Understanding and exploring the potentials of household water treatment methods for volatile disinfection by-products control: Kinetics, mechanisms, and influencing factors
Graphical abstract
Introduction
Disinfection via chlorine or alternative disinfectants (e.g., ozone) is a vital process for inactivating pathogens to provide safe drinking water. However, the disinfection process can produce undesirable disinfection by-products (DBPs) by the reaction of disinfectant with DBP precursors such as natural organic matter (NOM), bromide, and iodide [1]. Because trihalomethanes (THMs) were the first type of DBP detected [2] and are ubiquitously present in finished drinking water [3], [4], [5], [6], [7], they have received massive attention over the last several decades. In order to reduce carcinogenic risks [8], [9], four THMs (THM4) including trichloromethane (CHCl3, TCM), bromodichloromethane (CHCl2Br, BDCM), dibromochloromethane (CHClBr2, DBCM), and tribromomethane (CHBr3, TBM) are regulated in many countries [10]. For example, the maximum concentration level (MCL) for THM4 is 80 μg/L in the U.S. [11] and 100 μg/L in Europe (regulation no.: 98/83/EC), whereas the MCLs for TCM and BDCM are set at 60 μg/L each and BCDM and TBM at 100 μg/L each in China (regulation no.: GB5749-2006). In addition, as emerging DBPs such as iodinated halomethanes (IHMs) were identified in waters [12], [13], [14], [15] and shown to be more cytotoxic than regulated, chlorinated and brominated THMs [16], there are also increasing concerns toward these new IHMs.
In terms of treatment technology, the current strategy for DBP control is primarily conducted at the drinking water treatment plant (DWTP) by employing one or more of the following three approaches: 1) removal of DBP precursors, 2) use of alternative disinfectants, and 3) optimization of the disinfectant contact time and location of addition [17]. These approaches are based upon the knowledge that DBP formation is dependent upon the quantity and characteristics of the precursors, disinfectant reactivity, and reaction time [18]. However, because residual chlorine or chloramines is required to prevent pathogen regrowth in a distribution system and because only a portion of the NOM is removed by conventional water treatment processes and even advanced water treatment processes [19], [20], DBP formation occurs in the distribution system (e.g., chlorinated and brominated THMs in chlorinated systems, iodinated THMs in chloraminated systems) [1]. Thus, some utilities in the U.S. have been implementing air stripping [21] in parts of their distribution systems to reduce the concentrations of THMs. However, for utilities without such post-formation treatment systems, consumers are likely exposed to waters containing various types and levels of DBPs [22], [23], [24] of health concern. Even if the concentration level is in compliance, additional efforts to optimize the water quality may be needed.
Household water treatment (HWT) is a potential approach to supplement the existing DBP treatment strategy. For example boiling water, although it is usually perceived as a means of controlling pathogens in low-income, developing countries [25], [26] or a temporary strategy dealing with emerging events in industrialized nations, recent research has demonstrated its potentials on the control of DBPs in developed countries [27], [28], [29], [30], [31]. Thus, boiling may be employed to optimize water quality. However, earlier studies did not provide insights into many details regarding the heating/boiling process, such as what are the key forces responsible for the DBP loss, if hydrolysis and oxidation may contribute to DBP loss besides volatilization, and how much the preceding heating process, instead of boiling, accounts for the DBP loss. Therefore, the mechanisms are not fully understood. In addition, earlier studies usually boiled water for more than one min, which is much longer than a normal operation time provided by a contemporary electric boiler equipped with an automatic switch-off function (boiling usually lasts for only seconds), meaning that the treatability of a boiler on THMs may be overestimated based on boiling water studies.
From an epidemiology point of view, some studies examined the impact of exposure to THMs from ingestion, showering, bathing, and swimming, and suggested that the most important route of exposure to THMs may be either ingestion [32] or inhalation and dermal adsorption [33]. Meanwhile, there were other studies showing no or a weak association between THMs levels and certain health risks [34], [35]. The reason may be partially due to the lack of consideration of HWT methods that impact the actual intake levels, since some of the THM data were collected from the monitoring department [35]. Therefore, a better understanding of the water quality changes from tap to cup may be critical for epidemiology studies. However, except for boiling, limited research has paid attention to the capabilities of other types of HWT methods or water handling habits on DBP removal, such as microwave irradiation, pouring, stirring, and shaking water, nor to the influencing factors that might help recognize their potentials.
Based upon these considerations, this study systematically evaluated the effectiveness of five HWT methods, including two household appliances (i.e., boiler and microwave irradiation) and three handling methods (i.e., pouring, stirring, and shaking), on removing four regulated THM4 (i.e., TCM, BDCM, DBCM, TBM) and three emerging IHMs (IHM3, including triiodomethane [CHI3, TIM] [36], diiodomethane [CH2I2, DIM] [37], and chloroiodomethane [CH2ClI, CIM] [38]). The selected seven HMs (HM7) represent a diverse degree of halogenation and halogen types typically reported in disinfected waters. The HWT methods were mostly implemented under conditions simulative of residential uses with varying initial concentration, water matrix, and free chlorine content. In order to better understand the potential of HWT, a variety of operational parameters, including power input, operation time, water volume, heating speed, cooling technique, and capping conditions, were also examined. Moreover, the study examined the relative importance of different mechanisms (e.g., hydrolysis versus volatilization) and working modes (e.g., turbulence versus bubbling effects) on HM removal, which may enable an in-depth understanding of these processes.
Section snippets
Samples and chemicals
Except for TIM, which was in solid form, all other HMs were purchased in liquid forms (GC grade, >98% purity). TCM, TIM, DIM, CIM, TBM, and methyl-tert-butyl ether (MTBE) were bought from Aladdin, Inc., China, and BDCM and DBCM were purchased from Sigma-Aldrich, Co., USA. Prior to experiments, a mixture of HM7 was dissolved in methanol and then diluted with ultrapure water as a stock solution (each at 1 mg/L). Sodium hypochlorite (7.5%, Damao Inc., China) was used as the free chlorine reagent.
Reactors and operation procedures
In
HM removal by boiler
Fig. 1a displays the removals of HM7 by an automatic switch-off boiler, and Fig. S1 in the supporting information reports the temperature changes in the water during the heating process. As shown, heating water for 4.2 min (with boiling for only 10 s) removed on average 89% (Table S1) of THM4 from 1L of water containing 200 μg/L of each HM, which confirms that it is a very efficient way to reduce THM4 exposures. However, the removals were lower than the results reported in other literature
Conclusions
DBPs in drinking water are a serious concern around the world, and HWT processes may act as an important tool in reducing the exposures to volatile DBPs. In this study, we showed pronounced removals of HMs with five types of HWT methods where volatilization was the dominant mechanism responsible for HM loss. Some specific findings are shown below:
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Heating with a boiler is robust in controlling HMs through a combined effect of water turbulence and bubbling phenomena. The efficiency of the
Acknowledgements
The study was financially supported by the National Natural Science Foundation of China (Grant No: 51278144), Shenzhen Science & Technology R&D Funding (Grant No: JCYJ20120613150442560 and JCYJ20150902154515690). Thanks also to our colleagues in the laboratory (Junli Wang, Wendong Shi, and Ruinan Niu).
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