The necessity of performing this study that is the third phase of the project “investigation of the corrosion potential of the urban sewerage by examining the concentration of hydrogen sulfide and oxidation reduction potential” is due to the need of replacing the qualitative indices of the sewerage of Mashhad city with the oxidation reduction potential to achieve rapid results after modeling. In this study, the sewerage collectors in the west of Mashhad city with the length of 26 km were qualitatively assessed. These collectors are in two different paths of collecting the urban and industrial sewerage. In the first step, by selecting six qualitative stations in the urban sewerage path, four qualitative stations in the industrial sewerage path, and three qualitative stations in the section of transferring the combination of urban and industrial sewage were assessed. The six month studies with daily and weekly sequencing indicated that a significant relationship could be predicted between ORP and qualitative indices of urban sewerage, however there was no significant relationship between the qualitative indices of the industrial sewerage and the combination of urban and industrial sewerage. In the supplementary study of the quality monitoring stations, the collectors of the urban sewage transfer section with the length of 16 km increased to 11 stations. The studies showed that there was a significant relationship with the correlation coefficient of R=0.92 and determination coefficient of R=0.85 among the qualitative parameters of sewage including pH, ORP, and temperature with the COD response variable as well as a simple relationship with correlation coefficient of 0.83 and determination coefficient of R=0.7 between ORP and COD.

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