Formation, evolution and modeling of trihalomethanes in the drinking water of a town: I. At the municipal treatment utilities

Taking samples at eight points chosen from two conventional water treatment plants for the city of Salamanca, the formation and evolution of THM levels were studied on 11 different dates. The values obtained were correlated statistically with the following parameters: concentration of humic acids (only raw water), pre- and postchlorination dosages, UV absorbance (UV-254), pH and temperature. No statistical correlation was observed either with the humic acids content or with the organic matter measured as UV-254. A correlation was only found with the prechlorination dosage in the clarifiers of the old plant. However, in both plants there was a correlation with the postchlorination dosage although this was not very patent owing to the impossibility of knowing the contribution of each parameter at one of the sampling sites where postchlorination and pH correction are performed simultaneously. A clear linear correlation (r = 0.4345, P = 0.0001) was observed with temperature. Using stepwise regression (ANCOVA) a mathematical function was obtained (R = 0.8066, P = 0.0001) that relates the concentration of chloroform with temperature and the sampling points. From this it is deduced that both pH and temperature increase this concentration, although for each pH value all the In CHCl3 (μg/l) vs temperature curves showed a maximum (Tc = 18.97°C), after which chloroform levels decrease sharply. On attempting to quantify the contribution of the rest of the parameters studied here concerning the levels of THMs, it may be inferred that others should be considered, such as the design, the dimensions and the exploitation of the water treatment plants studied.