Bacterial diversity across four drinking water distribution systems in Croatia: impacts of water management practices and disinfection by-products.

Several factors may impact bacterial diversity in drinking water distribution systems (DWDSs) including the origin of the raw water, the water treatment technologies, and the disinfection practices applied. 16S rRNA metabarcoding was used for the in-depth characterization of bacterial communities in the four studied Croatian DWDSs (A, B, C, D) two of which had residual disinfectant (A, B) and two were without (C, D), while only B utilized the conventional water treatment technology. Significantly higher diversity and species richness were evidenced in non-disinfected DWDSs (p<0.05) compared to disinfected DWDSs. The phylum Proteobacteria was the most abundant in all the DWDSs, being proportionately higher in non-disinfected systems (p<0.05). The most abundant genera in DWDS-A Mycobacterium and Sphingomonas both positively correlated, whereas Lactobacillus negatively correlated with the concentration of disinfection by-products (DBPs) as a sum of haloacetic acids (HAAs). Conversely, the genus Ralstonia positively correlated with the individual DBP dichloroacetic acid. These results indicate that genera Sphingomonas, Mycobacterium, Lactobacillus and Ralstonia could have an effect on promoting the formation of DBPs, in a similar manner to how negatively correlated taxa may influence their degradation.

Mixing of arsenic-rich groundwater and surface water in drinking water distribution systems: Implications for contaminants, disinfection byproducts and organic components.

The utilization of groundwaters containing high levels of arsenic (As) for drinking water purposes presents major health and economic challenges for water utilities. One low-cost approach is to mix arsenic-rich groundwater (GW) with arsenic-free surface waters (SW) to achieve acceptable As levels. In this study we investigated the effect of different mixing ratios on water quality in an eastern Croatian water distribution system (WDS). To investigate the effects of mixing on drinking water quality, we measured the organic matter (OM) composition, disinfection byproduct (DBP) and metal concentrations in differently mixed ratios of GW and SW within the WDS. Fluorescence analysis revealed that the GW and SW had similar OM composition, with an almost equal ratio of humic- and protein-like OM throughout the WDS despite fluorescence indices revealing slightly different OM sources between the two water types. The tyrosine-like OM component was more variable, increasing during warmer months and towards the end of the WDS, most likely due to enhanced biofilm formation. Arsenic concentrations decreased to below 10 µg/L in the second half of the sampling campaign. Acceptable water quality was achieved after a period of destabilization and solubilization of loose deposits within the WDS resulting in their mobilization caused by water quality changes. Principal component and classification analysis, regression models and Spearman correlation coefficients revealed an association between As, OM and DBP concentrations with these correlations suggestive of their role in As mobilization in the WDS. Changing source waters, with different OM content and characteristics, corresponded to variable As release within the WDS.

Effects of changing supply water quality on drinking water distribution networks: Changes in NOM optical properties, disinfection byproduct formation, and Mn deposition and release.

Field studies were conducted in a Croatian city supplied by two distinct groundwater sources (referred to as A and B) to investigate both the effects of changing water source on the water quality in the drinking water supply system, as well as to further understand discoloration events that occurred in city locations that switched water from source A to B. The water treatment processes at site A were found to alter organic matter (OM) characteristics, removing humic substances while enhancing protein-derived (tryptophan) content. Although the humic-like component predominated in raw waters, microbially/protein-derived components were found to increase throughout the distribution networks of both systems. Disinfection byproducts (DBPs) such as total trihalomethane (TTHM) and total haloacetic acid (THAA) were prevalent in water distribution system (WDS)-A, which correlated with elevated OM content as well as re-chlorination with hypochlorite (NaOCl). Our field study revealed that THMs were more readily formed than HAAs during ClO2 treatment. Unsurprisingly, chlorite concentrations were generally higher than chlorate concentrations during ClO2 treatment, whereas (secondary) NaOCl disinfection contributed to higher chlorate production. Principal component analysis indicated that variable pH values and humic-like OM could affect Mn, As and Al concentrations at the consumer's tap. Our results suggested that although Mn concentrations complied with regulations at WDS-B and were below 50 µg/L after disinfection, Mn was oxidized and formed particulate Mn oxides capable of causing discoloration events depending on prevailing network physico-chemical and hydraulic conditions. Aluminium also appears to be released during hydraulic disturbances from extensive deposits within the network. Thermodynamic calculations showed that Mn-oxidation was strongly dependent upon the ORP, and to lesser extent the pH value. Collectively, our results confirm that ensuring the provision of safe drinking waters to consumers requires an understanding of water quality across entire distribution networks in addition to any routine post-treatment monitoring.

Disinfection by-products in Croatian drinking water supplies with special emphasis on the water supply network in the city of Zagreb.

The occurrence of disinfection by-products (DBPs) was investigated in 48 drinking water systems across Croatia. Eleven DBPs were studied: chlorite (ClO2-), chlorate (ClO3-), four trihalomethanes (THMs), and five haloacetic acids (HAAs). Furthermore, an intensive sampling program was conducted in the distribution system in the city of Zagreb where, aside from DBP analyses, natural organic matter (NOM) was characterized using fluorescence spectroscopy. In the drinking waters examined across Croatia, DBP values were found in the range from 0.7 µg/L to 32.8 µg/L for THMs, below LOQ to 17.2 µg/L for HAAs (primarily di- and trichloroacetic acids), below LOQ to 720 µg/L for ClO2- and below LOQ to 431 µg/L for ClO3-. The results obtained showed higher chlorite concentrations in the systems treated with hypochlorite compared to systems treated with chlorine dioxide. DBPs in the Zagreb distribution network were generally low (the average values were below 6 µg/L and 2 µg/L for total THM and total HAA respectively). In contrast to our observations throughout Croatia, dibromoacetic acid (DBAA) was found to be the predominant HAA within Zagreb, most likely due to the degradation of chlorinated carboxylates (di-/tri-chloroacetic) in the network. Characterization of NOM by Parallel Factor Analysis (PARAFAC) fluorescence spectroscopy across the Zagreb network showed distinct temporal variations arising from groundwater inputs, as evident from variable humic-, tyrosine-, and tryptophan-like peaks. Statistical correlations between fluorescence data and DBPs highlight its potential for monitoring the presence of DBPs in distribution networks.