Pressure Sewer System Review & I/I Study, APIA, SAMOA

Flooding in Apia, the capital city of Samoa, is a frequent problem due to high rainfall, inadequate drainage systems and a lack of town planning in the past. Flooding after heavy rain posed a health risk due to sewage effluent from septic tanks being released to the environment. A growing urban population density, adverse ground conditions, a high water table and regular flooding meant that individual septic tanks in the denser urban areas could no longer safely contain, treat and dispose of wastewater.The Government of Samoa recognized the need to address Apia's wastewater management, sanitation and drainage issues and co-financed the Samoa Sanitation and Drainage Project which involved the construction of a pressure sewer system (PSS) and a wastewater treatment plant (WWTP) in 2010. The WWTP was designed to treat flows of 1ML/day however it is currently only receiving approximately 500kL/day of dry weather flow. To make best use of this additional treatment capacity the Samoa Water Authority (SWA) commissioned a comprehensive review and modelling analysis of the PSS to determine the current available system capacity and to optimise the operational mode such that more connections could be made to the system.Unusually for a pressure sewer system, stormwater water inflow is a major issue due to stormwater entering via private properties into the PSS pump wet wells. During dry weather the WWTP is running at approximately 50% of capacity however during wet weather the peak flows to the plant are near capacity. The extent of future connections that could be made onto the system would ultimately be a function of how successful any inflow/infiltration reduction measures were in reducing wet weather flows.This paper provides details of the extensive and detailed review undertaken on the Apia pressure sewer system. The paper discusses a range of project tasks including physical inspections of the pumping stations and private drainage, I/I source detection, field testing, hydraulic modelling, system performance analysis and capacity assessment.