Introduction: The treated drinking water is delivered to consumers through pressurized distribution networks, and distribution of water is as important as production since quality of water delivered to consumers depends on the condition of pipes in the distribution network. Various chemical, physical and microbiological reactions occur in the water distribution system (WDS) and they can cause water quality deterioration. Disinfection by-products are formed by the reaction of organic matter and disinfectant. The most common disinfection by-products are trihalomethanes (THMs) (chloroform, bromoform, bromodichloromethane, and dibromochloromethane) and haloacetic acids (HAAs) (monochloro-, dichloro-, trichloro-, monobromo-, and dibromoacetic acid), and they are formed after chlorination of water. Chlorine dioxide (ClO2) is often used as an alternative disinfectant as it generates significantly fewer THMs and HAAs as compared to chlorine. However, about 60% of the applied ClO2 is reduced to chlorite ions, and about 10% is converted to chlorate ions. Therefore, chlorite and chlorate ions are disinfection by-products of ClO2. The maximum contaminant level (MCL) for chlorite and chlorate ions in Croatia is 400 μg/L and for total THMs 100 μg/L. Proposed MCL for total HAAs is 60 μg/L, however, their monitoring is still not mandatory. The amount and type of organic matter have the greatest effect on the formation of disinfection by-products. Organic matter is a complex mixture of pedogenic and anthropogenic material and it originates from the contact of water with organic matter in the hydrological cycle. Main components are carbohydrates, lipids, proteins and humic macromolecules. Fluorescence spectroscopy, especially excitation-emission matrices (EEMs) are one of the most popular techniques used for the determination and characterization of organic matter. EEMs provide information about relative concentration, structure, and characteristics of NOM (natural organic matter), with groups of fluorophores commonly named 'humic-like', 'fulvic-like', 'tyrosine-like', and 'tryptophan-like'. Parallel factor analysis (PARAFAC) is currently one of the most popular methods used to model generated data and to identify and quantify 'components' found in EEMs. Fluorescence spectroscopy is widely used in aqueous systems for monitoring the efficiency of water treatment steps, monitoring water quality at consumers tap or predicting the formation of disinfection by-products. It can be also used for real-time evaluation of the microbial quality of drinking water and for monitoring arsenic mobilization in groundwaters. Samples from WDS are rarely included in fluorescence spectroscopy research and it is important to explore the potential of fluorescence spectroscopy for monitoring changes in the WDS, as well as to establish correlation between organic matter and individual disinfection byproducts at the different parts of WDS. Drinking water entering the WDS contains different organic and inorganic matter and microbial cells which can accumulate in the WDS and later be released back in the bulk water. Pipe scales, biofilm and loose deposits develop and stabilize under historical conditions, however if destabilized they can be released in the water. Transition effects are physicochemical and microbiological water quality problems caused by irregular changes in supply water quality. Loose deposits and pipe scales consist mostly of iron, aluminium, manganese, arsenic and lead and can accumulate in the WDS even when their concentrations in treated water are very low. Organic matter can also accumulate in the WDS and form complexes with metals. These complexes could increase solubility and mobility of metal oxides in the WDS. For a better understanding of processes occurring in the WDS it is also important to characterize pipe scales. Release experiments under stagnant flow could help to predict metal release potential under different conditions, and to better understand the role of organic matter in the mobilization of different metals. This thesis aimed to determine disinfection by-products (THMs, HAAs, chlorite, and chlorate ions) and their spatial and temporal variability. The aim was also to characterize organic matter with fluorescence spectroscopy paired with parallel factor analysis (PARAFAC) and to determine metal concentrations in systems, their spatial and temporal variability, and the interaction of these parameters in the WDS. The aim was also to identify changes in the system that affects water quality (metal/metalloids and organic matter) on the example of WDSs that introduce water of different quality into the system Materials and methods: Preliminary research was conducted on 48 water distribution systems in Croatia. Of these, 35 WDSs used groundwater, 12 used surface water, and 1 used brackish water for supply. Twenty WDSs used Cl2, 16 WDSs used NaClO, 4 used a combination of Cl2 and NaClO, 6 WDSs used ClO2, and 2 WDSs used a combination of ClO2 and NaClO. Three water distribution systems were the focus of this research, WDS Zagreb, WDS Slavonski Brod, and WDS Osijek. WDS Zagreb is an example of a big WDS which supplies over 900 000 residents. Raw water contains low concentrations of organic matter and metals. WDS Slavonski Brod uses two different wellfields for water supply. Wellfield Jelas contained elevated concentrations of iron, manganese, and organic matter so was treated by a sequence of processes. In mid-2018 wellfield Sikirevci was put in use, which was connected to the eastern part of the city. Water from the new wellfield was not treated before distribution, just disinfected with ClO2. Groundwater quality in the three wells in wellfield B (Z6, Z7, Z8) that were used for water supply was not uniform and one of the wells contained elevated manganese concentrations (e.g. Z8 had Mn concentrations 107 ± 2 μg/L). Despite this, good water quality was achieved by mixing water from wells Z6 and Z7 before distribution. After a water discoloration event in June 2019 in the part of the city supplied by WDS-B, network flushing was conducted and samples were collected from fire hydrant faucets at 4 locations, including both first draw (2 samples) and after flushing (4 samples). The same day a control sample from the hydrant in the western part of the city supplied by WDS-A was also taken. WDSs Osijek was for decades supplied with groundwater containing elevated As, Fe, and organic matter concentrations. In 1986 a WTP was constructed to treat groundwater (GW) before its distribution. After the installation of the WTP, As concentrations dropped below 50 μg/L, the MCL for As proscribed by the Croatian Drinking water regulation (before 2013). As a result of the implementation of the EU Drinking Water Directive, Croatian regulations changed and the MCL for As was decreased to 10 μg/L. To be able to achieve the arsenic derogation limit of 30 μg/L, until the new plant becomes fully operational, a transitional solution of mixing GW and surface water (SW) was implemented under the sole discretion of the water utility. Three pipes from three different WDSs were collected (Osijek, Lipik-Pakrac and Baška). The pipe scales were collected and characterized by X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). Pipe scales were used for metal release experiments in stagnant conditions. Standard ISO methods were used for the determination of physico-chemical parameters like free chlorine, pH, turbidity, temperature, electrical conductivity, anions, cations, total organic carbon (TOC), THMs, metals and metalloids and microbial analyses. US EPA 552.3 method was used for determining HAAs. A fluorescence spectroscopy was used for organic matter characterization, after which PARAFAC modeling was performed. The X-ray diffraction (XRD) spectrums of scale samples were measured by an X-ray diffractometer, and the scanning electron microscope (SEM) images for surface analysis were obtained using a scanning electron microscope. Metal release experiments under stagnant conditions were carried in triplicates. 1.7 g of scale powder was weighted and placed on the bottom of experimental beaker with 1 L of experimental water added for release experiments. Tap water from three WDSs where pipes were collected was used as the experimental water. The experimental time was set at 96 hours, and the samples were taken at 0, 0,08 (5 min), 2, 4, 24, 72, 96 h Results and discussion: Three-component model was built for all three WDSs and it consists of humic-like organic matter, tyrosine-like and tryptophan-like organic matter. Humic organic matter intensity was stable across all three WDSs and did not show any seasonal changes. On the other hand, tyrosine intensity increased at the end of the network, probably as a result of biofilm formation. Tyrosine intensity increase was also observed in summer months, when the water temperatures are also higher. Preliminary research showed that the concentrations of measured disinfection by-products are lower than the MCL values. Generally, THMs are present in higher concentrations than HAAs in monitored WDSs, and THMs and HAAs concentrations are higher in surface water than in groundwater, probably because surface water often contains higher quantities of organic matter then groundwater. TCM was prevalent THM in all three systems. DBAA was prevalent HAA in WDS Zagreb, DCAA and DBAA in WDS Jelas, MBAA in fire hydrant samples from WDS Sikirevci and TCAA in WDS Osijek. HAAs and THMs concentrations increased in WDS Osijek after mixing groundwater with surface water which is richer with protein organic matter. THM and TCM correlated with humic organic matter, while DCAA and DBAA correlated with tyrosine. TCAA correlated with tryptophan in WDS Osijek. Introducing surface water to the WDS Osijek caused increase in tryptophan intensity, which directly caused higher amounts of TCAA. Based on the calculation of metal speciation and the balance of dissolution and precipitation (the calculation of the influence of oxidation-reduction potential values on pyrolusite (MnO2) concentrations) in the Jelas and Sikirevci systems and based on the analysis of hydrant flushes samples, it was determined that conditions in both systems are prone to the deposition of MnO2, but also iron and aluminum oxides. More rapid deposition of Mn(II) and the formation of larger quantities of MnO2 are expected in system Sikirevci. Firstly, because of higher pH values in system Sikirevci under which the oxidation of Mn(II) by ClO2 is kinetically more favourable, and secondly, because of higher Mn(II) concentrations in system Sikirevci in opposition to system Jelas. Even though Mn concentrations are under maximum contaminant level in system Sikirevci, disinfection by ClO2 under higher pH values caused Mn (II) oxidation and Mn particles formation and water discoloration events. First water discoloration event happened after change of water supply quality and higher concentrations of Mn entered the system. Our observations are probably the result of continuous Mn oxidation and particles formation, as well as desorption of loose deposits containing Mn and Fe combined with hydraulic changes. Transitional effects were also observed in WDS Osijek. Groundwater rich with As was mixed in different ratios with surface water which is As free to reduce As concentrations below 10 μg/L, which is MCL. It was achieved after six months of mixing groundwater and surface water. However, an increase in iron concentrations was also observed in the last months of the sampling campaign. The decrease in the efficiency of iron removal in the water treatment plant was probably related to the change in the composition of organic matter. The introduction of surface water in the system increased the amount of tryptophan, and the concentration of disinfection by-products. The correlation between the total number of bacteria and tyrosine indicates that this component could be used to assess bacteriological contamination of non-fecal origin, however with additional research. Three multiple regression models were built using organic matter components and disinfection by-products to predict As concentrations in WDS Osijek. These models highlight the impact of organic matter and DBPs on As concentrations in the WDS. All results from WDS Osijek were divided into three phases. The first phase lasted from approximately 2009 to April 2019 and only groundwater was used. During this phase, As, Fe, Mn and Al accumulated on the pipe walls in the system. The second phase lasted from May 2019 to July 2020 and during that phase mixed groundwater and surface water were used in various proportions. During that period, the concentrations of As, Fe and Al increased, and their release occurred. In the third phase, which covers August 2020 (last month of testing), water containing 40% groundwater and 60% surface water was used. As concentrations are below 10 μg/L. Accumulation of As and Fe and release of Mn and Al were observed through the network. Scale samples from three different water distribution systems were analyzed. Scale samples characterization showed that the samples mostly consist of iron oxides: magnetite, goethite and siderite. Iron is the main element present in the samples, followed by Al and Mn. As is present in larger quantities only in sample 1 (pipe scale from WDS Osijek). The monitored metals (Fe, Al, Mn, As, Pb) showed different release patterns of behavior in the three experiments. Fe, Al, As and Pb have the potential for co-release in experiment 1 (pipe scale from WDS Osijek). Mn and sulfate showed co-release in all three experiments, with the highest amount of release Mn found in experiment 3 (pipe scale from WDS Baška). This may indicate a potential problem with Mn in this system, especially with prolonged water retention time. Conclusions: Significant correlations between organic matter and disinfection by-products, metals, and the total number of colonies indicate the important role of organic matter in the processes in WDSs. These results indicate that potential problems in the WDS could be predicted by monitoring of organic matter, especially tyrosine.