DETERMINATION OF THE IMPACT OF REHABILITATION OF DIFFERENT COMPONENTS OF THE SEWER SYSTEM ON I/I REDUCTION

The LOTT Wastewater Partnership is undertaking a program to reduce infiltration/inflow in the tributary area of the Partnership's wastewater treatment plant in Olympia, Washington. The purposes of this program are to both reduce peak flows to avoid overflows in the conveyance system, and to reduce flows at the treatment plant to delay capacity upgrades.Rehabilitation of two basins in the City of Olympia offered the opportunity to determine the relative effectiveness of rehabilitating different components of the sewer system. Conflicting information suggests that effective reduction in I/I requires that private building sewers in a basin be rehabilitated—usually an unattractive option due to the difficulties of working on private property. In order to determine the relative effectiveness of rehabilitating public sewers within the public right-of-way and/or private building sewers, demonstration projects were initiated in these two basins. These demonstration projects included the following approaches:Rehabilitation of the public sewers and the portion of the building sewers within the public right-of-way (the lower laterals).After monitoring of flows to determine effectiveness of the above approach, selected upper laterals (building sewer from the property line to the building) in the same basin were replaced. Upper laterals were selected based on smoke testing results and mini-cam surveys made during the earlier work.Rehabilitation of only the upper building laterals in a different basin.These demonstration projects allow direct comparison of the effectiveness of rehabilitation of the upper lateral vs. the public sewer and lower laterals. In addition, the total removal that can be achieved when the entire system is rehabilitated is available.Using the techniques described in the paper, it was found that the first treatment (rehabilitation of sewers in the public right-of-way) reduced annual I/I volumes by 80 percent, but reduced peak hourly flows expected once in 10 years by only 17 percent. The presentation discusses the construction projects, the analysis techniques, results and costs for each treatment option.