(6) Evaluation of Sewerage Facilities 1) Sewer Network Results of hydraulic analysis (flow capacity) on sewer network of Panaji City to identify problems in existing condition, year 2001, are shown in this section. a. Study Methodology The steps of analysis are shown below. Step 1: Figure out sewer service area, population Step 2: Presume contributory population of target sewers of network Step 3: Presume design flow of each sewer Step 4: Figure out diameter, length, and slope of each sewer Step 5: Figure out flow velocity and flow capacity of each sewer (Manning’s formula) Step 6: Compare the flow capacity with the design flow and judge Assumptions as shown in Table 32.12 are set for the analysis on the sewer network for evaluation of sewer network capacity. Table 32.12 Assumptions for Sewer Network Analysis Item Assumption Population Adopt population mentioned in the Report for year 2001 Connection rate 100%, that is whole wastewater generated is discharged into sewers Contributory population of each sewer Distribute zone population to each sewer catchments proportional to its sewer length Sewer cross-section area reduction due to Not considered silting Flow capacity margin Not considered b. Sewerage Zone wise Population and Wastewater Quantity Sewerage zone wise population and generated wastewater quantity in year 2001 have been considered as shown in Table 32.6. c. Flow of Each Sewer As contributory area, population and flow of each sewer for year 2001 are not mentioned in the Report “Project Outline on Environmental Upgrading of Panaji City, Phase-1”, it was assumed 3 - 83 that population in the catchment area of each sewer is proportional to its sewer length. Then sewerage zone wastewater quantity is divided into sewer catchment area in same manner as the population. Sewers are designed for peak flow. According to the Manual on sewerage and sewage treatment or sewerage system of India, the peak factor should depend on the scale of contributory population in the sewer catchment area. In the case of Panaji City, with population 58,785 in year 2001, peak factor should be 2.25. See Volume IV Appendix M35.1 Flow Calculation Sheet for Main and Sub Main Sewers, Panaji City and M35.2 Flow Calculation Sheets for Branch Sewers, Panaji City for design flow of each sewer. d. Sewer Flow Capacity Flow capacity of main sewer, sub main sewers and branch sewers is estimated using following Manning’s formula. Roughness coefficient of 0.015 (fair condition of concrete pipe) has been adopted because almost all sewers were installed within last 20 years and these sewers are not in so bad physical condition according to PWD engineers. Manning’s formula: Q = AV V = R2/3･I1/2 / n Where: 3 Q : Flow capacity (m /sec) 2 A : Area of cross-section of sewer (m ) V : Flow velocity (m/sec) n : Roughness coefficient (=0.015) R : Hydraulic radius (m) I : Gradient e. Criteria for Evaluation on Flow Capacity The adequacy of sewer flow capacity at present condition has been evaluated with criteria shown in Table 32.13 comparing peak flow with flow capacity of the sewer. In the case of gravity sewer, the sewer was judged “adequate” when flow capacity is greater than peak flow, leaving desirable capacity margin out of consideration, as this stage is not for design. In the case of pressure main, it was judged “inadequate” when flow velocity exceeds 3.0 m/sec as preliminary evaluation. 3 - 84 Table 32.13 Criteria for Evaluation on Flow Capacity Investigation Type Condition Judgment Peak flow <= Flow capacity Adequate Gravity Sewer Peak flow > Flow capacity Inadequate Flow velocity <= 3.0 m/sec Adequate Pressure Main Flow Velocity > 3.0 m/sec Inadequate f. Result of Evaluation of Sewer Flow Capacity in Present Condition The result of evaluation is shown in Table 32.14. This Table includes the list of capacity gravity sewers and pressure main with inadequate flow capacity. Three reaches are detected as inadequate as shown in summary Table 32.14. See Figure 32.4 and Table in Volume IV Appendix M35.1 Flow Calculation Sheet for Main and Sub Main Sewers, Panaji City , M35.2 Flow Calculation Sheets for Branch Sewers, M35.5 Panaji City and Data of Flow Calculatoion Sheet of Sewers for details. Table 32.14 Summary of Result of Evaluation on Sewer Flow Capacity (List of Inadequate Capacity Sewers and Pressure Main in Present Condition) Node 1 Node 2 Estimated Flow Diameter Length peak flow Gradient Capacity Remarks From To From To (mm) (m) (lps) (lps) Zone III to Zone IV 23 42 66.5 22 42 44 70.2 67 44 63 79.8 35 4 3 63 68 83.0 300 27 200 59 Gravity (PS3) 68 53 87.2 22 Sewer 53 34F 88.8 28 34F PS3 102.6 10 Sub total 211 Pressure 4 5 PS3 21 102.6 200 320 Velocity: 3.3 m/sec Main Zone VII to Zone XII 223B 224A 239.2 130 224A 225 241.3 110 29 225 225A 259.2 44 13 600 500 238 Gravity (STP) 225A 232 269.9 640 Sewer 232 232C 294.9 76 232C STP 324.3 70 Sub total 1,070 Note: (1) Node 1: Node numbers in column “Node 1”is coordinated to Figure 3.2.xx (2) Node 2: Node numbers in column “Node 2” is coordinated to Sewerage Zone Map 3 - 85 Figure 32.4 Result of Evaluation on Flow Capacity, Panaji City 3 - 86 g. Physical Condition of Sewers and Manholes Many sewers and manholes in Zones I, II, III, IV, V and VII were installed in 1960’s, about 40 years ago. After that, those in Zones VI, VIII, IX, X, XI and XII were installed during the period between 1970s and 1995. According to an interview of a PWD engineer who is in charge of sewerage system in Panaji City, PWD has not experienced severe damages of sewers caused by sulphuric acid or heavy traffic load. But almost all manholes are masonry works, and some are physically damaged because of long use. These damaged manholes and joints are supposed to be one reason for high infiltration rate in rainy season. But infiltration flow runs up about 50 % of actual wastewater in rainy season according to PWD. 2) Pumping Station a. Evaluation of Pump Capacity Table 32.15 compared the existing pumping station capacity with the sewage inflow in 2001 reported by ”Project report on environmental upgradation of Panaji City”. Table 32.15 Pumping Station Capacity and Present Sewage Inflow Pumping Station Inflow (m3/sec) Pump Capacity (m3/sec) Evaluation P/S-1 0.0237 0.1364 OK P/S-2 0.0237 0.0414 OK P/S-3 0.0870 0.1278 OK P/S-5 0.2320 0.5095 + α*1 OK P/S-9 0.0170 0.2105 OK P/S-10 0.0111 0.0647 OK P/S-11 0.0240 0.1000 OK P/S-12 0.0020 0.0233 OK *1; No information of discharging capacity in No.1 Pump Photo 32.1 Outside View of P/S-3 Photo 32.2 Outside View of P/S-5 3 - 87 b. Existing Facility Evaluation Mechanical and electrical equipment of the existing pumping stations are evaluated by observation, operation records and age. The results were presented in Volume IV Appendix M35.6 Information of Pumping Stations, Panaji City. 3) Sewage Treatment Plant a. Evaluation of Treatment Capacity of the STP There are two sewage treatment facilities in Panaji STP, one is trickling filter and the other is SBR. The new STP (= SBR) capacity will be evaluated by its design capacity because the old trickling filter is not in operation now. The detailed process design calculation of SBR process is presented in Volume IV Appendix M35.7 Capacity Calculation of STP. The results of the design calculation are compared with capacity of the existing SBR in Table 32.16. The result shows that it has enough capacity. The quality of raw sewage and treated effluent were also shown in Table 32.16. The effluent qualities treated by SBR are less than 10 mg/L both BOD and SS, they met the effluent standards. Regarding the raw sewage quality, BOD concentrations were low as 53 mg/L and 82 mg/L, however, they are almost in the range of fluctuation, past data ranged 66 to 208 mg/L shown in Table 32.10. Table 32.16 Capacity and Water Quality of Existing STP (SBR Process) Capacity Facilities Design Requirement Existing Facility Evaluation Wet Well >5.0 min. 40.7 min. OK Pump New equipment settled in 2005 OK Grit Chamber 13.0 m2 23.0 m2 OK SBR Tank 6,250 m3 7,040 m3 OK Disinfection Tank 130 m3 200 m3 OK Sludge Dewatering 11.7 m3/hr 20.0 m3/hr OK Sludge Drying Bed 284 m2 2,025 m2 OK Water Quality, 2005 Parameters Season Sampling Law sewage Treated effluent Standard Panaji pH Dry 22/06/05 7.1 7.4 5.5 - 9.0 Rainy 17/07/056.3 6.9 BOD(mg/L) Dry 22/06/05 53 7.4 30 Rainy 17/07/0582 5.5 SS(mg/L) Dry 22/06/05 42 4.5 100 Rainy 17/07/0567 5.0 Coliform(MPN/100ml) Dry 22/06/05 46,000,000 1,100,000 Rainy 17/07/05 4,600,000 1,100,000 3 - 88 Photo 32.3 Preliminary Treatment Photo 32.4 SBR Reactor Tank b. Existing Facility Evaluation New SBR plant has no problem because it is just constructed in April 2005.