Research articleDisinfection by-products in Croatian drinking water supplies with special emphasis on the water supply network in the city of Zagreb
Graphical abstract
Introduction
Disinfection of drinking water is an important step in the assurance of safe drinking water in public water supply systems (Baldursson and Karanis, 2011). However, disinfectants, such as chlorine, ozone, chlorine dioxide and chloramine, are strong oxidants and, if natural organic matter (NOM) is present in water, the disinfectant and NOM react and disinfection by-products (DBPs) are formed (Richardson, 2011). Since 1974, when the first DBP was identified (Rook, 1974), more than 600 DBPs have been reported in the literature but only a few are regulated or studied (Richardson et al., 2007). Epidemiological studies have associated chlorinated drinking water with increased health risks including bladder cancer (Villanueva et al., 2015). Other studies have explored the relationship between DBPs and fetal growth restrictions (Nieuwenhuijsen et al., 2009) and extensive research has been conducted on DBP genotoxicity (Cortés and Marcos, 2018). Subchronic and acute exposure to chlorite and chlorate ions can induce hematological responses resulting in oxidative damage to erythrocytes causing methemoglobinemia and hemolytic anemia (WHO, 2011). Collectively, these findings warrant a precautionary risk management approach towards DBPs.
Trihalomethanes (THMs) and haloacetic acids (HAAs) are the most common classes of DBPs, formed during chlorination of drinking water supplies. The maximum contaminant level (MCL) for total THMs is 80 μg/L and for the sum of the five most common HAAs is 60 μg/L in the USA (US EPA, 1998). By contrast, in the European Union (EU) which includes Croatia, only total THMs are regulated with a MCL of 100 μg/L. Although HAAs remain currently unregulated, this will be shortly reversed across the EU (EC, 2018).
Chlorine dioxide (ClO2) is commonly used as an alternative disinfectant to chlorine as it has been shown to generate lesser quantities of halogenated DBPs including THMs and HAAs (Gan et al., 2016). However, various organic and inorganic components present in water influence the reduction of chlorine dioxide to the inorganic DBP chlorite and subsequent conversion to chlorate ions (Gan et al., 2019). In the absence of EU regulations, the limits for chlorite and chlorate are prescribed by Croatian legislation with the MCL set individually at 400 μg/L (OG 125/17, 2017). Chlorite and chlorate will be also shortly regulated across the EU (EC, 2018).
The presence of DBPs in drinking water is influenced by disinfectant type and concentration, as well as raw water characteristics (Krasner et al., 2006; Roth and Cornwell, 2018). Particular characteristics and other factors that influence DBP formation include pH, water temperature, concentration and type of natural organic matter (NOM) as well as conditions of disinfection such as dose, point of addition, contact time and residual concentration in the network (Chhipi-Shrestha et al., 2018; Richardson and Postigo, 2015). Of these factors, concentration and type of NOM have been found to be the main precursor influencing HAA and THM formation (Liang and Singer, 2003). Determining the molecular characteristics of NOM present in water supply networks would confer great advantage towards mitigating the creation of DBPs given the clear link between the two components. While there are many studies on NOM properties in drinking water supply systems (Baghoth et al., 2011; Bieroza et al., 2011; Li et al., 2020; Stedmon et al., 2011; Vera et al., 2017; Yang et al., 2015b, 2015a), investigations into NOM characteristics at distribution system points of use are rare (Heibati et al., 2017).
There are several tools available for the identification and characterization of NOM such as TOC (Leenheer and Croué, 2003) and specific ultraviolet absorbance (SUVA) (Edzwald et al., 1985). One of the most popular methods used to characterize NOM is fluorescence spectroscopy, particularly excitation emission matrices (EEMs) which have sensitivity and selectivity advantages (Bieroza et al., 2009). EEMs provide information about relative concentration, structure and characteristics of NOM, with groups of fluorophores commonly named ‘humic-like’, ‘fulvic-like’, ‘tyrosine-like’ and ‘tryptophan-like’ (Hudson et al., 2007). Parallel factor analysis (PARAFAC) is currently one of the most popular methods used to model generated data (Fellman et al., 2008) and to identify and quantify ‘components’ found in EEMs (Murphy et al., 2013).
The aim of this study was to profile the nation-wide occurrence of different kinds of DBPs in 48 different water supply systems across Croatia. Considerations were made to encompass different water types and disinfectants used, with the specific aim of determining HAAs for the first time in Croatia, along with THMs, chlorates, chlorites and other physico-chemical parameters. Special consideration was given to the city of Zagreb, in which the water distribution network serves one quarter of the Croatian population. The spatial and temporal variations and the relationship between the concentrations of individual species from the same, or different, DBP families were evaluated. Furthermore, NOM was characterized with fluorescence spectroscopy in order to investigate the possible correlations between DBPs and PARAFAC components in order to determine whether fluorescence spectroscopy could be used to predict the likely presence of DBPs in this large drinking water distribution system.
Section snippets
Sampling of water supply systems across Croatia
The study was conducted on 48 water distribution systems in Croatia of which 35 are supplied by groundwater, 12 by surface water and one (1) by brackish water. Due to different raw water quality, 19 source waters (9 surface water and 10 groundwater) required physico-chemical treatment prior to disinfection. The brackish water was treated at a desalinization plant while the remaining 28 source waters were only disinfected. Chlorine was used as the disinfectant in 20 systems, sodium hypochlorite
Drinking water quality in Croatia
Maximum, minimum, limit of quantification (LOQ) and MCL for measured parameters are listed in Table 1. All drinking water samples except two were found to comply with the MCLs stated in the Croatian Ordinance for drinking water safety (OG 125/17, 2017). One was identified as unsafe due to elevated levels of free chlorine (0.7 mg/L) and chlorite (720 μg/L) and the other from the desalination plant due to high boron (1.1 mg/L) and chlorite (431 μg/L) concentrations. All measured metals and
Conclusions
In this study, we provide the first nation-wide assessment of DBP presence in Croatian drinking water supply networks. Importantly, this included sampling feed waters from a wide range of surface- and ground-water-derived supplies, which were subject to different disinfection treatments, including both chlorination and chlorine dioxide (with and without sodium hypochlorite dosing). Low levels of DBPs were observed across the 48 water supply systems with maximum TTHM and THAA concentrations
CRediT authorship contribution statement
L. Kurajica: Investigation, Data curation, Writing - original draft, Validation, Visualization. M. Ujević Bošnjak: Conceptualization, Investigation, Methodology, Validation, Writing - original draft, Funding acquisition. M. Novak Stankov: Formal analysis. A.S. Kinsela: Formal analysis, Writing - review & editing. J. Štiglić: Investigation, Data curation. D.T. Waite: Writing - review & editing. K. Capak: Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work has been supported by the Croatian Science Foundation under the project number [UIP-2017-05-3088].
The authors would like to thank their colleagues from the Croatian Institute of Public Health, Department for Water Safety and Supply and Unit for Metals and Metalloids for their help in sampling and analysis of water. Our sincere gratitude is also extended to four anonymous reviewers for their helpful comments and suggestions.
Glossary
- BDCM
- Bromodichloromethane
- DBAA
- Dibromoacetic acid
- DBCM
- Dibromochloromethane
- DCAA
- Dichloroacetic acid
- EEM
- Excitation-emission matrices
- MBAA
- Monobromoacetic acid
- MCAA
- Monochloroacetic acid
- NC
- Number of colonies
- NOM
- Natural organic matter
- ORP
- Oxidation-reduction potential
- PARAFAC
- Parallel factor analysis
- TBM
- Tribromomethane
- TC
- Total coliforms
- TCAA
- Trichloroacetic acid
- TCM
- Trichloromethane (chloroform)
- THAA
- Total haloacetic acid
- TTHM
- Total trihalomethanes
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