According to Consolidation Resolution n.05/2017  , current law of water quality in Brazil, which sets forth the procedures of control and surveillance of water quality for human consumption and potable water standards, the water has to pass by a disinfection process that will inactivate pathogenic microorganisms. During the process the water may receive different chemical disinfectants; however, the chlorine is the most popular in the treatment of potable water and wastewater worldwide   .
The consequences of reactions between chlorine and natural organic compounds (NOCs) are the undesired byproducts of chlorination (as trihalomethanes and haloacetic acids), which occur since the treatment beginning, when there is the pre-chlorination in the Water Treatment Station-WTS   . The chlorination byproducts formation depends on several factors including the concentration of humic and fulvic acids, pH, temperature, contact time, concentration of chlorine and bromides dosage    . Although several are the byproducts of chlorination, the principals are trihalomethanes, haloacetic acids, haloacetonitrile, haloketones, halonitromethanes   .
The trihalomethanes (THMs) are the most significant group of chlorination byproducts, and the total trihalomethanes (THMt) correspond to the algebraic sum of THMs most commonly found in tap water, named trichloromethane (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and tribromomethane (CHBr3)   .
According to the United States Environmental Protection Agency  since the discovery of chlorination byproducts presence in potable water in 1974, several are the epidemiological studies that have investigated the relationship between exposure to disinfection byproducts and cancer. The International Agency for Research on Cancer (IARC) cites that THMt may cause diverse damages for human health, such as liver cancer, kidney cancer, effects on reproduction, and effects in the nervous system, according to the given classification. Trichloromethane (TCM), bromodichloromethane (BDCM) and tribromomethane (TBM) are rated 2B, which mean possible carcinogenic chlorination byproducts to humans, while dibromochloromethane (DBCM) has a rating 2A, which is a byproduct of the chlorination probably carcinogenic to humans  . Another study cites THMs in the incidence of colon, rectum and bladder cancer in laboratory animals when halogenated byproducts were administered to them  .
Many studies have been developed to analyze the levels, safe exposion to disinfection byproducts and the main vias of intake them. The authors  cite that indirect inhalation via evaporation from water, especially in bathrooms, was the major route of exposure to halogenated volatile compounds and accounted for 1.2 - 9 liter-equivalents/day for the median-exposure subpopulation. The ingestion of food was a major indirect route of exposure to haloacetic acids (HAAs). Contributions of direct water intake were the major route to trihalomethanes. In Ireland a large study in THM was conducted in country and indicated that chloroform contributed to the majority of the THMt in the drinking water supplies and the supply networks contributed to about 30 µg・L−1 of THMt  .
The THM formation occurs in two steps. Firstly, the kinetic is favored by the presence of non-ionized form of HOCl at acid pH, while in the second step, a catalytic hydrolysis occurs in basic medium, favored by higher pH  .The authors  have already shown that the THMs reaction rate increases as the temperature increases, leading to an increased concentration of THMt as well.
Given the importance of knowing these byproducts to the quality of treated water, and the high proportion of byproducts of chlorination in water, the objective of the study was to investigate the presence of THMs in potable water distributed in High and Fundamental Schools in Maringá-Brazil. In addition, it was investigated the influence of pH, and residual chlorine in order to compare the values found in treated water with those recommended in the Consolidation Resolution n.05/2017  to pH, residual chlorine and THMt. According to this resolution, the MAV to THMt is 100 µg/L, minimum residual chlorine is 0.2 mg/L and to pH the range of 6.0 to 9.5.
2. Materials and Methods
The two studies were performed in Maringá-Brazil, latitude 23˚25'31"S and longitude 51˚56'19"W, where points around the city that maximize the chlorination byproducts formation were selected for water samples. Each sampling point (eight in total) corresponds to municipal or state schools, which are located far from WTS. The WTS does not realize water analyzes for detection and quantification of chlorination byproducts levels in this points because the water is from internal reservoir. The Consolidation Resolution n.05/2017  sets that sampling by WTS must be done before water entering in the house/building, in the point called “cavalete”.
Two data set of water quality parameters were compared with focus on total trihalomethanes (THMt). These two data sets are from two different time periods (one in 2014 and other in 2017) separated by three years from each other. All the samples were collected in the same months, in both data sets. The samples were taken from its same corresponding sampling points in both periods of time. Figure 1 shows the location of sampling points in each school selected in the map of Maringá-Brazil.
Figure 1. Location of sampling points in Maringá-Brazil. Source: Google Earth, 2018.
Three important factors were considered when choosing the sampling point: location, reservoir storage capacity and cleaning time of the reservoir. Questionnaires were used for selection of sampling points so as to maximize the load of organic matter in the reservoir. Considering the cleaning time of reservoir, when not performed at least once in a semester may lead an accumulation of organic matter, which results in a likely of high concentrations of THMs.
For each sampling point the following analysis were performed: total trihalomethanes (THMt) by gas chromatography with detection by mass spectrometry (GC-MS); residual chlorine by the DPD colorimetric method and pH by the potentiometric method. Analytical methods were performed following the recommendations of Standard Methods for the Examination of Water and Wastewater  .
3. Results and Discussion
The pH values are shown in Figure 2(a) and Figure 2(b) to the first and second data set, respectively. All sampling points presented the pH neutral, with average value (arithmetic mean) of 7.3 to the first data set and 7.4 to the second data set. This values are close to that found by  , in a study conducted in Canada with an average of 7.37 in same conditions. All the pH values from the present study are within the parameters required by the Consolidation Resolution n.05/2017  which recommends that public water supply should have the pH maintained in the range of 6.0 to 9.5.
As higher is the dosage of chlorine, greater is the formation of THM and free chlorine has greater capacity to form THM than combined chlorine    . Furthermore, the World Health Organization (WHO) considers that for a satisfactory disinfection, 0.5 mg/L of free residual chlorine is enough, but adverse effects are not observed in the case of concentrations of 5 mg/L  .
Figure 2. (a) pH values of treated water to the first data set; (b) pH values of treated water to the second data set.
The Consolidation Resolution n.05/2017  considers that the residual chlorine parameter must to be maintained in values of 0.2 to 2 mg/L in distribution system, considering the network and the reservoir. Thus, the values obtained for this parameter are in accordance with current law and the average value (arithmetic mean) observed in this study was 0.38 mg/L to the first data set and 0.41 to the second data set. Figure 3(a) and Figure 3(b) show results to residual chlorine.
The THMt values correspond to the algebraic sum of four trihalomethanes concentrations analyzed: trichloromethane (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM) and tribromomethane (TBM). The results of individuals THMs which were found in the water samples collected in the selected schools are presented in Figure 4(a) and Figure 4(b). In both data sets only TCM and BDCM was detected. All the studied city regions have same conditions in altitude and weather.
Figure 3. (a) Residual chlorine concentration in the sample points to the first data set; (b) Residual chlorine concentration in the sample points to the second data set.
with the average percentage of 75% from the THMt found in all the samples. After this byproduct, the largest contribution to the THMt was the bromodichloromethane (BDCM), with the average percentage of 25%. The average values (arithmetic mean) of THMt found in all sampling points was 14.07 μg/L in the first data set and 18.15 to the second.
The high concentration of chloroform in the treated water can be explained by the higher residual chlorine content in the water due to the chlorination process used by the WTS and the probable low concentration of bromine in water, since the chlorine component, is more electronegative than bromine, tending to be more reactive. The author  states that water that shows high concentrations of bromide, the possibility of formation of larger amounts of brominated species, such as bromoform is also higher.
In a study  , chloroform was the THM that had the highest concentrations, reaching an average percentage of 95%, demonstrating the near absence of bromide ion in the treated water intended for public supply. The acceptable limit
Figure 4. (a) THMs concentrations in the sample points to the first data set; (b) THMs concentrations in the sample points to the second data set.
for THMt, according to the Consolidation Resolution n.05/2017  is 100 μg/L, all the concentrations are below the specified maximum value in both data sets. It was obtained a small increase in average THMt concentration in the second data set from the first. THMs are just some of the existing chlorination byproducts and their control in drinking water can help to reduce levels of other byproducts such as haloacetic acids.
1) From all sampling points of this study it was found a low concentration of THMs in the treated water from the public supply. The maximum value observed was 21.4 µg/L in the second data set, value below the MAV 100 μg/L established by Consolidation Resolution n.05/2017.
2) Among the four quantified trihalomethanes, the chloroform showed the highest concentrations, representing the average percentage of 75% of THMs found. The results suggest a minimal presence of brominated species in the source of water.
3) The values of residual chlorine were generally low when compared with the results of studies under similar conditions. For residual chlorine, concentrations below of 0.2 mg/L were found, which are preoccupant.
4) The present study also suggests an establishment of MAVs for specific THMs in Brazil, rather than the regulation only in THMt, since THMs may have different adverse effects to human health.
The authors thank to Centro Universitário de Maringá (UNICESUMAR) for the financial and technical support.