ABSTRACT
This study intends to monitor and evaluate the THMs concentration in the 6 different water districts in the city of Tehran as well as one district outside the city water district but inside the city limits. THMs are the main disinfectant by products [DBP] of chlorination, which are suspected to be human carcinogens. Formation of Trihalomethanes [THM] in drinking water depends mostly on raw water qualities such as pH, temperature, treatment process, residence time of water in the distribution system, as well as the bromine, chlorine and naturally occurring organic matter [NOM] concentrations. Because of the low cost of chlorine and convenience of application, chlorination is the most widely used disinfection process for drinking water. This led to a sharp reduction in mortality rate from many water born diseases, but the chlorine from the disinfection process reacts with NOMs in the raw water to form a series of undesired by-products. The most significant group of DBPs is trihalomethanes [THMs], which include chloroform [CHC13], bromodichloromethane [CHCI2Br], dibromochloromethane [CHClBr2], and bromoform [CHBr3]. The concentration of NOM in raw water is the most important parameter in the DBP formation. Increase in the bromine ion concentration shifts the DBPs from the chlorinated compounds to the brominated ones. Chlorination is used as the main disinfection process for the drinking water in Tehran, therefore; it is important to monitor and evaluate the THM's concentration in Tehran's drinking water. Samples were taken from twenty one locations across the city of Tehran in triplicates during the first week-end of April, May, June, July, August and September of 2009 They were collected directly from taps of consumers after letting the water run for several minutes before collecting the water in pre-cleaned glass containers with sodiumthio-sulfate preservative to eliminate any residual chlorine. The samples were analyzed for THMs using EPA method 524.2. A Purge and Trap device [Tekmar] was used to extract all the THMs and trap them in a sorbent tube. Using heat and inert N2 gas the trapped compounds were transferred to a gas chromatograph coupled with a mass spectrometer [6890 GC/5973 MSD, Agilent]. A 30 meter DB-5 capillary column with a 0.25 mm LD and 1 micro film thickness was used to separate the trihalomethanes before sending them to the mass spectrometer detector for identification and quantification. Spiked and duplicated samples were analyzed with every batch to determine the accuracy and precision of the analytical procedures. The data indicates that, the concentration of THMs are much higher in districts which are supplied mostly with surface water sources such as areas in the northern part of Tehran, compared to districts with well water as their main water source which are located mostly in the south and south west part of Tehran. The average concentration of chloroform, bromodichloromethane, dibromochloromethan and bromoform were 2.49 ppb, 2.08 ppb, 0.95 ppb and 0.15 ppb for the city of Tehran. The observed THMs concentrations are below the EPA's recommended MCL value of 80 ppb thus not causing a major adverse health effect on the population of Tehran. The highest average concentrations of chloroform during spring and summer seasons were observed in district 1, and 2 [Figure 1]. The measured concentrations for CHC13during the study period ranged between N.D in district 7 to 9.9 ppb in district 1