Quality change mechanism and drinking safety of repeatedly-boiled water and prolonged-boil water: A comparative study

Quality safety and potability of repeatedly-boiled water (RBW) and prolonged-boil water (PBW) lead to concern and even misgivings in the public from time to time, especially in China, and other societies have a habit of drinking boiled water, with improvements of living standards and increasing concerns for human health. This phenomenon is mainly attributed to the fact that the conclusions drawn from existing scienti ﬁ c experiments could not respond well to the concerns. In order to make up for this de ﬁ ciency, tap water was selected to carry out RBW and PBW experiments independently. The quality changes of RBW and PBW show very similar trends that are not as great as might be imagined, and both are impacted by the tap water quality and the physiochemical effects. The dominating physiochemical effects are the water evaporation and the resulting concentration of unreactive components (most dissolved components), which can easily be explained by the existing evaporation-concentration theory. The results show that tap water will be still safe and potable after being frequently boiled or after having undergone prolonged boiling, as long as it satis ﬁ es the sanitary standards of drinking water prior to heating. Therefore, there is no need to worry about drinking RBW or PBW in daily life. drinking RBW or PBW in daily life. (cid:129) The physicochemical effects controlling the boiling process were identi ﬁ ed, and the credibility of the results used to dispel misgivings was demonstrated. (cid:129) We sincerely hope that the publication of this manuscript will be helpful for more or less dispelling the publics misgivings about the safety of repeatedly-boiled water and the prolonged-boil water.


INTRODUCTION
Due to habits or history, residents in some countries such as drinking water more than once or unconsciously for a long period or even constantly in daily life (Luo ). Water that is boiled many times and is cooled to normal temperatures after each time is often called 'repeatedly-boiled water' (RBW), while water that is boiled for a long duration is often called 'prolonged-boil water' (PBW). The presence of RBW and PBW is quite normal and even inevitable in drinking fountains at home or in public (Luo ), which are even seen everywhere in the daily life of Chinese people ( Figure 1). This raises a practical issue in the public: whether the quality of RBW or PBW is safe for drinking, which has attracted increasing attention in the public with the improvement in people's living standards and the increasing concerns for human health (Song et al. ; Troldborg et al. ; Ma et al. ; Neil et al. ). Indeed, controversies remain among the public regarding this issue (Shen & Zhang ), while the academic community has not yet provided an adequate systematic and authoritative explanation for this issue.
The idea that RBW and PBW are not potable or are even carcinogenic has spread among the public of China in recent years and is even occasionally seen in news reports and in 'scientific tips' in newspapers and on the internet (Shen & Zhang ; Luo ), which has caused psychological burdens and anxiety for the public.
However, it is not difficult to find that almost all of the conclusions in these reports come from 'perceptual knowledge' or even 'parrot' ideas rather than relying on rigorous scientific experiments. This is a situation that neither the public nor the academic community would like to see.
These perceptions are generally based on the following logic: RBW and PBW can quickly cause an increase in salinity (known as TDS), hardness and heavy metals attributed to rapid water loss and thus the concentration of the solutes because of intensive water evaporation; a   Based on the above considerations, we identified the aim of this study, and it is to reveal the water quality change process and its mechanism of repeatedly-boiled water and prolonged-boil water, and further to determine the potability and safety for drinking of these two kinds of water in daily life. We sincerely hope that our efforts in this regard will be helpful for dispelling the misgivings of the public.

Experimental procedures
The boiling water experiments were carried out in the laboratory, and involved two independent experiments to emulate two common boiling habits of drinking water in daily life: RBW experiment (No. I) and PBW experiment (No. II), respectively. The experimental water (raw water) was taken directly from the urban tap water of Beijing, China, which entered the urban water distribution network after chlorination disinfection in the water plant. The boiling water container was a 15 L stainless steel steamer ( Figure 2) and was newly purchased for this experiment, and the heating device was a household induction cooker with a power of 2 kW. Before each experiment, the steamer was washed thoroughly using distilled water and then dried before adding the experimental water. , NH 4 þ -N, NO 3 --N and NO 2 --N were determined using ion chromatography (Thermo ICS-2100); HCO 3 À was determined using acid-base titration (0-50 mL burette); CHCl 3 and CHCl 4 were determined using meteorological chromatography (Varian CP-3800); COD Mn , volatile phenols and anionic synthetic detergent were determined using spectrophotometry (TU-1810DAPC); chroma was determined using platinum cobalt colorimetry; turbidity was determined using photoelectric colorimetry (GDS-3); smell and taste were determined through smell and taste; visible to the naked eye was determined through direct observation; total alpha radioactivity was determined using low background total alpha monitoring; total beta radioactivity was determined using a thin sample method; chlorine and free chlorine preparation (disinfection by-products) were determined using N, N-diethyl-p-phenylenediamine. In addition, the stable isotopes of water hydrogen and oxygen (D and 18 O) were also determined using a liquid water isotope analyzer (LGR912-0008) ( Table 1) to reveal the water evaporating process.
The equilibrium of anion and cation charge of each water sample was calculated to check the reliability of the measured data, and data were considered potentially valid only if the calculated result was neutral. The maximum difference of the replicate samples of all the samples was less than 0.5%, and thus all the measured data were valid.
The average value of the replicate samples was assigned to the corresponding sample for analyses. Thus, a total of 46 indicators/parameters (Table 1) were obtained for all collected samples and a total of 1,656 measured data points were obtained for analyses.

Analysis methods
The analysis methods in the study involved statistical analy- The details of all these methods and standards are omitted here given that they are very common and easily located, and related references can be used.
The relative value (relative to initial concentration) of the concentrating speed of chemical components in water during the test is explained by concentration ratio (CR 1 ).
the concentration rate (CR 2 ) is used to calculate the absolute value of the concentrating speed of chemical components in water during the test. The concentration rate (CR 2 ) is equal to the concentration ratio (CR 1 ) times the initial concentration. The two distinct equations of CR 1 and CR 2 can be written as: where CR 1 is the concentration ratio (%); CR 2 is the concen- is the initial concentration (ML -3 ); and N is the test count.

Change trends of the indicators
Of the 44 measured water quality indicators (Table 2), Fe, Mn, Al, Ba, Zn, As, Cd, Cr, Cu, Ni, Sb, Hg, Se, Pb, CN -,    It is worth noting that the same indicators from the two experiments generally presented similar change trends ( Figure 3) and had similar value ranges ( Table 2) (Table 2), therefore the water is safe for drinking.

Nevertheless, any microbial effects in the experiments
should be neglected considering the water temperature.
Thus, the revelation of the mechanism of quality change of repeatedly-boiled water and prolonged-boil water will proceed from the physical and chemical effects.

Physical effects
It can be confirmed that intensive water evaporation pro-  (Table 3) but also exhibited very similar increasing amplitudes ( Figure 5(a)). For example, the correlation coefficient of  (Table 3) but also had relatively similar decreasing amplitudes (Figure 5(a)). For example, the correlation coefficient of Ca 2þ and HCO 3 for the two experiments reached 0.79 (I) and 0.89 (II), respectively (Table 3). Furthermore, the linear correlations between the increasing and decreasing components were also clear (Table 3) in the experiments, it is clear that the phenomenon cannot be fully explained by physical effects but must be explained by considering chemical effects.

Chemical effects
First, it is well known that Ca 2þ and HCO 3 in water will undergo a thermal decomposition reaction when water is heated, in which CaCO 3 precipitates and CO 2 volatilizes and -4.54% (II) compared with the tap water ( Figure 5(b)).
It is also well known that NH 4 þ could be in the form of NH 3 (known as ammonia gas) (Wang et al. ). Thus, the measured concentrations of NH 4 þ -N contain both NH 4 þ and NH 3 , which is more complicated than and thus different from the case of Ca 2þ , HCO 3 and SO 4 2-(discussed above) due to of the existence of gas. NH 4 þ and HCO 3 in water also occur in a thermal decomposition reaction when the water is heated, in which both HN 3 and CO 2 volatilize as the pro- (I) and -3.00% (II) compared with the tap water, (Figure 5(b)).
The temporary hardness value also decreases quickly and is determined by Ca 2þ and HCO 3 -. Therefore, it is beneficial to reduce the temporary hardness through boiling the drinking water, as temporary hardness is also considered   The data in the lower left are from the repeatedly-boiled water experiment (I; n ¼ 18), and those in the upper right are from the prolonged-boil water experiment (II; n ¼ 16).
to be one of the harmful indicators if the concentration is relatively high in the drinking water. Different from the chemical components and the temporary hardness discussed above, TDS shows a fluctuation change rather than a trend change (Figure 3(e)). The phenomenon is synthetically and simultaneously determined by the concentration, and the volatilization and precipitation effects, of which the former causes a TDS increase, while the latter two cause TDS to decrease.
After the above analyses, the physicochemical effects Considering the undetectable components in the experiments, these phenomenon at least indicate that these components do not increase significantly caused by concentration effect or physiochemical dissolution from the wall of the steamer, or their concentrations were always too low to be detected although an increase actually took place.

Possible increases of harmful substances
Although the above has revealed the mechanism of water quality change, the public may still have doubts because of the absence of poisonous and harmful substances above, which are discussed in this section. The discussion is divided into two parts considering the actual results: inorganic and organic nitrogen species and other poisonous and harmful substances including heavy metals, trace organic substances, pathogenic microorganisms and radioactive substances.

N species
It is well known, or at least viewed by the public, that the concentration of nitrogen will obviously increase if the drinking water is boiled often or boiled for a long time (Liang & Chen ; Luo ). This is discussed in detail here based on experimental data. The possibility of an increase in harm from nitrogen is also discussed here considering the public concerns and misgivings and the complexity of the biochemical properties of nitrogen. The analysis in 'Mechanism of water quality change' above has shown that NO 3 --N and NO 2 --N present the same increasing trends as the other unreactive components in the experiments. CR 1 of NO 3 --N of the two experiments are 6.13% (I) and 5.62% (II), and the corresponding values of NO 2 --N are 6.43% (I) and 6.20% (II) (Figure 5(b)). CR 1 of NH 4 þ -N of the two experiments are 1.00% (I) and -3.00% (II) compared with the tap water, respectively ( Figure 5(b)). CR 1 of trinitrogen of the two experiments are 6.08% (I) and 5.50% (II), respectively. That is, trinitrogen levels did not appear to be abnormal except for a gradual increase during the experiments (Figure 7). Thus, we have to say that the changing law of trinitrogen is not different from the other components discussed above.
In addition to the physicochemical effects discussed other physiochemical and biological effects will not occur. Therefore, the law of the concentration increases of heavy metals will be at least similar to, if not the same as, that of Na þ .
High temperature is conducive to volatilization and degradation of trace organic substances (Li et al. b), so there is no doubt that boiling water is conducive to improving water quality. Moreover, the longer the boiling time is, the more the boiling times are, the more obvious the improvement effect is. As for the pathogenic microorganisms that may remain in tap water, they will be killed in the boiling process (Stijn et al. ), so we should not worry about it. As for radioactive substances, the situation will not be worse than that of heavy metals, because the decay of them is not affected by temperature, physiochemical and biological effects.

General applicability of the study findings
To make the results of this study more applicable and more broadly instructive, it is indispensable to demonstrate the credibility of the results obtained by this study. It can be seen from the above analyses that the water quality of RBW and PBW are determined by both the quality of the tap water (such as tap water) and the boiling habits. Therefore, demonstrating of the credibility of the results for guiding practices will also proceed from these two factors.

Representativeness of the municipal sample used in the experiment
Undoubtedly, the water quality change in both RBW and PBW originates from the tap water. If the tap water for boiling is from tap water, its quality is controlled by both the incoming water of the water and by the treatment process of the supply plant. In other words, the quality of the tap water varies from one plant to another. To verify the representativeness of the experimental water used in this study, water quality data from six urban plants in China were collected ( Type III means that new water is added continuously as long as there is space in the boiler. All the new water refers to the tap water, the quality of which is deservedly considered to satisfy the sanitary standards for drinking water. As for Type I, the water in the boiler will decrease quickly during heating and boiling because of the limited volume and the intensive evaporation of the water. The remaining water available for drinking in the boiler will be limited if it is boiled for a certain number of times (known  (Figure 5(b)) were adopted for calculation in order to reflect the worst scenario. TDS is largely determined by the macro components (Na þ þ K þ , Ca 2þ , Mg 2þ , HCO 3 -, Cland NO 3 --N), out of which Na þ þ K þ and Clare the fastest concentrated cations and anions, respectively ( Figure 5(b)). It is necessary to boil the tap water 268 and 152 times, respectively, in order to make the concentrations of Na þ þ K þ and Clin the tap water (11.73 and 21.61 mg/L) exceed the acceptable levels (200 and 250 mg/L). The corresponding times required for TDS is 73 if only the increasing components (known as TDS-(Ca 2þ þ 1/2 HCO 3 -) are considered, and the number will be much larger if the decreasing components are also considered. As to Fe, Mn and other possible heavy metals, the number of times cannot be calculated directly because they were always below the detectable limits in the experiments. In order to carry out the calculation, we assume that the concentrations of these heavy metals exactly equal the detectable limits (Table 4), and the concentration processes of them in the boiling process are similar to that of Na þ . Furthermore, we take the heavy metal with the smallest multiple between the acceptable value and the detectable limit as an example in order to reflect the worst scenario, and the heavy metal is Fe and the multiple is 10. Thus, the calculation for heavy metals can be performed and the corresponding number of times is 140. The corresponding numbers of times of NO 3 --N and NO 2 --N are 18 and 22, respectively. It can be seen from the above calculation that NO 3 --N and NO 2 --N are the easiest components to exceed the acceptable levels, because the numbers of times required of them are one order of magnitude smaller than those of the salinity and heavy metals.
That is to say, the RBW and the PBW are potable as long as the concentrations of NO 3 --N and NO 2 --N do not exceed the acceptable levels. In fact, the concentrations of NO 3 --N and NO 2 --N in tap water are generally lower than those of the experimental water (Table 4)

CONCLUSIONS
The two experiments (repeatedly-boiled water and prolonged-boil water experiments) successfully imitated two common habits of boiling water in daily life. The tap water used in the experiments was boiled for many more times than may happen in daily life, and the boiling time was also much longer. The water quality changes of the two experiments were very similar, which were modest in general and not as great as may be imagined by the public.
The water quality initially depended on the quality of the tap water, but was gradually dominated by the physiochem- concentration effects of nitrate, nitrite and heavy metals, as well as other unreactive chemicals, indeed occurred but was very limited. In fact, no other complicated or mysterious effects took place except the effects discussed above which could be explained by the existing simple evaporation-concentration theory. The theory can be expressed simply as, how much percentage of the water in the boiling container is evaporated, and how much percentage of the concentrations of the unreactive components in the water will increase accordingly. In general, the changes in the amplitudes of all dissolved chemicals in the water, including those harmful to human health, will be very limited, without degrading the water quality grade. That is, the quality of the tap water will be still safe for drinking after being frequently boiled or after prolonged boiling, as long as it satisfies the sanitary standards of drinking water prior to heating. Therefore, there is no need to worry about drinking repeatedlyboiled water or prolonged-boil water in daily life.