Variations of residual trihalomethane concentration in pipe-borne water during different in-house practices

Natural organic matter (NOM) is a precursor for the formation of disinfectant by-products (DBPs) by water chlorination. Even though more than 700 DBPs are identified, fewer than 30 DBPs are caused for carcinogenicity (Richardson et al. 2007; Mian et al. 2018). Among those DBPs, trihalomethanes (THMs) are the prevalent class of DBPs formed (King & Marrett 1996). The four ubiquitous THMs are chloroform (CHCl₃), bromodichloromethane (CHBrCl₂), dibromochloromethane (CHBr₂Cl), and bromoform (CHBr₃) (Al-Tmemy et al. 2018). THM formation continually occurs from the water treatment plant to consumer tap; the rate of THM formation depends on chlorine dose, retention time, type of NOM, pH, and bromide (Sadiq & Rodriguez 2004; Dion-Fortier et al. 2009; Weragoda et al. 2015; Amarasooriya et al. 2018).

THMs are cytotoxic and genotoxic to humans (Wagner & Plewa 2017). Moreover, long-term exposure to high levels of total THM (TTHM) caused carcinogenic defects such as intestine, renal, liver, thyroid, breast, and bladder cancer and leukaemia (Backer et al. 2000; Chaidez & Gerba 2004; Aslani et al. 2019; Diana et al. 2019). Further, mutagenic defects also could be resulted by long-term THM ingestion (Nieuwenhuijsen 2018). Exposure to THM could happen through consumption of drinking water, inhalation, and dermal contact too. Also, chlorinated drinking contributes approximately 50–70% of the TTHM risk (Jo et al. 1990; Lee et al. 2004). According to the United States Environmental Protection Agency (USEPA) defined TTHMs in drinking water should not exceed 80 μg/L as permissible levels (USEPA 2016).

In several parts of the world, numerous strategies such as refrigeration, boiling, and filtering of water are suggested to mitigate TTHM exposure, and those approaches reduced concentrations (Chowdhury et al. 2010). It was reported that commonly used domestic filtration devices consisting of activated carbon and ion exchange resins reduced THM up to 47% (Gibbons & Laha 1999). Physical methods such as boiling reduced about 83% of CHCl₃ (Wu et al. 2001; Dong et al. 2020), and 1–5 min of boiling removed 68–98% of THMs according to Krasner & Wright (2005). Levesque et al. (2006) had observed 30, 87, and 92% reduction of THMs through refrigeration, boiling, and filtering, respectively. Even though such inexpensive physical treatments were studied globally for removing the THMs in water (Ebrahim et al. 2016), no study was found reporting in the South Asian context.

Most of the countries in the South-East Asia region do not have in-house pipe-borne water (PBW) connections or uninterrupted water supply. Hence, the people in these regions usually store additional water required for their consumption (Äüôú 2003). These people commonly store their water in various types of vessels composed of different materials such as plastic (P), clay (C), stainless steel (SS), glass (G), and aluminium (Al), and storage of water for a longer period may facilitate or decrease the TTHM formation. Moreover, people store boiled and unboiled (UB) water before consumption too.

Up to date, the impact of in-house boiling, unboiling, and storage practices of PBW on THM formation has not been studied. Accordingly, for the first time, in this study, we examined the variation of THM concentration as a function of in-house practices to identify and evaluate the TTHMs formation in storage and other practices as well as best in-house practice to reduce the TTHM.

This research consists of the following laboratory studies: a study of THM variation in different PBW boiling practices in a SS kettle, a study of THM variation in boiled water storage in C, Al, SS, G, and P vessels with open and closed conditions, a study of THM variation in UB water stored in C, Al, SS, G, and P vessels with open and close conditions. For the aforementioned studies, PBW was collected from the Greater Kandy Water Treatment plant (GKWTP) distribution system, Kandy, Sri Lanka (location: 7°19′02.3″N 80°37′17.8″E) to meet the actual water receiving conditions by the consumers (i.e., residual chlorine (RCL), resident time, and temperature). Accordingly, samples were collected around the morning (the average seasonal temperature in the region was 23.5 °C (Climate data, 2022)), 1 km away from the water treatment plant in the year 2020. At the selected sampling point, the RCL measured was around 1.0 mg/L.

For the PBW storage experiments, storage vessels made out of C, Al, SS, G, and P were cleaned by detergent wash followed by a tap rinse, 5% HNO₃ rinse, three times distilled water rinse, ultra-pure water rinse, and final rinse with stored sample water. Furthermore, from each type of vessel, four PBW storage prototypes were set up and studied with boiled and UB water. The PBW storage prototypes were as follows: UB water stored in an opened vessel (UBOV) and in a closed vessel (UBCV), CBWH water stored in an opened vessel (BOV) and in a closed vessel (BCV). Samples were collected to test the initial THM concentration in PBW used to set up the UBOV and UBCV prototypes, before boiling the water and after boiling the water used to set up the BOV and BCV prototypes. The duration of the water boil was 20 min and then transferred immediately to prepare the BOV and BCV setups. After filling water, all vessels were stored at room temperature (25 °C), and samples for the THMs analysis were collected at 6, 24, and 48 h time intervals from each prototype. The stored water in the vessels was shaken, and 30 ml of the sample was collected.

Obtained data were analysed using Microsoft Excel 2010 and Minitab 16.1. Bar charts were generated through Excel 2010. The t-test was conducted with Minitab 16.1. The percentage differences of CHCl₃, CHBrCl₂, CHBr₂Cl, CHBr₃, and TTHM were calculated using Equation (1).

THMs were analysed based on the gas chromatography system equipped with an electron capture detector headspace method (Kuivinen & Johnsson 1999). A computerized headspace gas chromatography system (Thermo Scientific Trace 1300) equipped with an electron capture detector (GC-⁶³ Ni ECD) and a TRIPlus RSH autosampler (Thermo Fisher Scientific Inc, USA) was used for THMs analyses under split/split-less mode (TRACE-TR5). THM stock standard solution was a mixture of CHCl₃, CHBrCl₂, CHBr₂Cl, and CHBr₃, which contains equal concentrations from each compound (200 μg/mL) (Sigma Aldrich, USA). By diluting the appropriate volume from the stock solution, 1,000 μg/mL was prepared in high-pressure liquid chromatography grade methanol (Sigma Aldrich, USA). Working standards were prepared by the addition of an intermediate solution to boiled ultra-pure water (Thermo Fisher Scientific, USA) to reduce contamination from organic (conductivity 0.05 μS/cm). Data processing was done using Chromeleon 7, version 7.2 (USA) software.

The RCl was measured using the N,N-diethyl-p-phenylene diamine standard colorimetric method (Eaton et al. 1992; Rodger et al. 1990) with a digital chlorine meter (HACH, USA). pH was measured using a standard pH electrode (Hatch, USA) coupled with an auto titrator (Metrohm, Trinto Plus 848). EC was measured using a portable EC meter (Thermo Scientific). Plastic (P), clay (C), SS, glass (G), and aluminium (Al) pots were obtained from the local market in Sri Lanka.

TTHM variation with different PBW boiling practices was studied to find the best boiling practice. OB, CBH, and CBWH are the practices based on the open and closed conditions and with or without headspace in the closed condition in the boiling container (SS kettle). According to the results, CHCl₃, CHCl₂Br, and CHClBr₂ were the only DBPs found in the water collected from the GKWTP distribution system. Table 1 shows the TTHM variation in PBW water for different boiling methods. The highest TTHM reduction efficiency of 30.11% is observed in the sample of the OB method due to the escape of THMs by volatilization.

Past studies showed that 75% of volatilization losses of THMs approached when the water was boiled even for a short period (12.5 min) (Batterman et al. 2000). Since this experiment was performed in a highly elevated region, the boiling temperature was recorded at around 96 °C. The boiling points of CHCl₃, CHCl₂Br, CHClBr₂, and CHBr₃ are 61.3, 90, 119 °C, and 149–150 °C, respectively (World Health Organization & International Programme on Chemical Safety 1996). Therefore, the TTHM levels are decreased with the boiling of PBW (Xiao et al. 2022; Zhao et al. 2022). Furthermore, the rate of volatilization during the heating of water may be influenced by the kettle size, boiling time, water pouring time and height, and other factors (Batterman et al. 2000). CBH and CBWH samples showed 8.08 and −38.71% TTHM reduction and formation efficiencies, respectively. The relatively higher removal of THMs in CBH could result due to volatilization into the headspace in the kettle. According to Zhang et al. (2015), if the volatilization of THMs is blocked by enclosing the container with boiled water (90 °C), THMs increase during the first few hours. Therefore, the −38.71% in the CBWH method resulted due to the formation of THMs in the enclosed system where volatilization is restricted. Furthermore, in the CBWH method, the gas phase above the liquid phase inside the kettle was only opened to the atmosphere through the spout. As the evaporation from the water surface in the CBH kettle was easier than in the CBWH kettle, the OB kettle was exposed to the atmosphere more than the CBH and CBWH methods, and it easily vaporized the THMs from the liquid phase. Therefore, the evaporation through the boiled water varies as follows: OB > CBH > CBWH.

The RCl reduction due to boiling could result in a decrease in THM formation while storage of boiled water (Zhang 2013) for a considerable time period. Therefore, samples analysed from the storage prototype with UB water showed greater percentages of formation in each THM compound during the storage when compared to the CBWH water.

Open and closed vessels with UB and CBWH water and PBW were studied and compared to find the best practice. When comparing the open and closed vessels, the open vessels showed a comparatively high reduction of CHCl₃ (Figure 1), CHBrCl₂ (Figure 2), CHBr₂Cl (Figure 3), and TTHM (Figure 4). This could be associated with the evaporation of THM species under open conditions because all these compounds are highly volatile (Chowdhury et al. 2010). In general, THM was reduced with time in each prototype due to the volatility of THM compounds.

However, the production of the CHCl₃ was observed with time in the UBOV (Figure 1). The increment was observed as 1.6, 4.2, and 13.3% in Al, SS, and G vessels, respectively, for 6 h storage time. The G, UBOV showed a 13.5% increment for 12 h storage time. The increment of CHBrCl₂ was observed in the G, UBOV as 19.9 and 8.6% for 6 and 12 h storage time, respectively. A 36% CHBr₂Cl formation was observed in G, UBOV for 24 h storage time. This formation could have resulted due to the reaction of RCl and organic matter in the UB water. Furthermore, the space where the experiment was carried out was exposed to sunlight. Therefore, UV irradiation could enter the vessels and enhance the CHCl₃, CHBrCl₂, and CHBr₂Cl generation (Cassan et al. 2006).

Furthermore, the BCV prototype showed the formation of CHCl₃. The formation of CHCl₃ was observed as 11.3, 37.1, 21.6, 76.3, and 28.9% in P, C, SS, G, and Al vessels, respectively, for six storage time intervals (Figure 1(a)). The SS, G, and Al BCV prototypes showed a 28.1, 90.9, and 32.6% formation of CHCl₃ for 12 h storage time. Moreover, the G and Al BCV prototypes showed a 9.8 and 41.3% formation of CHCl₃ for 24 h storage time. This could be resulted due to THM formation in the first few hours of boiled water storage in an enclosed container (Zhang et al. 2015). CHBr₂Cl in each BOV and BCV storage showed a reduction throughout the storage time. Moreover, TTHM showed a reduction over time in the CBWH stored in both closed and opened vessels due to hydrolysis and evaporation of THMs, respectively (Batterman et al. 2000; Zhang et al. 2015; Zhao et al. 2022).

UB and CBWH, PBW stored in C, Al, SS, G, and P vessels in open and closed conditions were analysed to find the best type of vessel.

When considering the different types of vessels, the highest CHCl₃ reduction was recorded in the C, BOV. The percentage reductions were observed as 97.9, 98.7, and 99.6% in 6, 12, and 24 h storage time, respectively. According to Figure 2(b), the highest CHBrCl₂ reductions were also recorded in C, BOV vessel as 89.9, 97.8, and 100% for 6, 12, and 24 h storage time, respectively. The highest percentage of TTHM reductions in the C, BOV water was recorded as 98.2, 99, and 99.7% for 6, 12, and 24 h time periods, respectively. The percentage reduction of THMs was increased with the storage time. The boiled water showed the highest reduction of THMs due to the evaporation and hydrolysis of THMs (Batterman et al. 2000; Zhang et al. 2015; Zhao et al. 2022). Furthermore, the porous structure of C vessels could enhance the evaporation of THMs through the wall of the vessels (Hunt 2017). Moreover, the overall reduction of each compound varied as CHBr₂Cl > CHBrCl₂ > CHCl₃. That variation could be resulted due to a greater hydrolysis rate constant of the brominated THMs than the chlorinated THMs in boiled water stored in enclosed containers (Zhang et al. 2015).

UBOV prototype of C vessel showed the highest CHCl₃ reductions of 27.9, 64.35, and 77.3%, respectively, for 6, 12, and 24 h time intervals. Further, the C, UBOV showed the highest CHBrCl₂ reduction of 46% after 24 h storage. The highest CHBr₂Cl reduction was observed in the C, UBOV after 24 h of storage time. The TTHM reduction in C, UBOV varied as 27.06, 65, and 79.9% for 6, 12, and 24 h time intervals, respectively. When compared to other storage vessel materials, C showed a higher porosity in its structure. Porous structures keep the content cool by evaporation in C vessel (Hunt 2017). It possesses a better adsorption capability for organic contaminants due to the negative charge on the clay (Boyd et al. 1988). Moreover, it controlled the THM formation with the presence of RCl.

When considering the overall reduction of TTHM in each vessel type, the prototypes with CBWH water showed reduction as follows in 6 h time intervals: C > G > Al > P > SS in the BOV and as C > G > P > SS > Al in the BCV. The aforementioned THM variations could be resulted due to the hydrolysis of THMs by the thermal conductivity of storage vessel material, which evaporate through the surface and wall of the storage vessel.

UB water showed an overall reduction of THMs as follows in 6 h time intervals: C > P > Al > SS > G in the UBCV and SS > C > Al > P > G in the UBOV. THM concentrations can increase in heating water at lower temperatures than boiling (Weisel & Chen 1994; Batterman et al. 2000). Each material can conduct thermal energy from a light source (sunlight or fluorescence light bulb) (Sioshansi & Denholm 2010; Tan & Panda 2011) to store water through the thermal conductivity of the material. Moreover, UV irradiation could enhance the CHCl₃ and CHBrCl₂ formation via increased active chlorine (Cassan et al. 2006). Thermal conductivity at room temperature (value) of each storage material varies as follows: P < C < G < SS < Al (Childs et al. 1973; Woodcraft 2005; Xu et al. 2019). Therefore, the temperature variation could act on each vessel accordingly. Further, the UB water could be reacted with the remaining RCl from the heat obtained from thermal conductivity and formed THMs accordingly.

According to the variation mentioned earlier, G vessel showed the minimum reduction of THMs when storing UB water. The highest light penetration occurred in G vessels due to the transparency. That could result due to high light penetration through the wall of G vessel when compared to other vessels. Therefore, THMs could be formed during the storage of UB water in G vessels.

According to Mohanan et al. (2017), the best storage vessel material based on pH, EC, total dissolved solids, dissolve oxygen, hardness, alkalinity, biological oxygen demand, and chemical oxygen demand was given as follows: C > G > SS > Al > P. However, that study did not consider the THMs in stored water. Therefore, when considering this study, the overall highest reduction percentage of TTHM in 6 h varies as follows: BOV storage in C > G > Al > P > SS vessels > BCV storage in C > G > P > SS > Al vessels > UBOV storage in SS > C > Al > P > G vessels > UBCV storage in C > P > Al > SS > G vessels.

This study was conducted to investigate the effect of different in-house practices on residual TTHM levels in PBW in South Asia. The impact on common in-house practices such as boiling, unboiling, storage vessel materials, open and closed storage practices, and the storage times were investigated to monitor residual TTHM levels. Vessels made out of commonly used materials such as P, C, SS, G, and Al were used for the study. According to the variation of TTHM in different boiling methods, it can be concluded that the formation of THMs increased in the system where volatilization is restricted. Considering the variation OB > CBH > CBWH of TTHM reduction efficiency during different boiling practices, OB is recommended as the best practice for boiling water. According to the comparison of boiled and UB water storage in CBWH and UB storage prototypes, storage of UB water in all types of vessels has increased the formation of THM compounds, while CBWH water storage has reduced the formed TTHM. During the comparison of THM concentrations in open and closed vessel storage, the open vessels showed a comparatively high reduction of TTHM. Furthermore, comparing the type of vessel data, the highest THM reduction was recorded in the C for both BOV and UB, while the highest THM reduction was recorded in the SS for UBOV. TTHM formation was observed in glass vessel storage. Accordingly, it can be concluded that boiled water storage in C is appropriate.

However, further research is recommended to find the THM reduction mechanism from Clay vessels. In order to find the mechanism, it is suggested to conduct adsorption studies considering the material properties.

The authors would acknowledge the Prof. Rohan Weerasooriya at National Institute of Fundamental Studies for guidance and National water supply and drainage board and Ministry of city planning and water supply for providing analytical facilities and funding. Furthermore, we would like to express our gratitude to all the laboratory staff in China Sri Lanka Research Project, especially A. Kularathne, C. Upeka, L. Bandara, R. S. De Silva, C. Madushani, I. Pathirathne, D. Kandeyaya, and A. M. R. C. Attanayake for research support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.