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Sewer pipe improvement projects by build-transfer-lease (BTL) method were consecutively completed after 2008, and the effectiveness of the completed projects have been reported. However, certain areas with completed projects suffered from reduced effectiveness due to some factors. Therefore, the aim of this study is to investigate the factors that contributed to the reduction of project effectiveness. Three study areas which the effectiveness of the project was reducing were chosen for the cause analysis of the effectiveness reduction based on the investigation of biochemical oxygen demand (BOD) and infiltration/inflow (I/I) from inside and outside area of BTL project. The BOD and I/I were obtained from field investigation and operation data. The water quality investigation showed that the BOD outside the BTL project area was 38.1–136.1 mg/L; the BOD inside the project area (performance guarantee point) was 14.0–171.8 mg/L. The results of I/I investigation showed that the penetration rate of infiltration outside the BTL project area was 3.6–76.2% and the rate inside the project area (performance guarantee point) was 1.6–29.2%. Inflow volume outside the BTL project area was 2.8–1,720.7 m 3 while the volume inside the project area (performance guarantee point) was 0–2,124.0 m 3 . Inflow rate per rainfall was 0.2–24 m 3 /mm outside the BTL project area and 0–16.8 m 3 /mm. In summary, the reduction in project effectiveness was mainly influenced by factors from outside the project area. Thus, we have to establish a systematic plan for maintenance and replacement of sewer systems through consistent investigation and analysis.

Investigation and Analysis for Outside Project Area for Understanding the Case of Effectiveness Reduction of BLT Project