles and the associated bacteria primarily originated from the distribution network, which were confirmed by the increased contributions from loose deposits and biofilm determined by SourceTracker2. Temporally, daily peaks in the water quality, including particle-associated bacterial quantity, observed operational taxonomic unit (OTU) number, and contributions of biofilm and loose deposits, were sensitively captured during the high water demand (morning/evening peaks). The temporal results revealed clear dynamic interactions between the liquid (i.e., bulk water, suspended particles) and solid (i.e., biofilm, loose deposits) phases in DWDSs.
Driven by increasingly stringent drinking water regulations and challenges to drinking water quality, efforts are underway to further improve water quality. These initiatives include source water switching, upgrading treatment processes, and implementing changes to disinfectant strategies. Such actions change the quality and composition of the treated water that enters the DWDS. This may have transition effects, which in this thesis refers to the water quality deteriorations contributed by the release of cells and particles from biofilm and/or loose deposits due to the irregular changes in supply-water quality. It is largely unknown whether, where and when the transition effects will happen. In Chapter 4, transition effects were investigated through characterizing the particles before (T₀), during (T_3-weeks) and after (T_6-months) introducing additional treatment steps (softening, second rapid sand filtration and adding carbon dioxide) to the existing treatment. The results showed that the upgraded treatment significantly improved the water quality after 6 months’ time. However, significant water quality deterioration was observed at the initial stage (T_3-weeks) when the quality-improved treated water entered into the network. This manifested as a significant increase in total suspended solids (TSS) by 50-260%, active biomass (ATP) by 95-230%, and Mn by 130-250%. Furthermore, pyrosequencing results revealed sharp differences in microbial community composition and structure of the bacteria associated with particles between T₀ and T_3-weeks, implying the potential contributions from biofilm or loose deposits in the DWDS. Interestingly, the domination of Nitrospira spp. and Polaromonas spp. in the distribution system at T_3-weeks, which were detected at rather low relative abundance at treatment plant, further confirmed the potential contributions from biofilm or loose deposits.
Though the study in Chapter 4 confirmed the occurrence of the transition effects, the question how fast/how long the transition effects will occur/last, where the deteriorations originate from, and what actions can be carried out to minimize the transition effects is not clear. The sampling was conducted in a relatively short time frame (i.e., 6 months), with only a few time points (i.e., T₀, T_3-weeks, T_6-months) and without the collection of biofilm and loose deposit samples. Additionally, as what we can see from the results from Chapter 3, it could be imagined that the transition effects might be enhanced during high water demand when shear forces are high. In order to fill the knowledge gaps, the OMSS was applied, accompanied with SourceTracker2, in an unchlorinated DWDS where partial RO was introduced (Chapter 5). The study was conducted before (T_B), immediately after (T₀), one month (T_1M), two month (T_2M), one year (T_1Y) and two years (T_2Y) after the partial RO introduction. Noticeably, significant transition effects in DWDS were captured right after the RO introduction, with increases in the particle loads, bacterial quantity, community diversity, and significant differences between bacterial communities in particles at treatment plant and distribution network. The disturbances lasted one month until T_1M, after which they ceased to be observable around T_2M. The captured deteriorations were confirmed by the increased contributions of loose deposits and biofilm (both the number of the immigrants and their abundance) at T₀ and T_1M determined by SourceTracker2 and neutral community model. While the peak transition window spanned about one month, it took considerably longer, until one year (T_1Y) and two years (T_2Y) later, for the microbial ecology to re-stabilize and for improvements in water quality to become noticeable. In addition, the peaks in the water quality deteriorations were enlarged during the high water demand (morning/evening peaks), which implies that current monitoring could potentially underestimate the extent of the quality deterioration. Remarkably, the observation that loose deposits contributed more to the transition effects than biofilm challenges the traditional standpoint, and provided new insights into the management of the transition effects, where the risks of the transition effects can be largely reduced by conducting flushing before the introduction of treatment changes to remove the loose deposits. In light of the destabilization caused by the changed water quality, flushing with new-quality water might be more rewarding.
To conclude, through conducting studies at both field and pilot scales, the effects of the (changes in) operational conditions on the microbial drinking water quality in DWDSs were comprehensively explored. The findings in the thesis offer novel insights into the drinking water quality management. The knowledge gained from investigating the biofilm succession dynamics under different disinfectant regimes has significantly deepened our understanding of managing drinking water biofilms. These insights serve as valuable information when making informed decisions about the appropriate strategies to employ. The implementation of the developed OMSS is capable of capturing both periodic and aperiodic changes in drinking water quality, making it an essential tool in minimizing assessment deviations and ensuring accurate evaluations of drinking water quality. Moreover, the established methodology holds promise for application in various systems, including those that utilize chlorination. By identifying and characterizing the transition effects resulting from changes in supply water quality, such as treatment upgrades or the introduction of reverse osmosis, the study highlights the significance of considering these effects in water management practices. These observations underscore the importance of addressing the impact of transition effects on drinking water quality and provide practical implications for minimizing their negative consequences.

Microorganisms may cause problems in drinking water distribution system (DWDS). It is hard to control and always forms the biofilm, which can result in the deterioration of drinking water quality. At household’s tap, due to the favorable conditions, which incurs an easier formation of biofilm. Therefore, the idea of secondary disinfection at households to control the drinking water quality has come up. Ultrasound disinfection provides an easy-operated way for point-of-use disinfection, which can be influenced by the local hydraulic conditions. Previous studies have monitored the spatial and temporal variations in the microbial community in DWDS but most of cases were based on long-term effect. In this study, a simulated household piping system was sampled intensively over short time scales. Several parameters on the dynamics of microbiological processes such as tATP, cATP, TOC and total cell concentration of drinking water samples were examined at four sampling points under various flow rate at different disinfection time. The results highlighted that ultrasound disinfection can restrain cell activity but the effect was also influenced by flow rate. Both FCM and ATP data described the microbiological dynamics in the drinking water samples. The cell concentrations decreased when the flow rate was lower. Moreover, operation time of the ultrasonic device showed significant influence on the disinfection efficiency. At the same sampling point, when the system was operated at a constant flow rate, the cell activity decreased with the increase of the operation time of ultrasonic device. Considering the time that a cell travel through the system subjected to ultrasonic effects, the so called exposure time, it is observed that the cell activity decreased with the increase of time that the cell experienced ultrasound. As a result, the cell at the furthest sampling point under the most significant flow rate with longest ultrasound disinfection time showed the lowest activity.

For drinking water companies in Netherlands, the current conventional treatment technique may not be fully capable to cope with potential future problems resulted from climate change, salinization and social developments, such as population growth. Innovative technologies like reverse osmosis and more strict drinking water standards are expected to be applied to provide better water quality to customers. In Oasen drinking water company, reverse osmosis has been applied in Lekkerkerk drinking water treatment station since 2016 to prepare for the future challenges, while potential transition effects like biofilms detachment and resuspension of loose deposits in the drinking water distribution systems might occur under irregular changes caused by supply water switch. A setup named Smart Water Meter was developed and used with integrated functions, such as seizing particles inside distribution networks and monitoring several parameters, to investigate this phenomenon. Although slight and temporary transition effects were captured with the help of this device, the amount of retained materials were not sufficient for transition effects study. According to the company, new treatment stations are being planned where background information is required to be collected and still potential transition effects might occur. The main objective of this research was to optimize the Smart Water Meter for future transition effects study by re-selecting the pore size of the filter bag to make it more sensitive and install the upgraded setups in four different locations under different water quality to have a better understanding on its performance and characteristics. Before being applied into the fields, a stagnation (water retained inside the setup for a long time) test was conducted as well to figure out the feasibility of experiments simplification where water is kept flowing through the setup instead of intermittent mode and possible deterioration of the water quality. 10 microns filter bag is selected in this research after three stages tests. Results reveal that the optimized filter bag with new pore size is able to intercept more particles without arousing sudden pressure drop under normal conditions. However, if this setup could set an early alarm, like a sudden pressure drop, when serious transition effects occur remains to be checked in the future. According to the stagnation test, chemical elements concentration and biological activity increased inside the filter bag, meaning that the water quality was indeed deteriorated when it was stagnated inside the filter bag for 23 hours while the quality could go back to normal after opening both influent and effluent valves on the setup for a while. This also indicates that the simplification of experiments is acceptable. Pore size of the filter bag may require to be reconsidered when the influent pressure in households is not sufficient, otherwise great pressure loss is inevitable. For different water quality in four locations provided by three drinking water companies, the performance of the setup is relatively reliable with a stable 100% removal rate of the particles larger than 29 microns inside the water while for the five elements, little relationship could be found. Besides, the filter bag could remove part of the living cells inside it when water keeps flowing through it. Observation of the filter bag inner surface could provide a general idea of the filtrates composition and by making chemical and biological analysis of them, possible origins of the distribution network harbored materials are conjectured. In this research, most of the distribution network harbored materials might come from detached biofilms as calcium is crucial for biofilm formation while part of the retained materials may also originate from loose deposits due to As and Aeromonas existence in some locations. With the optimized setups and a deeper understanding of its performance under different water quality, the setup could be better applied in the future study of transition effects and early warning systems.

Driven by solving water shortage problems and meeting the increasing strict regulations of water quality, changes of supply water quality are commonly happening worldwide. These kinds of irregular changes can have impacts on drinking water distribution systems with accumulated material, leading to chemical, physical and microbiological deteriorations in water quality with (in)organics release and biofilm detachment. Household connection pipes (HCPs) are the last obstacle reaching customers’ taps. The long stagnation time, non- continuous flow, high surface to volume ratio and high water quality deterioration potential make the HCPs deserve more attention. In this study, the impacts of changing supply water quality from conventional treated groundwater to reverse osmosis treated water on aged HCPs were studied.
Aged HCPs were collected from customers and used for building test rigs in the lab. Three types of water quality were supplied to these HCPs, including Kamerik water by conventional treatment (KA), remineralized RO permeate (RE) and mixed water (MX, 70% RO permeate and 30% KA). Water samples were taken every week and the biofilm were compared before and after switching supply water from KA to MX and RE to reveal the potential changes on water quality and accumulated material in pipes. Whether and to what extent the deterioration would occur in the aged HCPs could therefore be determined.
Changes of water quality revealed that the release of ATP, Aeromonas, Fe and Al may happen in the first 8 weeks and then become stabilized again, no effects on As and Mn, and no release of Ca were observed. From the study related to changes of pipe harbored material, the results showed that less bacterial activity and elements’ accumulation (except for Ca) were observed, suggesting the impacts of changing supply water not only represented by the accumulation of bacterial, but also by inorganic material.

Driven by the fast development of water purification technologies, tightening of water quality regulations and the increasing public concern on water-related health problems, drinking water companies have spared no efforts on treatment processes upgradation as well as striving on maintenance aspects. In the event of perturbation such as supply-water change in this study, historically harbored materials in drinking water distribution systems (DWDS), can potentially be converted into suspended materials and consequently delivered to end users, leading to a reduction on hygienic water quality.
In order to capture the potential occurrence of transition effects in a research area and figure out the impact as well as trace back its potential origination, Smart Water Meter is designed and developed with the integrated functions of on-line monitoring, sampling and real-time data communication detect and monitor potentially occurred physiochemical and microbiological water quality deterioration during supply-water change period. The improved Smart Water Meter consists of pressure sensors, temperature sensors, filter bag with 50-micron pore size, a normal water meter and a monitor box. Meanwhile, 3 batch of pipes sampling and 2 batch of filter bags sampling were carried out in time series to analysis and confirm the development and behavior of transition effect.
The results showed that new supplied water with low nutrients and particle load in the research area has triggered the occurrence of transition effects while the improved Smart Water Meters are capable to well capture the destabilized distribution network harbored materials (DNHM) in DWDS and prevent undesirable large particulate matter from reaching the consumers’. Besides, the triggered transition effect in this case was not violate and the temporal tendency of the microbiological and elemental parameters indicated the gradual settle-down of the transition effect. The cross comparison between the elements structure from pipe samples and filter bags, Ca and Fe are the dominate compounds in filtrates and indicates the destabilized DNHM could probably originated from detached biofilm and resuspended loose deposits since they the hotspots for calcium and iron respectively.

The drinking water safety has been paid close attention, and the water purification development and distribution cleaning have been widely studied. To meet stricter water quality regulations and fight new emerging pollutants, upgrading treatment processes to improve treated water quality has become common practices worldwide. However, the quality improved drinking water still needs to be delivered by the old distribution system with harboured material which may cause serious water quality deterioration during the shifting of supply water quality. Until now, the accompanying transition effects within distribution networks caused by changes of water quality or hydraulic condition have been poorly documented. These irregular operations will impact water quality
negatively through the associated physiochemical and microbiological destabilization of accumulated materials such as loose deposits resuspension and biofilm detachment.
The destabilized materials would be released as suspended solids into bulk water and travel throughout distribution networks. Therefore, the investigation on changes of suspended solids characteristics is the key to understand transition effects. To study the transition effects through suspended solids, there are challenges: 1) suspended solids are diluted in the big volume of water, resulting in low concentration if take water samples; 2) the detection limit of regular parameters makes the changes hard to be noticed; 3) the
release of suspended particles is non-periodicity, which leads to the difficulty of catching the moment.
In my studies, the online particle sampling system (OPSS) was applied in three distribution systems to collect and monitor suspended solids, which successfully overcame all of the three challenges mentioned above. The main objective of this report is to study water quality changes under regular operations, and transition effects under irregular operations such as the upgraded treatment and pipe cleaning by using OPSS. At the end of the report, the problems were diagnosed and the management strategies were proposed for different systems.