Review & Design of the Apia Pressure Sewer System Presented by Jammie Saena - Samoa Water Authority & Paul Edwards Urban Water Solutions 1
Contents Project Background Project Team Data Collection Physical Inspections Modelling Existing System Analysis On Property Design Reticulation Design 2
Introduction to Pressure Sewers A pressure sewer is a network of fully sealed pipes, which are fed by grinder pump located at each property. The grinder pump grinds the solids into a slurry, which is then pumped through a small diameter pipeline from the house to the reticulation pressure sewer in the street Pipes are sized to accommodate a limited number of pumps operating at one time. As a result the pipelines required for pressure sewers are considerably smaller than those required for traditional gravity sewers. 3
Pressure Sewer Configuration 4
Apia Pressure Sewer System Client Samoa Water Authority (SWA) SWA provides water to 80% of Samoa s population. SWA provides wastewater services to 120 commercial customers in the CBD of Apia, Samoa s capital. Population of Apia 37,000 Population of Samoa 180,000 5
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Project Background Flooding frequent problem in Apia due to high rainfall and inadequate drainage. Septic tanks releasing wastewater into the environment. Adverse ground conditions & high water table. Pressure Sewer System and WWTP constructed in 2010. 8
Project Drivers WWTP designed to treat 1ML/day based on design assumptions by previous consultant. Average flows of 500kL/day currently received at WWTP. Large wet weather inflow to the system. Need for comprehensive review of the system performance & operation. Assessment of system capacity so more connections be made to the system. Knowledge transfer to SWA Modelling, design, analysis & operation 9
Project Scope Existing System Conduct a physical survey of the existing system. Develop a hydraulic model in InfoWorks CS of the existing PSS Determine system constraints and recommend improvements. Assessment of system capacity so more connections be made to the system. Future System Review & Update O&M Procedures Provide training to SWA staff in modelling, design and operation of the PSS. Design future extensions to the system 10
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Project Team Urban Water Solutions Modelling of existing system in InfoWorks System Performance Analysis Training of SWA staff in modelling & PS ReticDesign Design of extensions to the pressure sewer system Pressure Sewer Services Australia Specialist Subconsultant Physical inspections & pump diagnostics checks Review of current O&M procedures Training of SWA staff in O&M procedures On property design for future extensions 12
Physical Surveys & Inspections UWS team & SWA staff carried diagnostic checks on all pump units in the system. Verification of Voltage and Amperage against the pump curve. Verification of audible & visual alarm system Verification of level sensing devices. Inspection of control panel condition. Inspection of pump well, pump and cutter condition. Inspection of boundary valve and operation of boundary valve. 13
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Smoke Testing & Drainage Inspections UWS team & SWA staff carried out smoke testing to locate entry points for storm water inflows. Identification of direct cross connections into the PSS. On property drainage inspections undertaken to identify drainage system infiltration points. 16
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Infiltration Points Manhole covers -Plastic manhole covers used at new government building. Rodding Points - Unsealed covers. Grease Traps (Septic Tanks) Low points on properties 20
Cross Connections Roof drainage directly connected to pump wet well. Direct connection of a roof drainage down pipe into the overflow relief gully. 21
Outputs from Physical Inspections Defects Summary List Property Defect Forms Infiltration Defects Summary Updated Asset Register 22
Data Collection Water Usage & Pump Run Times To determine system inflows, water meters, electricity meters and pump run meters were read over an 8 day period. High pump run hours noted 23
Data Collection Inlet Flow Data from the WWTP Flow Data extracted from SCADA at the WWTP 8 days at 90 second timestep 6 months of historical flow data extracted at 10 min timestep 24
Inlet Flow Data 25
Data Collection Pressure Data For use in model calibration. UWS installed 6 pressure loggers across the system. Set to record pressures at a 30 second timestep Installed over an 8 day period. 26
Pressure Logger Locations 27
Pressure Logger Installation 28
Pressure Data 29
Rainfall Data Installed a rain gauge at the WWTP Obtained historical rainfall data from the Samoa Met Office 30
Model Development SWA provided the following the data for modelling: CAD layout of the pressure sewer system. Cadastral mapping. Contour data. Make/model and locations/size of the pump units. Pump curves. Water meter billing data/8 day pump and meter reads. 31
Model Configuration 100mm PVC Pressure sewer main Manufacturer's pump curve links each wet well node to the service line 90mm HDPE service line Pump wet well modelled as a node Sanitary flows from the building are loaded to the pump wet well node Each property modelled as a subcatchment 32
Model Layout 33
Model Calibration (Red = WWTP Inlet Meter Data, Green = Model Data) 34
Pressure Verification (Red = Pressure Logger Data, Green = Model Data) 35
System Analysis Calibrated model used to analysis system performance. Model run for 7 days to assess peak velocities. The WSAA pressure sewer code states the minimum flow velocity, which shall be achieved at least once in every 24 h, shall be 0.60 m/s. 36
Predicted Velocities 37
Wet Weather Response Wet flows can be double the daily volume during heavy rainfall Average daily volume at WWTP 450-500 kl/day Rainfall event 26/03/12 Total 36mm rainfall 38
Wet Weather Response Small rainfall event recorded on 31/3/12 (5mm) Pressure increase on foreshore line. Increase in flows at the WWTP Increase in pressure during storm event due to pumps operating. WWTP Flows Pressure increase on foreshore line 39
Target I/I Removal Foreshore Line Tooa Salamasina Blg Seal Manhole Diabetes Clinic Low Gully Seal rag screen Central Bank Infiltration into tank TTTE Building Direct Roof Connection RSA Club Infiltration through pump lid Library Tree root intrusion Low Gully 40
Summary of Existing Review In Dry Weather, the WWTP is only running at approx 50% capacity. Appeared to be plenty of capacity in major reticulation network. In major wet weather the WWTP receives flows close to capacity. On property defects and major locations of I/I identified. 41
Design Process Two design streams: On property design Reticulation design and analysis Future Connections: 108 new properties connected 3 mains extensions 42
Reticulation Design Survey Surveyor engaged to undertake topographic survey. Model Update New pipelines added. Future properties digitised into the model. Pump locations & service lines modelled. 43
Reticulation Design 44
Reticulation Design Sewage Loading Estimates Water consumption data sourced and average long term water usage estimated. 45
Reticulation Design Design Criteria Sewage estimates generally 80% ave daily water consumption. Seasonal facilities (eg. hotels) used peak 4 month average daily water consumption. Diurnal profile assigned to reflect business type 46
Design Criteria Reticulation Design Aquatec OGP pumps exclusively across the system. All new mains PE100 PN16 pipe in sizes 63mm, 90mm and 110mm. Minimum velocities as per WSAA standards 47
Reticulation Design Model Design Process Model run for 60 day simulation to randomise pump wet well levels at start of design run. Design run using 7 day simulation to cover full weekly range flows. Model simulated iteratively to optimise pipe selection while maintaining velocity and pressure requirements. Pump station sizing confirmed in conjunction with property design review and SWA preferences. 48
Daily Scouring Velocity 49
Future Flows to WWTP 50
Daily Flows to WWTP 51
Reticulation Design Plans 52
Summary Operation, performance and capacity assessment successfully undertaken on existing PSS. Calibrating the model against flow and pressure data provides confidence in system capacity analysis. Using InfoWorksCS enables pressure sewer design to be undertaken with confidence (predicted velocities and pressure etc.) Range of connection/flow scenarios can be tested. 53
Thank You 54