3.4 Routing of Interceptors Options of possible routes of interceptors have been developed and assessed in this Section. All of the outlets are to be intercepted and collected wastewater is to be conveyed to Nong Bon WWTP which will be constructed at the site adjacent to Rama IX Park in all options. 3.4.1 Comparison of Routes The following prerequisite conditions are established for selecting interceptor routes. 1) All of the existing outlets are intercepted. 2) Interceptors are laid under klongs or roads. 3) Outlets are connected to WWTP by shortest possible routes since the topography of the treatment area is almost flat. Among the above conditions, (1) is thought to be natural because all wastewater is to be collected. The reasons for condition (2) are described in the former Section 3.3.3. The reasons for condition (3) are to minimize construction cost by making interceptors as shallow as possible. Taking into account the above conditions, the following 2 options are developed as trial options. Option 1: Almost all interceptors are laid under klongs Option 2: Almost all interceptors are laid under roads Routes of these two options are shown in Figure 3.4.1 and 3.4.2 respectively. Features of these options are as follows. 1) A total length of interceptors of Option 1 is approximately 66 km which is shorter than that of Option 2 (78 km) by approximately 12 km, since klongs run in all directions in the treatment area. 2) Wastewater can be received directly from the existing outlets in Option 1. However, connecting pipe lines from outlets which are currently discharging to klongs, to the interceptor are necessary in Option 2, and this is the main reason for longer total length of interceptors. Option 3 was worked out based on the Option 1 which has shorter length of interceptors and easiness of receiving wastewater, and a part of interceptors are laid under roads. Interceptors are laid under roads, where the roads run in parallel with klongs or klongs run zigzag. Because of easiness of construction and maintenance, interceptors are laid under roads at these points. Further, Option 4 was worked out by changing downstream end of interceptor near WWTP from 46 one line to two lines. After discussion with DDS, Option 4 was selected because lower construction cost due to smaller diameter of downstream end of interceptor. Options 3 and 4 are shown in Figure 3.4.3 and 3.4.4 respectively. Comparison of Options 3 and 4 is described in Table 3.4.1 Table 3.4.1 Comparison of Options 3 and 4 Option 3 Option 4 Total length of interceptor 63.70 km 64.27 km Average depth of interceptor 10.9 m 10.3 m Depth at inlet of WWTP GL -21 to 22 m GL -20 to 21 m Estimate of construction cost 3,701 million Baht 3,642 million Baht Diameter of pipe near WWTP is larger than that of Option 4. Sizes of Influence on surrounding roads Less influence on surrounding roads vertical shaft and working yard are (Neighborhood of WWTP) than Option 3. larger than those of Option 4. Longer time is required for construction.. Evaluation △ ○ Source: JST Total length of the interceptor of Option 3 is shorter than that of Option 4. However, since diameters of pipes are smaller and depth of pipes is shallower, construction cost for Option 4 is lower than that of Option 3. Also, more influence on surrounding roads is envisaged for Option 3. Taking into consideration the above, Option 4 is selected for the project. Construction cost in the above table was estimated by the following formula developed for pipe jacking method under F/S for Klong Toie Project. C = {3305.9H – 7738.1 + D (987.72H + 19580)} L Where C: Construction Cost (1,000 BAHT) H: Average Depth of Interceptor (m) D: Diameter of Interceptor (m) L: Length of Interceptor (m) 47 Source: JST Figure 3.4.1 Routes of Interceptors, Option 1 48 Source: JST Figure 3.4.2 Routes of Interceptors, Option 2 49 Source: JST Figure 3.4.3 Routes of Interceptors, Option 3 50 (43.60km) (20.67km) Source: JST Figure 3.4.4 Routes of Interceptors, Option 4 51 3.4.2 Runoff Analysis and Flow Calculation Diameters, gradients and depths of interceptor pipes were determined based on the runoff analysis for Option 4. In wet weather, downstream portion of interceptor is pressurized, but spilling over is prevented. For this reason, diameter of a part of interceptor can be made smaller. Results of runoff analysis is presented in Appendix 4 Software “MOUSE” developed by DHI Company was used for runoff analysis. 3.4.3 Longitudinal Sections of Interceptor Longitudinal section of interceptor was determined based on the runoff analysis and flow calculation mentioned above. Longitudinal section of interceptor is presented in Drawings. 52 3.5 Summary of Interceptor and Pumping Station Interceptors and pumping station preliminary designed in this Chapter are summarized in this Section. 1) Interceptors Sizes, lengths and other conditions of interceptors and vertical shafts for construction are summarized in Table 3.5.1 below. Table 3.5.1 Summary of Interceptors Interceptor Vertical Shaft Diameter Length（m） Number Average depth（m） （mm） Under road Under klong Under road Under klong Under road Under klong 300 480 0 4 0 6.9 - 450 0 1,455 0 11 - 7.8 600 4,970 24,900 36 172 7.5 8.1 800 4,025 9,720 27 50 14.9 10.0 1,000 4,270 755 22 5 11.7 13.3 1,200 1,595 4,125 11 23 16.7 14.8 1,500 4,970 2,645 21 19 19.0 15.6 2,000 385 0 3 0 20.6 - Total 20,665 43,600 124 280 - - Source: JST All interceptors will be constructed by pipe jacking method. 2) Interceptor Chambers 136 points (can be reduced to 121 points by integration of chambers) 3) Pumping Station None 53 3.6 Phased Construction of Interceptors (1) Division of Interceptor Construction Total length of interceptors in Nong Bon treatment area is approximately 64 km. It is expected to take long time to construct interceptors. Therefore, construction work is to be divided into a few lots and to be implemented in parallel. Construction of interceptors of which total length is approximately 30 km is divided into two lots in case of Bang Sue project which is currently under construction. Taking Bang Sue project into account, construction of interceptors in Nong Bon treatment area is divided into four lots. Interceptors are classified into four groups as shown in Table 3.6.1 and Figure 3.6.1 for smooth implementation. Table 3.6.1 Four Groups of Interceptors in Nong Bon Treatment Area Pop. Construction Area Served density Group Features of the Name of trunk Length (km) cost (million (ha) population No. (person/ service area Baht) (persons) ha) Urbanization 20.2 North Nong advanced mostly 1 （Trunk 9.8 , 811 1,910 81,000 42 Bon in the treatment Branch 10.4） area Commercial 18.2 area and South Nong 2 （Trunk 8.6 , 707 1,768 79,000 45 congested Bon Branch 9.6） residential area are mixed Congested 17.6km residential area 3 Prawet （Trunk 9.0, 684 1,913 75,000 39 with a new Branch 8.6） railway station 8.3 Rural area with 4 Dokmai （Trunk 7.3, 266 794 30,000 38 scattered Branch 1.0） residents 64.3 Total (Trunk 34.7, 2,468 6,385 265,000 Branch 29.6) Source: JST 54 North Nong Bon Prawet Trunk Line Trunk Line TP South Nong Bon Dok Mai Trunk Line Trunk Line 0 1.0 2.0km North Nong Bon Trunk Line Branch Line Catchment Area South Nong Bon Trunk Line Branch Line Catchment Area Prawet Trunk Line Branch Line Catchment Area Dok Mai Trunk Line Branch Line Catchment Area Source: JST Figure 3.6.1 Division of Interceptors into Four Groups In Nong Bon Treatment Area (2) Priority of Interceptor Groups DDS has the intention to implement all interceptors at once. However, phased construction might be required according to circumstances, e.g. restriction of budget. In this case, implementation should be commenced from the priority group. The following two groups are considered to be priority groups for the reasons mentioned in 55 Table 3.6.2. Table 3.6.2 Priority Interceptor Groups Name of Trunk（Group） Reasons North Nong Bon Urbanization has been progressed most South Nong Bon Catchment of significantly polluted Klong Bang Na Source: JST In case phased construction is implemented, two groups, i.e. North Nong Bon and South Nong Bon should be constructed first followed by the remaining two groups, i.e. Prawet and Dokmai. Construction schedules for both cases are presented later in Chapter 4 of this report. 56 3.7 Issues and Technical Countermeasures in Future Interceptor sewerage system has been adopted in Nong Bon treatment area as well as in other treatment areas in BMA, for which interceptor chambers are constructed in neighborhood of existing outlet to collect wastewater and convey it to WWTP. However, as described in Conceptual Master Plan Report, there are many issues regarding the interceptor sewerage system. These issues should be resolved on medium- and long-term basis after implementation of the proposed project including construction of WWTP, interceptors and interceptor chambers under this F/S. Three issues to be resolved and countermeasures on mid- and long-term basis are described below. 3.7.1 Prevention of Back Flow from Klong Topography of the Nong Bon treatment area is flat and low lying, and existing drain pipes are often constructed below water level in klong as can be seen in existing treatment areas in BMA.