آب و فاضلاب (Feb 2022)
Groundwater Infiltration and Rainfall-Derived Inflow and Infiltration Assessment in Separate Sewer Systems (Case Study: Tehran Urban Area)
Abstract
Separate sewer networks are designed and built to collect foul sewage from buildings and convey it to wastewater treatment plants (WWTPs). However, due to some holes and cracks on the pipes or unsealed pipe joints, a volume of ground or subsurface water that is placed in saturated or unsaturated zones intrudes sewers and WWTPs. In a condition that the wastewater level in the sewer stands higher than the groundwater level, wastewater leaks into the soil and groundwater and this phenomenon is called sewer exfiltration. Moreover, during a rain event, a part of the precipitation gets into the sewer system through the illicit connection of the yard drains and roof downspouts to the building’s lateral, and this part of the flow is called rainfall-derived inflow and infiltration (RDII). In this research, infiltration, exfiltration, and RDII were evaluated in a 16-month period to show the range of inflow and infiltration in the sewer network under the service of the South Tehran WWTP. Results show that the infiltration constitutes the major volume of the total inflow and infiltration. But the RDII leads to more operational problems due to its nature as an intense flow. After calculating the range of inflow and infiltration, results show that the average infiltration rate that entered the WWTP in the year 2014 was 0.0175 L/s/ha that is much less than the values mentioned in Criterion No. 118. Furthermore, total inflow and infiltration in five hours of the study period crossed the upper limit of the inflow and infiltration range (0.6 L/s/ha) in Criterion No. 118 that shows the high percentage of illicit connections in Tehran and a high volume of rainfall that enters the sewer network. Accordingly, the inflow and infiltration range in the Criterion No. 118 seems to be an unsound range and it needs some revisions.
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