Abstract

Sewer rehabilitation BTL projects, which started with 17 project businesses in 2005, had recorded a total of 97 project businesses up to the end of 2013 (the total project cost is approximately 7 trillion won). Of them, about 70 have already been completed and in operation now. A sewer rehabilitation BTL project business goes through regular performance evaluation throughout operation period and gets compensated according to performance. From the fourth year of a project, I/I analysis is supposed to be part of performance index. As for I/I index, which is applied from the fourth year of operation, it is true that considerable extent of controversy and difficulty surround the use of I/I index and its application to performance evaluation due to the lack of reliability in flow data, unpreparedness of specific evaluation method and standard, necessity of improving the current I/I calculation standard manual of the Ministry of Environment, various business conditions by region, generation of I/I not attributable to business operator, insufficient cases of actual application of I/I analysis to performance evaluation and so on.

Therefore, this study surveyed the current status of the fixed flowmeters installed in 15 sewer rehabilitation BTL businesses and reviewed improvement of flowmeter installation in order to enhance flow data reliability. And, for 13 points of 4 businesses including JC county, the present study examined the appropriateness of I/I computation and application procedures of the current Ministry of Environment with the actual data of flow rate and precipitation and reviewed improvement of I/I computation and application method. In addition, I/I analysis data of 4-year operation in 5 points of JC county were used to investigate the characteristics of I/I generation by project location and year. The results of I/I performance evaluation of the 4th operation year were reviewed to forecast potential problems that other businesses may face for years to come and improvement plans were proposed. The conclusion of the present study is summarized as follows.
The current status of installation and operation of fixed flow meters of 15 sewer rehabilitation BTL businesses showed that data of flow rate measured in 54 (61.4%) out of total 88 points are considered valid or reliable to performance evaluation. However, data measured in 34 points (38.6%) turned out to have problem in effective data acquisition rate. The survey of installation and operation of fixed flowmeters revealed the impact of operation of pumping station and facility (32%) is most problematic and it is followed by inflow of sewage from areas out of the project locations (29%), insufficient rate of effective data collection in low-water level areas (22%) and impact of intercepting trunk sewer and sewage treatment plant (13%).

The improvement for the problems involved with the installation and operation of fixed flowmeter is as follows. 1) relocation of fixed flowmeters to non-interference areas out of pumping station, 2) relocation of fixed flowmeters to where there is no generation of sewage flow from non-project areas or reduction of flow rate in non-project areas through additional installation of flowmeters, 3) relocation of fixed flowmeters to areas where minimum level of water for flowmeter to measure is secured or alternation of flowmeter type, 4) relocation of fixed flowmeters to areas not intefered by trunk sewer and sewage treatment plant, 5) improvement of installing flowmeter sensor and relocation of fixed flowmeters to areas suitable for required conditions. In addition, it was also confirmed that the current status of night minimum sewage flow rate generation at installation area should be more carefully examined in designing and applying water level - flow velocity equation flowmeter (ex. ultrasonic flowmeter).

The review of the main procedures of I/I calculation and application of the current Ministry of Environment was found that a proper calculation method for infiltration requires a change of minimum sewage standard (every 2 hours by daily) to every hour. It was also suggested that it is more valid to use 25 to 75% (50% of the total data) of yearly estimation result of infiltration(generation of daily minimum flow rate) excluding top-bottom 25% as an alternative of “seasonal estimation method of infiltration”. In addition, it turned out more valid to include infiltration per household(m3/day/location) along with infiltration per sewer extension and diameter(m3/day/cm/km) in the comparative index of infiltration. Furthermore, it was known that using infiltration and inflow data until a year prior to the year of evaluation as reference period is more valid than data of the first 3 years of operation in the phase of “annual relative growth rate of infiltration/inflow”

The analysis of infiltration for 4-year operation in 5 locations of JC county showed the ratio of average infiltration to daily average flow rate is 20.9%; average generation of infiltration flow per sewer extension and sewer diameter is 0.25m3/day/cm/km; and average generation of infiltration flow per location with drainage facility is 0.10m3/day/household. The average generation of infiltration flow per sewer extension and sewer diameter of the research locations turned out much lower than that(1.438m3/day/cm/km) of 15 metropolitan cities in Korea. In addition, the present study derived ‘equation of rainfall-inflow’ by year and location from the analytic data of inflow during operation period. It showed that the research areas with as follows; average y is equal to 5.377x(y=rainfall-inflow rate, x=rainfall, y-intercept is constant at 0) and correlation coefficient(R2) to be 0.817. It also revealed that the average inflow rate per extended sewer is 0.867m3/mm-km, which is much lower generation of inflow rate than that(5.208m3/mm-km) of 15 metropolitan cities in Korea.

When reviewing the performance index of the fourth year I/I (2012) of the concerned project by item, 7.1 was given out of 8.75 to the infiltration flow generation while 6.55 out of 8.75 was given to inflow generation. In total, 13.65 (about 78 point) was assigned to infiltration/inflow generation. Since the result of I/I evaluation of Sewer rehabilitation BTL project in the research targets turned out much lower than the average point(90 point) of other indexes and criterion for the reduction of operation cost, disputes and difficulties are foreseen to take place due to I/I evaluation in the process of applying the same evaluation criteria to other projects. Therefore, this study applied improvement plans to the result of I/I evaluation of the 4th operation year and obtained 15.05(about 86 point) out of 17.5 in I/I index score of the concerned target businesses. It was found that the introduction of QA/QC inspection(e.g. sewer rehabilitation survey) to the areas with low reliability inflow rate data is much more favorable to I/I performance evaluation.